| Message ID | 20220228110822.491923-3-jakobkoschel@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Remove usage of list iterator past the loop body | expand |
> On 28. Feb 2022, at 12:20, Greg KH <gregkh@linuxfoundation.org> wrote: > > On Mon, Feb 28, 2022 at 12:08:18PM +0100, Jakob Koschel wrote: >> If the list does not contain the expected element, the value of >> list_for_each_entry() iterator will not point to a valid structure. >> To avoid type confusion in such case, the list iterator >> scope will be limited to list_for_each_entry() loop. >> >> In preparation to limiting scope of a list iterator to the list traversal >> loop, use a dedicated pointer to point to the found element. >> Determining if an element was found is then simply checking if >> the pointer is != NULL. >> >> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com> >> --- >> arch/x86/kernel/cpu/sgx/encl.c | 6 +++-- >> drivers/scsi/scsi_transport_sas.c | 17 ++++++++----- >> drivers/thermal/thermal_core.c | 38 ++++++++++++++++++---------- >> drivers/usb/gadget/configfs.c | 22 ++++++++++------ >> drivers/usb/gadget/udc/max3420_udc.c | 11 +++++--- >> drivers/usb/gadget/udc/tegra-xudc.c | 11 +++++--- >> drivers/usb/mtu3/mtu3_gadget.c | 11 +++++--- >> drivers/usb/musb/musb_gadget.c | 11 +++++--- >> drivers/vfio/mdev/mdev_core.c | 11 +++++--- >> 9 files changed, 88 insertions(+), 50 deletions(-) > > The drivers/usb/ portion of this patch should be in patch 1/X, right? I kept them separate since it's a slightly different case. Using 'ptr' instead of '&ptr->other_member'. Regardless, I'll split this commit up as you mentioned. > > Also, you will have to split these up per-subsystem so that the > different subsystem maintainers can take these in their trees. Thanks for the feedback. To clarify I understand you correctly: I should do one patch set per-subsystem with every instance/(file?) fixed as a separate commit? If I understand correctly, I'll repost accordingly. > > thanks, > > greg k-h thanks, Jakob Koschel
On Mon, Feb 28, 2022 at 4:19 AM Christian König <christian.koenig@amd.com> wrote: > > I don't think that using the extra variable makes the code in any way > more reliable or easier to read. So I think the next step is to do the attached patch (which requires that "-std=gnu11" that was discussed in the original thread). That will guarantee that the 'pos' parameter of list_for_each_entry() is only updated INSIDE the for_each_list_entry() loop, and can never point to the (wrongly typed) head entry. And I would actually hope that it should actually cause compiler warnings about possibly uninitialized variables if people then use the 'pos' pointer outside the loop. Except (a) that code in sgx/encl.c currently initializes 'tmp' to NULL for inexplicable reasons - possibly because it already expected this behavior (b) when I remove that NULL initializer, I still don't get a warning, because we've disabled -Wno-maybe-uninitialized since it results in so many false positives. Oh well. Anyway, give this patch a look, and at least if it's expanded to do "(pos) = NULL" in the entry statement for the for-loop, it will avoid the HEAD type confusion that Jakob is working on. And I think in a cleaner way than the horrid games he plays. (But it won't avoid possible CPU speculation of such type confusion. That, in my opinion, is a completely different issue) I do wish we could actually poison the 'pos' value after the loop somehow - but clearly the "might be uninitialized" I was hoping for isn't the way to do it. Anybody have any ideas? Linus
On Mon, Feb 28, 2022 at 11:56 AM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > I do wish we could actually poison the 'pos' value after the loop > somehow - but clearly the "might be uninitialized" I was hoping for > isn't the way to do it. Side note: we do need *some* way to do it. Because if we make that change, and only set it to another pointer when not the head, then we very much change the semantics of "list_for_each_head()". The "list was empty" case would now exit with 'pos' not set at all (or set to NULL if we add that). Or it would be set to the last entry. And regardless, that means that all the if (pos == head) kinds of checks after the loop would be fundamentally broken. Darn. I really hoped for (and naively expected) that we could actually have the compiler warn about the use-after-loop case. That whole "compiler will now complain about bad use" was integral to my clever plan to use the C99 feature of declaring the iterator inside the loop. But my "clever plan" was apparently some ACME-level Wile E. Coyote sh*t. Darn. Linus
Am 28.02.22 um 20:56 schrieb Linus Torvalds: > On Mon, Feb 28, 2022 at 4:19 AM Christian König > <christian.koenig@amd.com> wrote: >> I don't think that using the extra variable makes the code in any way >> more reliable or easier to read. > So I think the next step is to do the attached patch (which requires > that "-std=gnu11" that was discussed in the original thread). > > That will guarantee that the 'pos' parameter of list_for_each_entry() > is only updated INSIDE the for_each_list_entry() loop, and can never > point to the (wrongly typed) head entry. > > And I would actually hope that it should actually cause compiler > warnings about possibly uninitialized variables if people then use the > 'pos' pointer outside the loop. Except > > (a) that code in sgx/encl.c currently initializes 'tmp' to NULL for > inexplicable reasons - possibly because it already expected this > behavior > > (b) when I remove that NULL initializer, I still don't get a warning, > because we've disabled -Wno-maybe-uninitialized since it results in so > many false positives. > > Oh well. > > Anyway, give this patch a look, and at least if it's expanded to do > "(pos) = NULL" in the entry statement for the for-loop, it will avoid > the HEAD type confusion that Jakob is working on. And I think in a > cleaner way than the horrid games he plays. > > (But it won't avoid possible CPU speculation of such type confusion. > That, in my opinion, is a completely different issue) Yes, completely agree. > I do wish we could actually poison the 'pos' value after the loop > somehow - but clearly the "might be uninitialized" I was hoping for > isn't the way to do it. > > Anybody have any ideas? I think we should look at the use cases why code is touching (pos) after the loop. Just from skimming over the patches to change this and experience with the drivers/subsystems I help to maintain I think the primary pattern looks something like this: list_for_each_entry(entry, head, member) { if (some_condition_checking(entry)) break; } do_something_with(entry); So the solution should probably not be to change all those use cases to use more temporary variables, but rather to add a list_find_entry(..., condition) macro and consistently use that one instead. Regards, Christian. > > Linus
On Mon, Feb 28, 2022 at 12:03 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Side note: we do need *some* way to do it. Ooh. This patch is a work of art. And I mean that in the worst possible way. We can do typeof(pos) pos in the 'for ()' loop, and never use __iter at all. That means that inside the for-loop, we use a _different_ 'pos' than outside. And then the compiler will not see some "might be uninitialized", but the outer 'pos' *will* be uninitialized. Unless, of course, the outer 'pos' had that pointless explicit initializer. Here - can somebody poke holes in this "work of art" patch? Linus
On Mon, Feb 28, 2022 at 12:10 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > We can do > > typeof(pos) pos > > in the 'for ()' loop, and never use __iter at all. > > That means that inside the for-loop, we use a _different_ 'pos' than outside. The thing that makes me throw up in my mouth a bit is that in that typeof(pos) pos the first 'pos' (that we use for just the typeof) is that outer-level 'pos', IOW it's a *different* 'pos' than the second 'pos' in that same declaration that declares the inner level shadowing new 'pos' variable. If I was a compiler person, I would say "Linus, that thing is too ugly to live", and I would hate it. I'm just hoping that even compiler people say "that's *so* ugly it's almost beautiful". Because it does seem to work. It's not pretty, but hey, it's not like our headers are really ever be winning any beauty contests... Linus
On Mon, Feb 28, 2022 at 12:10:24PM -0800, Linus Torvalds wrote: > We can do > > typeof(pos) pos > > in the 'for ()' loop, and never use __iter at all. > > That means that inside the for-loop, we use a _different_ 'pos' than outside. Then we can never use -Wshadow ;-( I'd love to be able to turn it on; it catches real bugs. > +#define list_for_each_entry(pos, head, member) \ > + for (typeof(pos) pos = list_first_entry(head, typeof(*pos), member); \ > + !list_entry_is_head(pos, head, member); \ > pos = list_next_entry(pos, member))
On Mon, 2022-02-28 at 20:16 +0000, Matthew Wilcox wrote: > On Mon, Feb 28, 2022 at 12:10:24PM -0800, Linus Torvalds wrote: > > We can do > > > > typeof(pos) pos > > > > in the 'for ()' loop, and never use __iter at all. > > > > That means that inside the for-loop, we use a _different_ 'pos' than outside. > > Then we can never use -Wshadow ;-( I'd love to be able to turn it on; > it catches real bugs. > I was just going to say the same thing... If we're willing to change the API for the macro, we could do list_for_each_entry(type, pos, head, member) and then actually take advantage of -Wshadow? johannes
On Mon, Feb 28, 2022 at 12:16 PM Matthew Wilcox <willy@infradead.org> wrote: > > Then we can never use -Wshadow ;-( I'd love to be able to turn it on; > it catches real bugs. Oh, we already can never use -Wshadow regardless of things like this. That bridge hasn't just been burned, it never existed in the first place. The whole '-Wshadow' thing simply cannot work with local variables in macros - something that we've used since day 1. Try this (as a "p.c" file): #define min(a,b) ({ \ typeof(a) __a = (a); \ typeof(b) __b = (b); \ __a < __b ? __a : __b; }) int min3(int a, int b, int c) { return min(a,min(b,c)); } and now do "gcc -O2 -S t.c". Then try it with -Wshadow. In other words, -Wshadow is simply not acceptable. Never has been, never will be, and that has nothing to do with the typeof(pos) pos kind of thing. Your argument just isn't an argument. Linus
On Mon, Feb 28, 2022 at 12:29 PM Johannes Berg <johannes@sipsolutions.net> wrote: > > If we're willing to change the API for the macro, we could do > > list_for_each_entry(type, pos, head, member) > > and then actually take advantage of -Wshadow? See my reply to Willy. There is no way -Wshadow will ever happen. I considered that (type, pos, head, member) kind of thing, to the point of trying it for one file, but it ends up as horrendous syntax. It turns out that declaring the type separately really helps, and avoids crazy long lines among other things. It would be unacceptable for another reason too - the amount of churn would just be immense. Every single use of that macro (and related macros) would change, even the ones that really don't need it or want it (ie the good kinds that already only use the variable inside the loop). So "typeof(pos) pos" may be ugly - but it's a very localized ugly. Linus
On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: > Am 28.02.22 um 20:56 schrieb Linus Torvalds: > > On Mon, Feb 28, 2022 at 4:19 AM Christian König > > <christian.koenig@amd.com> wrote: > > > I don't think that using the extra variable makes the code in any > > > way > > > more reliable or easier to read. > > So I think the next step is to do the attached patch (which > > requires > > that "-std=gnu11" that was discussed in the original thread). > > > > That will guarantee that the 'pos' parameter of > > list_for_each_entry() > > is only updated INSIDE the for_each_list_entry() loop, and can > > never > > point to the (wrongly typed) head entry. > > > > And I would actually hope that it should actually cause compiler > > warnings about possibly uninitialized variables if people then use > > the > > 'pos' pointer outside the loop. Except > > > > (a) that code in sgx/encl.c currently initializes 'tmp' to NULL > > for > > inexplicable reasons - possibly because it already expected this > > behavior > > > > (b) when I remove that NULL initializer, I still don't get a > > warning, > > because we've disabled -Wno-maybe-uninitialized since it results in > > so > > many false positives. > > > > Oh well. > > > > Anyway, give this patch a look, and at least if it's expanded to do > > "(pos) = NULL" in the entry statement for the for-loop, it will > > avoid the HEAD type confusion that Jakob is working on. And I think > > in a cleaner way than the horrid games he plays. > > > > (But it won't avoid possible CPU speculation of such type > > confusion. That, in my opinion, is a completely different issue) > > Yes, completely agree. > > > I do wish we could actually poison the 'pos' value after the loop > > somehow - but clearly the "might be uninitialized" I was hoping for > > isn't the way to do it. > > > > Anybody have any ideas? > > I think we should look at the use cases why code is touching (pos) > after the loop. > > Just from skimming over the patches to change this and experience > with the drivers/subsystems I help to maintain I think the primary > pattern looks something like this: > > list_for_each_entry(entry, head, member) { > if (some_condition_checking(entry)) > break; > } > do_something_with(entry); Actually, we usually have a check to see if the loop found anything, but in that case it should something like if (list_entry_is_head(entry, head, member)) { return with error; } do_somethin_with(entry); Suffice? The list_entry_is_head() macro is designed to cope with the bogus entry on head problem. James
On Mon, Feb 28, 2022 at 12:14:44PM -0800, Linus Torvalds wrote: > On Mon, Feb 28, 2022 at 12:10 PM Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > We can do > > > > typeof(pos) pos > > > > in the 'for ()' loop, and never use __iter at all. > > > > That means that inside the for-loop, we use a _different_ 'pos' than outside. > > The thing that makes me throw up in my mouth a bit is that in that > > typeof(pos) pos > > the first 'pos' (that we use for just the typeof) is that outer-level > 'pos', IOW it's a *different* 'pos' than the second 'pos' in that same > declaration that declares the inner level shadowing new 'pos' > variable. The new "pos" has not yet been declared, so this has to refer to the outer "pos", it cannot be the inner one. Because it hasn't been declared yet :-) Compare this to typeof (pos) pos = pos; where that last "pos" *does* refer to the newly declared one: that declaration has already been done! (So this code is UB btw, 6.3.2.1/2). > If I was a compiler person, I would say "Linus, that thing is too ugly > to live", and I would hate it. I'm just hoping that even compiler > people say "that's *so* ugly it's almost beautiful". It is perfectly well-defined. Well, it would be good if we (GCC) would document it does work, and if someone tested it on LLVM as well. But it is really hard to implement it to *not* work :-) > Because it does seem to work. It's not pretty, but hey, it's not like > our headers are really ever be winning any beauty contests... It is very pretty! Needs a comment though :-) Segher
Am 28.02.22 um 21:42 schrieb James Bottomley: > On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: >> Am 28.02.22 um 20:56 schrieb Linus Torvalds: >>> On Mon, Feb 28, 2022 at 4:19 AM Christian König >>> <christian.koenig@amd.com> wrote: >>> [SNIP] >>> Anybody have any ideas? >> I think we should look at the use cases why code is touching (pos) >> after the loop. >> >> Just from skimming over the patches to change this and experience >> with the drivers/subsystems I help to maintain I think the primary >> pattern looks something like this: >> >> list_for_each_entry(entry, head, member) { >> if (some_condition_checking(entry)) >> break; >> } >> do_something_with(entry); > > Actually, we usually have a check to see if the loop found anything, > but in that case it should something like > > if (list_entry_is_head(entry, head, member)) { > return with error; > } > do_somethin_with(entry); > > Suffice? The list_entry_is_head() macro is designed to cope with the > bogus entry on head problem. That will work and is also what people already do. The key problem is that we let people do the same thing over and over again with slightly different implementations. Out in the wild I've seen at least using a separate variable, using a bool to indicate that something was found and just assuming that the list has an entry. The last case is bogus and basically what can break badly. If we would have an unified macro which search for an entry combined with automated reporting on patches to use that macro I think the potential to introduce such issues will already go down massively without auditing tons of existing code. Regards, Christian. > > James > >
On Mon, 2022-02-28 at 21:56 +0100, Christian König wrote: > > Am 28.02.22 um 21:42 schrieb James Bottomley: > > On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: > > > Am 28.02.22 um 20:56 schrieb Linus Torvalds: > > > > On Mon, Feb 28, 2022 at 4:19 AM Christian König > > > > <christian.koenig@amd.com> wrote: > > > > [SNIP] > > > > Anybody have any ideas? > > > I think we should look at the use cases why code is touching > > > (pos) > > > after the loop. > > > > > > Just from skimming over the patches to change this and experience > > > with the drivers/subsystems I help to maintain I think the > > > primary pattern looks something like this: > > > > > > list_for_each_entry(entry, head, member) { > > > if (some_condition_checking(entry)) > > > break; > > > } > > > do_something_with(entry); > > > > Actually, we usually have a check to see if the loop found > > anything, but in that case it should something like > > > > if (list_entry_is_head(entry, head, member)) { > > return with error; > > } > > do_somethin_with(entry); > > > > Suffice? The list_entry_is_head() macro is designed to cope with > > the bogus entry on head problem. > > That will work and is also what people already do. > > The key problem is that we let people do the same thing over and > over again with slightly different implementations. > > Out in the wild I've seen at least using a separate variable, using > a bool to indicate that something was found and just assuming that > the list has an entry. > > The last case is bogus and basically what can break badly. Yes, I understand that. I'm saying we should replace that bogus checks of entry->something against some_value loop termination condition with the list_entry_is_head() macro. That should be a one line and fairly mechanical change rather than the explosion of code changes we seem to have in the patch series. James
On Mon, Feb 28, 2022 at 3:45 PM James Bottomley <James.Bottomley@hansenpartnership.com> wrote: > ... > > Just from skimming over the patches to change this and experience > > with the drivers/subsystems I help to maintain I think the primary > > pattern looks something like this: > > > > list_for_each_entry(entry, head, member) { > > if (some_condition_checking(entry)) > > break; > > } > > do_something_with(entry); > > > Actually, we usually have a check to see if the loop found anything, > but in that case it should something like > > if (list_entry_is_head(entry, head, member)) { > return with error; > } > do_somethin_with(entry); Borrowing from c++, perhaps an explicit end should be used: if (list_entry_not_end(entry)) do_somethin_with(entry) It is modelled after c++ and the 'while(begin != end) {}' pattern. Jeff
> On 28. Feb 2022, at 21:10, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Mon, Feb 28, 2022 at 12:03 PM Linus Torvalds > <torvalds@linux-foundation.org> wrote: >> >> Side note: we do need *some* way to do it. > > Ooh. > > This patch is a work of art. > > And I mean that in the worst possible way. > > We can do > > typeof(pos) pos > > in the 'for ()' loop, and never use __iter at all. > > That means that inside the for-loop, we use a _different_ 'pos' than outside. > > And then the compiler will not see some "might be uninitialized", but > the outer 'pos' *will* be uninitialized. > > Unless, of course, the outer 'pos' had that pointless explicit initializer. The goal of this is to get compiler warnings right? This would indeed be great. Changing the list_for_each_entry() macro first will break all of those cases (e.g. the ones using 'list_entry_is_head()). I assumed it is better to fix those cases first and then have a simple coccinelle script changing the macro + moving the iterator into the scope of the macro. > > Here - can somebody poke holes in this "work of art" patch? With this you are no longer able to set the 'outer' pos within the list iterator loop body or am I missing something? Like this it stays uninitialized but you'll probably want to set it from within the loop. You would then yet again need a variable with another name to use after the loop. I fail to see how this will make most of the changes in this patch obsolete (if that was the intention). > > Linus > <patch.diff> - Jakob
On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley <James.Bottomley@HansenPartnership.com> wrote: >On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: >> Am 28.02.22 um 20:56 schrieb Linus Torvalds: >> > On Mon, Feb 28, 2022 at 4:19 AM Christian König >> > <christian.koenig@amd.com> wrote: >> > > I don't think that using the extra variable makes the code in any >> > > way >> > > more reliable or easier to read. >> > So I think the next step is to do the attached patch (which >> > requires >> > that "-std=gnu11" that was discussed in the original thread). >> > >> > That will guarantee that the 'pos' parameter of >> > list_for_each_entry() >> > is only updated INSIDE the for_each_list_entry() loop, and can >> > never >> > point to the (wrongly typed) head entry. >> > >> > And I would actually hope that it should actually cause compiler >> > warnings about possibly uninitialized variables if people then use >> > the >> > 'pos' pointer outside the loop. Except >> > >> > (a) that code in sgx/encl.c currently initializes 'tmp' to NULL >> > for >> > inexplicable reasons - possibly because it already expected this >> > behavior >> > >> > (b) when I remove that NULL initializer, I still don't get a >> > warning, >> > because we've disabled -Wno-maybe-uninitialized since it results in >> > so >> > many false positives. >> > >> > Oh well. >> > >> > Anyway, give this patch a look, and at least if it's expanded to do >> > "(pos) = NULL" in the entry statement for the for-loop, it will >> > avoid the HEAD type confusion that Jakob is working on. And I think >> > in a cleaner way than the horrid games he plays. >> > >> > (But it won't avoid possible CPU speculation of such type >> > confusion. That, in my opinion, is a completely different issue) >> >> Yes, completely agree. >> >> > I do wish we could actually poison the 'pos' value after the loop >> > somehow - but clearly the "might be uninitialized" I was hoping for >> > isn't the way to do it. >> > >> > Anybody have any ideas? >> >> I think we should look at the use cases why code is touching (pos) >> after the loop. >> >> Just from skimming over the patches to change this and experience >> with the drivers/subsystems I help to maintain I think the primary >> pattern looks something like this: >> >> list_for_each_entry(entry, head, member) { >> if (some_condition_checking(entry)) >> break; >> } >> do_something_with(entry); > > >Actually, we usually have a check to see if the loop found anything, >but in that case it should something like > >if (list_entry_is_head(entry, head, member)) { > return with error; >} >do_somethin_with(entry); > >Suffice? The list_entry_is_head() macro is designed to cope with the >bogus entry on head problem. Won't suffice because the end goal of this work is to limit scope of entry only to loop. Hence the need for additional variable. Besides, there are no guarantees that people won't do_something_with(entry) without the check or won't compare entry to NULL to check if the loop finished with break or not. >James
> On 28. Feb 2022, at 21:56, Christian König <christian.koenig@amd.com> wrote: > > > > Am 28.02.22 um 21:42 schrieb James Bottomley: >> On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: >>> Am 28.02.22 um 20:56 schrieb Linus Torvalds: >>>> On Mon, Feb 28, 2022 at 4:19 AM Christian König >>>> <christian.koenig@amd.com> wrote: >>>> [SNIP] >>>> Anybody have any ideas? >>> I think we should look at the use cases why code is touching (pos) >>> after the loop. >>> >>> Just from skimming over the patches to change this and experience >>> with the drivers/subsystems I help to maintain I think the primary >>> pattern looks something like this: >>> >>> list_for_each_entry(entry, head, member) { >>> if (some_condition_checking(entry)) >>> break; >>> } >>> do_something_with(entry); There are other cases where the list iterator variable is used after the loop Some examples: - list_for_each_entry_continue() and list_for_each_entry_from(). - (although very rare) the head is actually of the correct struct type. (ppc440spe_get_group_entry(): drivers/dma/ppc4xx/adma.c:1436) - to use pos->list for example for list_add_tail(): (add_static_vm_early(): arch/arm/mm/ioremap.c:107) If the scope of the list iterator is limited those still need fixing in a different way. >> >> Actually, we usually have a check to see if the loop found anything, >> but in that case it should something like >> >> if (list_entry_is_head(entry, head, member)) { >> return with error; >> } >> do_somethin_with(entry); >> >> Suffice? The list_entry_is_head() macro is designed to cope with the >> bogus entry on head problem. > > That will work and is also what people already do. > > The key problem is that we let people do the same thing over and over again with slightly different implementations. > > Out in the wild I've seen at least using a separate variable, using a bool to indicate that something was found and just assuming that the list has an entry. > > The last case is bogus and basically what can break badly. > > If we would have an unified macro which search for an entry combined with automated reporting on patches to use that macro I think the potential to introduce such issues will already go down massively without auditing tons of existing code. Having a unified way to do the same thing would indeed be great. > > Regards, > Christian. > >> >> James >> >> > - Jakob
On Mon, 2022-02-28 at 23:59 +0200, Mike Rapoport wrote: > > On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley < > James.Bottomley@HansenPartnership.com> wrote: > > On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: [...] > > > > I do wish we could actually poison the 'pos' value after the > > > > loop somehow - but clearly the "might be uninitialized" I was > > > > hoping for isn't the way to do it. > > > > > > > > Anybody have any ideas? > > > > > > I think we should look at the use cases why code is touching > > > (pos) after the loop. > > > > > > Just from skimming over the patches to change this and experience > > > with the drivers/subsystems I help to maintain I think the > > > primary pattern looks something like this: > > > > > > list_for_each_entry(entry, head, member) { > > > if (some_condition_checking(entry)) > > > break; > > > } > > > do_something_with(entry); > > > > Actually, we usually have a check to see if the loop found > > anything, but in that case it should something like > > > > if (list_entry_is_head(entry, head, member)) { > > return with error; > > } > > do_somethin_with(entry); > > > > Suffice? The list_entry_is_head() macro is designed to cope with > > the bogus entry on head problem. > > Won't suffice because the end goal of this work is to limit scope of > entry only to loop. Hence the need for additional variable. Well, yes, but my objection is more to the size of churn than the desire to do loop local. I'm not even sure loop local is possible, because it's always annoyed me that for (int i = 0; ... in C++ defines i in the outer scope not the loop scope, which is why I never use it. However, if the desire is really to poison the loop variable then we can do #define list_for_each_entry(pos, head, member) \ for (pos = list_first_entry(head, typeof(*pos), member); \ !list_entry_is_head(pos, head, member) && ((pos = NULL) == NULL; \ pos = list_next_entry(pos, member)) Which would at least set pos to NULL when the loop completes. > Besides, there are no guarantees that people won't > do_something_with(entry) without the check or won't compare entry to > NULL to check if the loop finished with break or not. I get the wider goal, but we have to patch the problem cases now and a simple one-liner is better than a larger patch that may or may not work if we ever achieve the local definition or value poisoning idea. I'm also fairly certain coccinelle can come up with a use without checking for loop completion semantic patch which we can add to 0day. James
Hi 2022. február 28., hétfő 23:28 keltezéssel, James Bottomley írta: > [...] > Well, yes, but my objection is more to the size of churn than the > desire to do loop local. I'm not even sure loop local is possible, > because it's always annoyed me that for (int i = 0; ... in C++ defines > i in the outer scope not the loop scope, which is why I never use it. It is arguably off-topic to the discussion at hand, but I think you might be thinking of something else (or maybe it was the case in an ancient version of C++) because that does not appear to be case. If it were, for (int i ...) { ... } for (int i ...) { ... } would have to trigger a redeclaration error, but that happens neither in C++ nor in C. The variable is also inaccessible outside the loop. > [...] Regards, Barnabás Pőcze
On Mon, Feb 28, 2022 at 12:37:15PM -0800, Linus Torvalds wrote: > On Mon, Feb 28, 2022 at 12:16 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > Then we can never use -Wshadow ;-( I'd love to be able to turn it on; > > it catches real bugs. > > Oh, we already can never use -Wshadow regardless of things like this. > That bridge hasn't just been burned, it never existed in the first > place. > > The whole '-Wshadow' thing simply cannot work with local variables in > macros - something that we've used since day 1. > > Try this (as a "p.c" file): > > #define min(a,b) ({ \ > typeof(a) __a = (a); \ > typeof(b) __b = (b); \ > __a < __b ? __a : __b; }) > > int min3(int a, int b, int c) > { > return min(a,min(b,c)); > } > > and now do "gcc -O2 -S t.c". > > Then try it with -Wshadow. #define ___PASTE(a, b) a##b #define __PASTE(a, b) ___PASTE(a, b) #define _min(a, b, u) ({ \ typeof(a) __PASTE(__a,u) = (a); \ typeof(b) __PASTE(__b,u) = (b); \ __PASTE(__a,u) < __PASTE(__b,u) ? __PASTE(__a,u) : __PASTE(__b,u); }) #define min(a, b) _min(a, b, __COUNTER__) int min3(int a, int b, int c) { return min(a,min(b,c)); } (probably there's a more elegant way to do this)
On Mon, Feb 28, 2022 at 05:28:58PM -0500, James Bottomley wrote: > On Mon, 2022-02-28 at 23:59 +0200, Mike Rapoport wrote: > > > > On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley < > > James.Bottomley@HansenPartnership.com> wrote: > > > On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: > [...] > > > > > I do wish we could actually poison the 'pos' value after the > > > > > loop somehow - but clearly the "might be uninitialized" I was > > > > > hoping for isn't the way to do it. > > > > > > > > > > Anybody have any ideas? > > > > > > > > I think we should look at the use cases why code is touching > > > > (pos) after the loop. > > > > > > > > Just from skimming over the patches to change this and experience > > > > with the drivers/subsystems I help to maintain I think the > > > > primary pattern looks something like this: > > > > > > > > list_for_each_entry(entry, head, member) { > > > > if (some_condition_checking(entry)) > > > > break; > > > > } > > > > do_something_with(entry); > > > > > > Actually, we usually have a check to see if the loop found > > > anything, but in that case it should something like > > > > > > if (list_entry_is_head(entry, head, member)) { > > > return with error; > > > } > > > do_somethin_with(entry); > > > > > > Suffice? The list_entry_is_head() macro is designed to cope with > > > the bogus entry on head problem. > > > > Won't suffice because the end goal of this work is to limit scope of > > entry only to loop. Hence the need for additional variable. > > Well, yes, but my objection is more to the size of churn than the > desire to do loop local. I'm not even sure loop local is possible, > because it's always annoyed me that for (int i = 0; ... in C++ defines > i in the outer scope not the loop scope, which is why I never use it. In C its scope is the rest of the declaration and the entire loop, not anything after it. This was the same in C++98 already, btw (but in pre-standard versions of C++ things were like you remember, yes, and it was painful). Segher
On Mon, Feb 28, 2022 at 1:47 PM Jakob Koschel <jakobkoschel@gmail.com> wrote: > > The goal of this is to get compiler warnings right? This would indeed be great. Yes, so I don't mind having a one-time patch that has been gathered using some automated checker tool, but I don't think that works from a long-term maintenance perspective. So if we have the basic rule being "don't use the loop iterator after the loop has finished, because it can cause all kinds of subtle issues", then in _addition_ to fixing the existing code paths that have this issue, I really would want to (a) get a compiler warning for future cases and (b) make it not actually _work_ for future cases. Because otherwise it will just happen again. > Changing the list_for_each_entry() macro first will break all of those cases > (e.g. the ones using 'list_entry_is_head()). So I have no problems with breaking cases that we basically already have a patch for due to your automated tool. There were certainly more than a handful, but it didn't look _too_ bad to just make the rule be "don't use the iterator after the loop". Of course, that's just based on that patch of yours. Maybe there are a ton of other cases that your patch didn't change, because they didn't match your trigger case, so I may just be overly optimistic here. But basically to _me_, the important part is that the end result is maintainable longer-term. I'm more than happy to have a one-time patch to fix a lot of dubious cases if we can then have clean rules going forward. > I assumed it is better to fix those cases first and then have a simple > coccinelle script changing the macro + moving the iterator into the scope > of the macro. So that had been another plan of mine, until I actually looked at changing the macro. In the one case I looked at, it was ugly beyond belief. It turns out that just syntactically, it's really nice to give the type of the iterator from outside the way we do now. Yeah, it may be a bit odd, and maybe it's partly because I'm so used to the "list_for_each_list_entry()" syntax, but moving the type into the loop construct really made it nasty - either one very complex line, or having to split it over two lines which was even worse. Maybe the place I looked at just happened to have a long typename, but it's basically always going to be a struct, so it's never a _simple_ type. And it just looked very odd adn unnatural to have the type as one of the "arguments" to that list_for_each_entry() macro. So yes, initially my idea had been to just move the iterator entirely inside the macro. But specifying the type got so ugly that I think that typeof (pos) pos trick inside the macro really ends up giving us the best of all worlds: (a) let's us keep the existing syntax and code for all the nice cases that did everything inside the loop anyway (b) gives us a nice warning for any normal use-after-loop case (unless you explicitly initialized it like that sgx_mmu_notifier_release() function did for no good reason (c) also guarantees that even if you don't get a warning, non-converted (or newly written) bad code won't actually _work_ so you end up getting the new rules without any ambiguity or mistaken > With this you are no longer able to set the 'outer' pos within the list > iterator loop body or am I missing something? Correct. Any assignment inside the loop will be entirely just to the local loop case. So any "break;" out of the loop will have to set another variable - like your updated patch did. > I fail to see how this will make most of the changes in this > patch obsolete (if that was the intention). I hope my explanation above clarifies my thinking: I do not dislike your patch, and in fact your patch is indeed required to make the new semantics work. What I disliked was always the maintainability of your patch - making the rules be something that isn't actually visible in the source code, and letting the old semantics still work as well as they ever did, and having to basically run some verification pass to find bad users. (I also disliked your original patch that mixed up the "CPU speculation type safety" with the actual non-speculative problems, but that was another issue). Linus
On Mon, Feb 28, 2022 at 3:26 PM Matthew Wilcox <willy@infradead.org> wrote: > > #define ___PASTE(a, b) a##b > #define __PASTE(a, b) ___PASTE(a, b) > #define _min(a, b, u) ({ \ Yeah, except that's ugly beyond belief, plus it's literally not what we do in the kernel. Really. The "-Wshadow doesn't work on the kernel" is not some new issue, because you have to do completely insane things to the source code to enable it. Just compare your uglier-than-sin version to my straightforward one. One does the usual and obvious "use a private variable to avoid the classic multi-use of a macro argument". And the other one is an abomination. Linus
On Mon, Feb 28, 2022 at 4:38 PM Segher Boessenkool <segher@kernel.crashing.org> wrote: > > In C its scope is the rest of the declaration and the entire loop, not > anything after it. This was the same in C++98 already, btw (but in > pre-standard versions of C++ things were like you remember, yes, and it > was painful). Yeah, the original C++ model was just unadulterated garbage, with no excuse for it, and the scope was not the loop, but the block the loop existed in. That would never have been acceptable for the kernel - it's basically just an even uglier version of "put variable declarations in the middle of code" (and we use "-Wdeclaration-after-statement" to disallow that for kernel code, although apparently some of our user space tooling code doesn't enforce or follow that rule). The actual C99 version is the sane one which actually makes it easier and clearer to have loop iterators that are clearly just in loop scope. That's a good idea in general, and I have wanted to start using that in the kernel even aside from some of the loop construct macros. Because putting variables in natural minimal scope is a GoodThing(tm). Of course, we shouldn't go crazy with it. Even after we do that -std=gnu11 thing, we'll have backports to worry about. And it's not clear that we necessarily want to backport that gnu11 thing - since people who run old stable kernels also may be still using those really old compilers... Linus
On Mon, Feb 28, 2022 at 4:45 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Yeah, except that's ugly beyond belief, plus it's literally not what > we do in the kernel. (Of course, I probably shouldn't have used 'min()' as an example, because that is actually one of the few places where we do exactly that, using our __UNIQUE_ID() macros. Exactly because people _have_ tried to do -Wshadow when doing W=2). Linus
From: Linus Torvalds > Sent: 28 February 2022 19:56 > On Mon, Feb 28, 2022 at 4:19 AM Christian König > <christian.koenig@amd.com> wrote: > > > > I don't think that using the extra variable makes the code in any way > > more reliable or easier to read. > > So I think the next step is to do the attached patch (which requires > that "-std=gnu11" that was discussed in the original thread). > > That will guarantee that the 'pos' parameter of list_for_each_entry() > is only updated INSIDE the for_each_list_entry() loop, and can never > point to the (wrongly typed) head entry. > > And I would actually hope that it should actually cause compiler > warnings about possibly uninitialized variables if people then use the > 'pos' pointer outside the loop. Except > > (a) that code in sgx/encl.c currently initializes 'tmp' to NULL for > inexplicable reasons - possibly because it already expected this > behavior > > (b) when I remove that NULL initializer, I still don't get a warning, > because we've disabled -Wno-maybe-uninitialized since it results in so > many false positives. > > Oh well. > > Anyway, give this patch a look, and at least if it's expanded to do > "(pos) = NULL" in the entry statement for the for-loop, it will avoid > the HEAD type confusion that Jakob is working on. And I think in a > cleaner way than the horrid games he plays. > > (But it won't avoid possible CPU speculation of such type confusion. > That, in my opinion, is a completely different issue) > > I do wish we could actually poison the 'pos' value after the loop > somehow - but clearly the "might be uninitialized" I was hoping for > isn't the way to do it. > > Anybody have any ideas? > > Linus diff --git a/include/linux/list.h b/include/linux/list.h index dd6c2041d09c..bab995596aaa 100644 --- a/include/linux/list.h +++ b/include/linux/list.h @@ -634,10 +634,10 @@ static inline void list_splice_tail_init(struct list_head *list, * @head: the head for your list. * @member: the name of the list_head within the struct. */ -#define list_for_each_entry(pos, head, member) \ - for (pos = list_first_entry(head, typeof(*pos), member); \ - !list_entry_is_head(pos, head, member); \ - pos = list_next_entry(pos, member)) +#define list_for_each_entry(pos, head, member) \ + for (typeof(pos) __iter = list_first_entry(head, typeof(*pos), member); \ + !list_entry_is_head(__iter, head, member) && (((pos)=__iter),1); \ + __iter = list_next_entry(__iter, member)) /** * list_for_each_entry_reverse - iterate backwards over list of given type. I think you actually want: !list_entry_is_head(__iter, head, member) ? (((pos)=__iter),1) : (((pos) = NULL),0); Which can be done in the original by: !list_entry_is_head(pos, head, member) ? 1 : (((pos) = NULL), 0); Although it would be safer to have (without looking up the actual name): for (item *__item = head; \ __item ? (((pos) = list_item(__item, member)), 1) : (((pos) = NULL), 0); __item = (pos)->member) The local does need 'fixing' to avoid shadowing for nested loops. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
From: Matthew Wilcox > Sent: 28 February 2022 20:16 > > On Mon, Feb 28, 2022 at 12:10:24PM -0800, Linus Torvalds wrote: > > We can do > > > > typeof(pos) pos > > > > in the 'for ()' loop, and never use __iter at all. > > > > That means that inside the for-loop, we use a _different_ 'pos' than outside. > > Then we can never use -Wshadow ;-( I'd love to be able to turn it on; > it catches real bugs. > > > +#define list_for_each_entry(pos, head, member) \ > > + for (typeof(pos) pos = list_first_entry(head, typeof(*pos), member); \ > > + !list_entry_is_head(pos, head, member); \ > > pos = list_next_entry(pos, member)) Actually can't you use 'pos' to temporarily hold the address of 'member'. Something like: for (pos = (void *)head; \ pos ? ((pos = (void *)pos - offsetof(member)), 1) : 0; \ pos = (void *)pos->next) So that 'pos' is NULL if the loop terminates. No pointers outside structures are generated. Probably need to kill list_entry_is_head() - or it just checks for NULL. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Mon, 28 Feb 2022 16:41:04 -0800 Linus Torvalds wrote: > So yes, initially my idea had been to just move the iterator entirely > inside the macro. But specifying the type got so ugly that I think > that > > typeof (pos) pos > > trick inside the macro really ends up giving us the best of all worlds: > > (a) let's us keep the existing syntax and code for all the nice cases > that did everything inside the loop anyway > > (b) gives us a nice warning for any normal use-after-loop case > (unless you explicitly initialized it like that > sgx_mmu_notifier_release() function did for no good reason > > (c) also guarantees that even if you don't get a warning, > non-converted (or newly written) bad code won't actually _work_ > > so you end up getting the new rules without any ambiguity or mistaken I presume the goal is that we can do this without changing existing code? Otherwise actually moving the iterator into the loop body would be an option, by creating a different hidden variable: #define list_iter(head) \ for (struct list head *_l = (head)->next; _l != (head); _l = _l->next) #define list_iter_entry(var, member) \ list_entry(_l, typeof(*var), member) list_iter(&p->a_head) { struct entry *e = list_iter_entry(e, a_member); /* use e->... */ } Or we can slide into soft insanity and exploit one of Kees'es tricks to encode the type of the entries "next to" the head: #define LIST_HEAD_MEM(name, type) \ union { \ struct list_head name; \ type *name ## _entry; \ } struct entry { struct list_head a_member; }; struct parent { LIST_HEAD_MEM(a_head, struct entry); }; #define list_for_each_magic(pos, head, member) \ for (typeof(**(head ## _entry)) *pos = list_first_entry(head, typeof(**(head ## _entry)), member); \ &pos->member != (head); \ pos = list_next_entry(pos, member)) list_for_each_magic(e, &p->a_head, a_member) { /* use e->... */ } I'll show myself out...
Am 28.02.22 um 22:13 schrieb James Bottomley: > On Mon, 2022-02-28 at 21:56 +0100, Christian König wrote: >> Am 28.02.22 um 21:42 schrieb James Bottomley: >>> On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote: >>>> Am 28.02.22 um 20:56 schrieb Linus Torvalds: >>>>> On Mon, Feb 28, 2022 at 4:19 AM Christian König >>>>> <christian.koenig@amd.com> wrote: >>>>> [SNIP] >>>>> Anybody have any ideas? >>>> I think we should look at the use cases why code is touching >>>> (pos) >>>> after the loop. >>>> >>>> Just from skimming over the patches to change this and experience >>>> with the drivers/subsystems I help to maintain I think the >>>> primary pattern looks something like this: >>>> >>>> list_for_each_entry(entry, head, member) { >>>> if (some_condition_checking(entry)) >>>> break; >>>> } >>>> do_something_with(entry); >>> Actually, we usually have a check to see if the loop found >>> anything, but in that case it should something like >>> >>> if (list_entry_is_head(entry, head, member)) { >>> return with error; >>> } >>> do_somethin_with(entry); >>> >>> Suffice? The list_entry_is_head() macro is designed to cope with >>> the bogus entry on head problem. >> That will work and is also what people already do. >> >> The key problem is that we let people do the same thing over and >> over again with slightly different implementations. >> >> Out in the wild I've seen at least using a separate variable, using >> a bool to indicate that something was found and just assuming that >> the list has an entry. >> >> The last case is bogus and basically what can break badly. > Yes, I understand that. I'm saying we should replace that bogus checks > of entry->something against some_value loop termination condition with > the list_entry_is_head() macro. That should be a one line and fairly > mechanical change rather than the explosion of code changes we seem to > have in the patch series. Yes, exactly that's my thinking as well. Christian. > > James > >
> On 1. Mar 2022, at 01:41, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Mon, Feb 28, 2022 at 1:47 PM Jakob Koschel <jakobkoschel@gmail.com> wrote: >> >> The goal of this is to get compiler warnings right? This would indeed be great. > > Yes, so I don't mind having a one-time patch that has been gathered > using some automated checker tool, but I don't think that works from a > long-term maintenance perspective. > > So if we have the basic rule being "don't use the loop iterator after > the loop has finished, because it can cause all kinds of subtle > issues", then in _addition_ to fixing the existing code paths that > have this issue, I really would want to (a) get a compiler warning for > future cases and (b) make it not actually _work_ for future cases. > > Because otherwise it will just happen again. > >> Changing the list_for_each_entry() macro first will break all of those cases >> (e.g. the ones using 'list_entry_is_head()). > > So I have no problems with breaking cases that we basically already > have a patch for due to your automated tool. There were certainly > more than a handful, but it didn't look _too_ bad to just make the > rule be "don't use the iterator after the loop". > > Of course, that's just based on that patch of yours. Maybe there are a > ton of other cases that your patch didn't change, because they didn't > match your trigger case, so I may just be overly optimistic here. Based on the coccinelle script there are ~480 cases that need fixing in total. I'll now finish all of them and then split them by submodules as Greg suggested and repost a patch set per submodule. Sounds good? > > But basically to _me_, the important part is that the end result is > maintainable longer-term. I'm more than happy to have a one-time patch > to fix a lot of dubious cases if we can then have clean rules going > forward. > >> I assumed it is better to fix those cases first and then have a simple >> coccinelle script changing the macro + moving the iterator into the scope >> of the macro. > > So that had been another plan of mine, until I actually looked at > changing the macro. In the one case I looked at, it was ugly beyond > belief. > > It turns out that just syntactically, it's really nice to give the > type of the iterator from outside the way we do now. Yeah, it may be a > bit odd, and maybe it's partly because I'm so used to the > "list_for_each_list_entry()" syntax, but moving the type into the loop > construct really made it nasty - either one very complex line, or > having to split it over two lines which was even worse. > > Maybe the place I looked at just happened to have a long typename, but > it's basically always going to be a struct, so it's never a _simple_ > type. And it just looked very odd adn unnatural to have the type as > one of the "arguments" to that list_for_each_entry() macro. > > So yes, initially my idea had been to just move the iterator entirely > inside the macro. But specifying the type got so ugly that I think > that > > typeof (pos) pos > > trick inside the macro really ends up giving us the best of all worlds: > > (a) let's us keep the existing syntax and code for all the nice cases > that did everything inside the loop anyway > > (b) gives us a nice warning for any normal use-after-loop case > (unless you explicitly initialized it like that > sgx_mmu_notifier_release() function did for no good reason > > (c) also guarantees that even if you don't get a warning, > non-converted (or newly written) bad code won't actually _work_ > > so you end up getting the new rules without any ambiguity or mistaken > >> With this you are no longer able to set the 'outer' pos within the list >> iterator loop body or am I missing something? > > Correct. Any assignment inside the loop will be entirely just to the > local loop case. So any "break;" out of the loop will have to set > another variable - like your updated patch did. > >> I fail to see how this will make most of the changes in this >> patch obsolete (if that was the intention). > > I hope my explanation above clarifies my thinking: I do not dislike > your patch, and in fact your patch is indeed required to make the new > semantics work. ok it's all clear now, thanks for clarifying. I've defined all the 'tmp' iterator variables uninitialized so applying your patch on top of that later will just give the nice compiler warning if they are used past the loop body. > > What I disliked was always the maintainability of your patch - making > the rules be something that isn't actually visible in the source code, > and letting the old semantics still work as well as they ever did, and > having to basically run some verification pass to find bad users. Since this patch is not a complete list of cases that need fixing (30%) I haven't included the actual change of moving the iterator variable into the loop and thought that would be a second step coming after this is merged. With these changes alone, yes you still rely on manual verification passes. > > (I also disliked your original patch that mixed up the "CPU > speculation type safety" with the actual non-speculative problems, but > that was another issue). > > Linus - Jakob
On Tue, Mar 01, 2022 at 12:28:15PM +0100, Jakob Koschel wrote: > > > > On 1. Mar 2022, at 01:41, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > > > On Mon, Feb 28, 2022 at 1:47 PM Jakob Koschel <jakobkoschel@gmail.com> wrote: > >> > >> The goal of this is to get compiler warnings right? This would indeed be great. > > > > Yes, so I don't mind having a one-time patch that has been gathered > > using some automated checker tool, but I don't think that works from a > > long-term maintenance perspective. > > > > So if we have the basic rule being "don't use the loop iterator after > > the loop has finished, because it can cause all kinds of subtle > > issues", then in _addition_ to fixing the existing code paths that > > have this issue, I really would want to (a) get a compiler warning for > > future cases and (b) make it not actually _work_ for future cases. > > > > Because otherwise it will just happen again. > > > >> Changing the list_for_each_entry() macro first will break all of those cases > >> (e.g. the ones using 'list_entry_is_head()). > > > > So I have no problems with breaking cases that we basically already > > have a patch for due to your automated tool. There were certainly > > more than a handful, but it didn't look _too_ bad to just make the > > rule be "don't use the iterator after the loop". > > > > Of course, that's just based on that patch of yours. Maybe there are a > > ton of other cases that your patch didn't change, because they didn't > > match your trigger case, so I may just be overly optimistic here. > > Based on the coccinelle script there are ~480 cases that need fixing > in total. I'll now finish all of them and then split them by > submodules as Greg suggested and repost a patch set per submodule. > Sounds good? Sounds good to me! If you need help carving these up and maintaining them over time as different subsystem maintainers accept/ignore them, just let me know. Doing large patchsets like this can be tough without a lot of experience. thanks, greg k-h
On Mon, Feb 28, 2022 at 01:06:57PM +0100, Jakob Koschel wrote: > > > > On 28. Feb 2022, at 12:20, Greg KH <gregkh@linuxfoundation.org> wrote: > > > > On Mon, Feb 28, 2022 at 12:08:18PM +0100, Jakob Koschel wrote: > >> If the list does not contain the expected element, the value of > >> list_for_each_entry() iterator will not point to a valid structure. > >> To avoid type confusion in such case, the list iterator > >> scope will be limited to list_for_each_entry() loop. > >> > >> In preparation to limiting scope of a list iterator to the list traversal > >> loop, use a dedicated pointer to point to the found element. > >> Determining if an element was found is then simply checking if > >> the pointer is != NULL. > >> > >> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com> > >> --- > >> arch/x86/kernel/cpu/sgx/encl.c | 6 +++-- > >> drivers/scsi/scsi_transport_sas.c | 17 ++++++++----- > >> drivers/thermal/thermal_core.c | 38 ++++++++++++++++++---------- > >> drivers/usb/gadget/configfs.c | 22 ++++++++++------ > >> drivers/usb/gadget/udc/max3420_udc.c | 11 +++++--- > >> drivers/usb/gadget/udc/tegra-xudc.c | 11 +++++--- > >> drivers/usb/mtu3/mtu3_gadget.c | 11 +++++--- > >> drivers/usb/musb/musb_gadget.c | 11 +++++--- > >> drivers/vfio/mdev/mdev_core.c | 11 +++++--- > >> 9 files changed, 88 insertions(+), 50 deletions(-) > > > > The drivers/usb/ portion of this patch should be in patch 1/X, right? > > I kept them separate since it's a slightly different case. > Using 'ptr' instead of '&ptr->other_member'. Regardless, I'll split > this commit up as you mentioned. > > > > > Also, you will have to split these up per-subsystem so that the > > different subsystem maintainers can take these in their trees. > > Thanks for the feedback. > To clarify I understand you correctly: > I should do one patch set per-subsystem with every instance/(file?) > fixed as a separate commit? Yes, each file needs a different commit as they are usually all written or maintained by different people. As I said in my other response, if you need any help with this, just let me know, I'm glad to help. thanks, greg k-h
> On 1. Mar 2022, at 18:36, Greg KH <greg@kroah.com> wrote: > > On Tue, Mar 01, 2022 at 12:28:15PM +0100, Jakob Koschel wrote: >> >> >>> On 1. Mar 2022, at 01:41, Linus Torvalds <torvalds@linux-foundation.org> wrote: >>> >>> On Mon, Feb 28, 2022 at 1:47 PM Jakob Koschel <jakobkoschel@gmail.com> wrote: >>>> >>>> The goal of this is to get compiler warnings right? This would indeed be great. >>> >>> Yes, so I don't mind having a one-time patch that has been gathered >>> using some automated checker tool, but I don't think that works from a >>> long-term maintenance perspective. >>> >>> So if we have the basic rule being "don't use the loop iterator after >>> the loop has finished, because it can cause all kinds of subtle >>> issues", then in _addition_ to fixing the existing code paths that >>> have this issue, I really would want to (a) get a compiler warning for >>> future cases and (b) make it not actually _work_ for future cases. >>> >>> Because otherwise it will just happen again. >>> >>>> Changing the list_for_each_entry() macro first will break all of those cases >>>> (e.g. the ones using 'list_entry_is_head()). >>> >>> So I have no problems with breaking cases that we basically already >>> have a patch for due to your automated tool. There were certainly >>> more than a handful, but it didn't look _too_ bad to just make the >>> rule be "don't use the iterator after the loop". >>> >>> Of course, that's just based on that patch of yours. Maybe there are a >>> ton of other cases that your patch didn't change, because they didn't >>> match your trigger case, so I may just be overly optimistic here. >> >> Based on the coccinelle script there are ~480 cases that need fixing >> in total. I'll now finish all of them and then split them by >> submodules as Greg suggested and repost a patch set per submodule. >> Sounds good? > > Sounds good to me! > > If you need help carving these up and maintaining them over time as > different subsystem maintainers accept/ignore them, just let me know. > Doing large patchsets like this can be tough without a lot of > experience. Very much appreciated! There will probably be some cases that do not match one of the pattern we already discussed and need separate attention. I was planning to start with one subsystem and adjust the coming ones according to the feedback gather there instead of posting all of them in one go. > > thanks, > > greg k-h - Jakob
On Tue, Mar 01, 2022 at 06:40:04PM +0100, Jakob Koschel wrote: > > > > On 1. Mar 2022, at 18:36, Greg KH <greg@kroah.com> wrote: > > > > On Tue, Mar 01, 2022 at 12:28:15PM +0100, Jakob Koschel wrote: > >> > >> > >>> On 1. Mar 2022, at 01:41, Linus Torvalds <torvalds@linux-foundation.org> wrote: > >>> > >>> On Mon, Feb 28, 2022 at 1:47 PM Jakob Koschel <jakobkoschel@gmail.com> wrote: > >>>> > >>>> The goal of this is to get compiler warnings right? This would indeed be great. > >>> > >>> Yes, so I don't mind having a one-time patch that has been gathered > >>> using some automated checker tool, but I don't think that works from a > >>> long-term maintenance perspective. > >>> > >>> So if we have the basic rule being "don't use the loop iterator after > >>> the loop has finished, because it can cause all kinds of subtle > >>> issues", then in _addition_ to fixing the existing code paths that > >>> have this issue, I really would want to (a) get a compiler warning for > >>> future cases and (b) make it not actually _work_ for future cases. > >>> > >>> Because otherwise it will just happen again. > >>> > >>>> Changing the list_for_each_entry() macro first will break all of those cases > >>>> (e.g. the ones using 'list_entry_is_head()). > >>> > >>> So I have no problems with breaking cases that we basically already > >>> have a patch for due to your automated tool. There were certainly > >>> more than a handful, but it didn't look _too_ bad to just make the > >>> rule be "don't use the iterator after the loop". > >>> > >>> Of course, that's just based on that patch of yours. Maybe there are a > >>> ton of other cases that your patch didn't change, because they didn't > >>> match your trigger case, so I may just be overly optimistic here. > >> > >> Based on the coccinelle script there are ~480 cases that need fixing > >> in total. I'll now finish all of them and then split them by > >> submodules as Greg suggested and repost a patch set per submodule. > >> Sounds good? > > > > Sounds good to me! > > > > If you need help carving these up and maintaining them over time as > > different subsystem maintainers accept/ignore them, just let me know. > > Doing large patchsets like this can be tough without a lot of > > experience. > > Very much appreciated! > > There will probably be some cases that do not match one of the pattern > we already discussed and need separate attention. > > I was planning to start with one subsystem and adjust the coming ones > according to the feedback gather there instead of posting all of them > in one go. That seems wise. Feel free to use USB as a testing ground for this if you want to :) thanks, greg k-h
On Mon, Feb 28, 2022 at 04:45:11PM -0800, Linus Torvalds wrote: > Really. The "-Wshadow doesn't work on the kernel" is not some new > issue, because you have to do completely insane things to the source > code to enable it. The first big glitch with -Wshadow was with shadowed global variables. GCC 4.8 fixed that, but it still yells about shadowed functions. What _almost_ works is -Wshadow=local. At first glace, all the warnings look solvable, but then one will eventually discover __wait_event() and associated macros that mix when and how deeply it intentionally shadows variables. :) Another way to try to catch misused shadow variables is -Wunused-but-set-varible, but it, too, has tons of false positives. I tried to capture some of the rationale and research here: https://github.com/KSPP/linux/issues/152
On Tue, Mar 01, 2022 at 12:28:15PM +0100, Jakob Koschel wrote: > Based on the coccinelle script there are ~480 cases that need fixing > in total. I'll now finish all of them and then split them by > submodules as Greg suggested and repost a patch set per submodule. > Sounds good? To help with this splitting, see: https://github.com/kees/kernel-tools/blob/trunk/split-on-maintainer It's not perfect, but it'll get you really close. For example, if you had a single big tree-wide patch applied to your tree: $ rm 0*.patch $ git format-patch -1 HEAD $ mv 0*.patch treewide.patch $ split-on-maintainer treewide.patch $ ls 0*.patch If you have a build log before the patch that spits out warnings, the --build-log argument can extract those warnings on a per-file basis, too (though this can be fragile).
On Tue, Mar 1, 2022 at 10:14 AM Kees Cook <keescook@chromium.org> wrote: > > The first big glitch with -Wshadow was with shadowed global variables. > GCC 4.8 fixed that, but it still yells about shadowed functions. What > _almost_ works is -Wshadow=local. Heh. Yeah, I just have long memories of "-Wshadow was a disaster". You looked into the details. > Another way to try to catch misused shadow variables is > -Wunused-but-set-varible, but it, too, has tons of false positives. That on the face of it should be an easy warning to get technically right for a compiler. So I assume the "false positives" are simply because we end up having various variables that really don't end up being used - and "intentionally" so). Or rather, they might only be used under some config option - perhaps the use is even syntactically there and parsed, but the compiler notices that it's turned off under some if (IS_ENABLED(..)) option? Because yeah, we have a lot of those. I think that's a common theme with a lot of compiler warnings: on the face of it they sound "obviously sane" and nobody should ever write code like that. A conditional that is always true? Sounds idiotic, and sounds like a reasonable thing for a compiler to warn about, since why would you have a conditional in the first place for that? But then you realize that maybe the conditional is a build config option, and "always true" suddenly makes sense. Or it's a test for something that is always true on _that_architecture_ but not in some general sense (ie testing "sizeof()"). Or it's a purely syntactic conditional, like "do { } while (0)". It's why I'm often so down on a lot of the odd warnings that are hiding under W=1 and friends. They all may make sense in the trivial case ("That is insane") but then in the end they happen for sane code. And yeah, -Wshadow has had tons of history with macro nesting, and just being badly done in the first place (eg "strlen" can be a perfectly fine local variable). That said, maybe people could ask the gcc and clan people for a way to _mark_ the places where we expect to validly see shadowing. For example, that "local variable in a macro expression statement" thing is absolutely horrendous to fix with preprocessor tricks to try to make for unique identifiers. But I think it would be much more syntactically reasonable to add (for example) a "shadow" attribute to such a variable exactly to tell the compiler "yeah, yeah, I know this identifier could shadow an outer one" and turn it off that way. Linus
On Tue, Mar 01, 2022 at 10:14:07AM -0800, Kees Cook wrote: > On Mon, Feb 28, 2022 at 04:45:11PM -0800, Linus Torvalds wrote: > > Really. The "-Wshadow doesn't work on the kernel" is not some new > > issue, because you have to do completely insane things to the source > > code to enable it. > > The first big glitch with -Wshadow was with shadowed global variables. > GCC 4.8 fixed that, but it still yells about shadowed functions. What > _almost_ works is -Wshadow=local. At first glace, all the warnings > look solvable, but then one will eventually discover __wait_event() > and associated macros that mix when and how deeply it intentionally > shadows variables. :) Well, that's just disgusting. Macros fundamentally shouldn't be referring to things that aren't in their arguments. The first step to cleaning this up is ... I'll take a look at the rest of cleaning this up soon. From 28ffe35d56223d4242b915832299e5acc926737e Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" <willy@infradead.org> Date: Tue, 1 Mar 2022 13:47:07 -0500 Subject: [PATCH] wait: Parameterize the return variable to ___wait_event() Macros should not refer to variables which aren't in their arguments. Pass the name from its callers. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> --- include/linux/swait.h | 12 ++++++------ include/linux/wait.h | 32 ++++++++++++++++---------------- include/linux/wait_bit.h | 4 ++-- 3 files changed, 24 insertions(+), 24 deletions(-) diff --git a/include/linux/swait.h b/include/linux/swait.h index 6a8c22b8c2a5..5e8e9b13be2d 100644 --- a/include/linux/swait.h +++ b/include/linux/swait.h @@ -191,14 +191,14 @@ do { \ } while (0) #define __swait_event_timeout(wq, condition, timeout) \ - ___swait_event(wq, ___wait_cond_timeout(condition), \ + ___swait_event(wq, ___wait_cond_timeout(condition, __ret), \ TASK_UNINTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_timeout_exclusive(wq, condition, timeout) \ ({ \ long __ret = timeout; \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __swait_event_timeout(wq, condition, timeout); \ __ret; \ }) @@ -216,14 +216,14 @@ do { \ }) #define __swait_event_interruptible_timeout(wq, condition, timeout) \ - ___swait_event(wq, ___wait_cond_timeout(condition), \ + ___swait_event(wq, ___wait_cond_timeout(condition, __ret), \ TASK_INTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\ ({ \ long __ret = timeout; \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __swait_event_interruptible_timeout(wq, \ condition, timeout); \ __ret; \ @@ -252,7 +252,7 @@ do { \ } while (0) #define __swait_event_idle_timeout(wq, condition, timeout) \ - ___swait_event(wq, ___wait_cond_timeout(condition), \ + ___swait_event(wq, ___wait_cond_timeout(condition, __ret), \ TASK_IDLE, timeout, \ __ret = schedule_timeout(__ret)) @@ -278,7 +278,7 @@ do { \ #define swait_event_idle_timeout_exclusive(wq, condition, timeout) \ ({ \ long __ret = timeout; \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __swait_event_idle_timeout(wq, \ condition, timeout); \ __ret; \ diff --git a/include/linux/wait.h b/include/linux/wait.h index 851e07da2583..890cce3c0f2e 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -271,7 +271,7 @@ static inline void wake_up_pollfree(struct wait_queue_head *wq_head) __wake_up_pollfree(wq_head); } -#define ___wait_cond_timeout(condition) \ +#define ___wait_cond_timeout(condition, __ret) \ ({ \ bool __cond = (condition); \ if (__cond && !__ret) \ @@ -386,7 +386,7 @@ do { \ }) #define __wait_event_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_UNINTERRUPTIBLE, 0, timeout, \ __ret = schedule_timeout(__ret)) @@ -413,13 +413,13 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_timeout(wq_head, condition, timeout); \ __ret; \ }) #define __wait_event_freezable_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_INTERRUPTIBLE, 0, timeout, \ __ret = freezable_schedule_timeout(__ret)) @@ -431,7 +431,7 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_freezable_timeout(wq_head, condition, timeout); \ __ret; \ }) @@ -503,7 +503,7 @@ do { \ }) #define __wait_event_interruptible_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_INTERRUPTIBLE, 0, timeout, \ __ret = schedule_timeout(__ret)) @@ -531,7 +531,7 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_interruptible_timeout(wq_head, \ condition, timeout); \ __ret; \ @@ -698,7 +698,7 @@ do { \ } while (0) #define __wait_event_idle_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_IDLE, 0, timeout, \ __ret = schedule_timeout(__ret)) @@ -725,13 +725,13 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_idle_timeout(wq_head, condition, timeout); \ __ret; \ }) #define __wait_event_idle_exclusive_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_IDLE, 1, timeout, \ __ret = schedule_timeout(__ret)) @@ -762,7 +762,7 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_idle_exclusive_timeout(wq_head, condition, timeout);\ __ret; \ }) @@ -932,7 +932,7 @@ extern int do_wait_intr_irq(wait_queue_head_t *, wait_queue_entry_t *); }) #define __wait_event_killable_timeout(wq_head, condition, timeout) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ TASK_KILLABLE, 0, timeout, \ __ret = schedule_timeout(__ret)) @@ -962,7 +962,7 @@ extern int do_wait_intr_irq(wait_queue_head_t *, wait_queue_entry_t *); ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_killable_timeout(wq_head, \ condition, timeout); \ __ret; \ @@ -1107,7 +1107,7 @@ do { \ }) #define __wait_event_lock_irq_timeout(wq_head, condition, lock, timeout, state) \ - ___wait_event(wq_head, ___wait_cond_timeout(condition), \ + ___wait_event(wq_head, ___wait_cond_timeout(condition, __ret), \ state, 0, timeout, \ spin_unlock_irq(&lock); \ __ret = schedule_timeout(__ret); \ @@ -1141,7 +1141,7 @@ do { \ timeout) \ ({ \ long __ret = timeout; \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_lock_irq_timeout( \ wq_head, condition, lock, timeout, \ TASK_INTERRUPTIBLE); \ @@ -1151,7 +1151,7 @@ do { \ #define wait_event_lock_irq_timeout(wq_head, condition, lock, timeout) \ ({ \ long __ret = timeout; \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_event_lock_irq_timeout( \ wq_head, condition, lock, timeout, \ TASK_UNINTERRUPTIBLE); \ diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h index 7dec36aecbd9..227e6a20a978 100644 --- a/include/linux/wait_bit.h +++ b/include/linux/wait_bit.h @@ -292,7 +292,7 @@ do { \ }) #define __wait_var_event_timeout(var, condition, timeout) \ - ___wait_var_event(var, ___wait_cond_timeout(condition), \ + ___wait_var_event(var, ___wait_cond_timeout(condition, __ret), \ TASK_UNINTERRUPTIBLE, 0, timeout, \ __ret = schedule_timeout(__ret)) @@ -300,7 +300,7 @@ do { \ ({ \ long __ret = timeout; \ might_sleep(); \ - if (!___wait_cond_timeout(condition)) \ + if (!___wait_cond_timeout(condition, __ret)) \ __ret = __wait_var_event_timeout(var, condition, timeout); \ __ret; \ })
On Mon, Feb 28, 2022 at 2:29 PM James Bottomley <James.Bottomley@hansenpartnership.com> wrote: > > However, if the desire is really to poison the loop variable then we > can do > > #define list_for_each_entry(pos, head, member) \ > for (pos = list_first_entry(head, typeof(*pos), member); \ > !list_entry_is_head(pos, head, member) && ((pos = NULL) == NULL; \ > pos = list_next_entry(pos, member)) > > Which would at least set pos to NULL when the loop completes. That would actually have been excellent if we had done that originally. It would not only avoid the stale and incorrectly typed head entry left-over turd, it would also have made it very easy to test for "did I find an entry in the loop". But I don't much like it in the situation we are now. Why? Mainly because it basically changes the semantics of the loop _without_ any warnings about it. And we don't actually get the advantage of the nicer semantics, because we can't actually make code do list_for_each_entry(entry, ....) { .. } if (!entry) return -ESRCH; .. use the entry we found .. because that would be a disaster for back-porting, plus it would be a flag-day issue (ie we'd have to change the semantics of the loop at the same time we change every single user). So instead of that simple "if (!entry)", we'd effectively have to continue to use something that still works with the old world order (ie that "if (list_entry_is_head())" model). So we couldn't really take _advantage_ of the nicer semantics, and we'd not even get a warning if somebody does it wrong - the code would just silently do the wrong thing. IOW: I don't think you are wrong about that patch: it would solve the problem that Jakob wants to solve, and it would have absolutely been much better if we had done this from the beginning. But I think that in our current situation, it's actually a really fragile solution to the "don't do that then" problem we have. Linus
On Tue, Mar 1, 2022 at 11:06 AM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > So instead of that simple "if (!entry)", we'd effectively have to > continue to use something that still works with the old world order > (ie that "if (list_entry_is_head())" model). Just to prove my point about how this is painful, that doesn't work at all. If the loop iterator at the end is NULL (good, in theory), we can't use "list_entry_is_head()" to check whether we ended. We'd have to use a new thing entirely, to handle the "list_for_each_entry() has the old/new semantics" cases. That's largely why I was pushing for the "let's make it impossible to use the loop iterator at all outside the loop". It avoids the confusing case, and the patches to move to that stricter semantic can be merged independently (and before) doing the actual semantic change. I'm not saying my suggested approach is wonderful either. Honestly, it's painful that we have so nasty semantics for the end-of-loop case for list_for_each_entry(). The minimal patch would clearly be to keep those broken semantics, and just force everybody to use the list_entry_is_head() case. That's the "we know we messed up, we are too lazy to fix it, we'll just work around it and people need to be careful" approach. And laziness is a virtue. But bad semantics are bad semantics. So it's a question of balancing those two issues. Linus
From: Linus Torvalds > Sent: 01 March 2022 19:07 > On Mon, Feb 28, 2022 at 2:29 PM James Bottomley > <James.Bottomley@hansenpartnership.com> wrote: > > > > However, if the desire is really to poison the loop variable then we > > can do > > > > #define list_for_each_entry(pos, head, member) \ > > for (pos = list_first_entry(head, typeof(*pos), member); \ > > !list_entry_is_head(pos, head, member) && ((pos = NULL) == NULL; \ > > pos = list_next_entry(pos, member)) > > > > Which would at least set pos to NULL when the loop completes. > > That would actually have been excellent if we had done that > originally. It would not only avoid the stale and incorrectly typed > head entry left-over turd, it would also have made it very easy to > test for "did I find an entry in the loop". > > But I don't much like it in the situation we are now. > > Why? Mainly because it basically changes the semantics of the loop > _without_ any warnings about it. And we don't actually get the > advantage of the nicer semantics, because we can't actually make code > do > > list_for_each_entry(entry, ....) { > .. > } > if (!entry) > return -ESRCH; > .. use the entry we found .. > > because that would be a disaster for back-porting, plus it would be a > flag-day issue (ie we'd have to change the semantics of the loop at > the same time we change every single user). > > So instead of that simple "if (!entry)", we'd effectively have to > continue to use something that still works with the old world order > (ie that "if (list_entry_is_head())" model). > > So we couldn't really take _advantage_ of the nicer semantics, and > we'd not even get a warning if somebody does it wrong - the code would > just silently do the wrong thing. > > IOW: I don't think you are wrong about that patch: it would solve the > problem that Jakob wants to solve, and it would have absolutely been > much better if we had done this from the beginning. But I think that > in our current situation, it's actually a really fragile solution to > the "don't do that then" problem we have. Can it be resolved by making: #define list_entry_is_head(pos, head, member) ((pos) == NULL) and double-checking that it isn't used anywhere else (except in the list macros themselves). The odd ones I just found are fs/locks.c mm/page_reporting.c security/apparmor/apparmorfs.c (3 times) net/xfrm/xfrm_ipcomp.c#L244 is buggy. (There is a WARN_ON() then it just carries on regardless!) There are only about 25 uses of list_entry_is_head(). There are a lot more places where these lists seem to be scanned by hand. I bet a few of those aren't actually right either. (Oh at 3am this morning I thought it was a different list type that could have much the same problem!) Another plausible solution is a variant of list_foreach_entry() that does set the 'entry' to NULL at the end. Then code can be moved over in stages. I'd reorder the arguments as well as changing the name! David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Tue, Mar 1, 2022 at 2:58 PM David Laight <David.Laight@aculab.com> wrote: > > Can it be resolved by making: > #define list_entry_is_head(pos, head, member) ((pos) == NULL) > and double-checking that it isn't used anywhere else (except in > the list macros themselves). Well, yes, except for the fact that then the name is entirely misleading... And somebody possibly uses it together with list_first_entry() etc, so it really is completely broken to mix that change with the list traversal change. Linus Linus
From: Linus Torvalds > Sent: 01 March 2022 23:03 > > On Tue, Mar 1, 2022 at 2:58 PM David Laight <David.Laight@aculab.com> wrote: > > > > Can it be resolved by making: > > #define list_entry_is_head(pos, head, member) ((pos) == NULL) > > and double-checking that it isn't used anywhere else (except in > > the list macros themselves). > > Well, yes, except for the fact that then the name is entirely misleading... > > And somebody possibly uses it together with list_first_entry() etc, so > it really is completely broken to mix that change with the list > traversal change. Probably true :-( Actually adding list_entry_not_found() as a synonym for list_entry_is_head() and changing the 25ish places that use it after a loop might work. Once that is done the loop can be changed at the same time as list_entry_not_found(). That won't affect the in-tree callers. (and my out of tree modules don't use those lists - so I don't care about that!) Having said that there are so few users of list_entry_is_head() it is reasonable to generate two new names. One for use after list_for_each_entry() and one for list_next_entry(). Then the change all the call sites. After that list_entry_is_head() can be deleted - breaking out of tree compiles. Finally list_for_each_entry() can be rewritten to set NULL at the end of the list. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Tue, Mar 1, 2022 at 3:19 PM David Laight <David.Laight@aculab.com> wrote: > > Having said that there are so few users of list_entry_is_head() > it is reasonable to generate two new names. Well, the problem is that the users of list_entry_is_head() may be few - but there are a number of _other_ ways to check "was that the HEAD pointer", and not all of them are necessarily correct. IOW, different places do different random tests for "did we walk the whole loop without breaking out". And many of them happen to work. In fact, in practice, pretty much *all* of them happen to work, and you have to have the right struct layout and really really bad luck to hit a case of "type confusion ended up causing the test to not work". And *THAT* is the problem here. It's not the "there are 25ish places that current use list_entry_is_head()". It's the "there are ~480 places that use the type-confused HEAD entry that has been cast to the wrong type". And THAT is why I think we'd be better off with that bigger change that simply means that you can't use the iterator variable at all outside the loop, and try to make it something where the compiler can help catch mis-uses. Now, making the list_for_each_entry() thing force the iterator to NULL at the end of the loop does fix the problem. The issue I have with it is really just that you end up getting no warning at all from the compiler if you mix old-style and new-style semantics. Now, you *will* get an oops (if using a new-style iterator with an old-style check), but many of these things will be in odd driver code and may happen only for error cases. And if you use a new-style check with an old-style iterator (ie some backport problem), you will probably end up getting random memory corruption, because you'll decide "it's not a HEAD entry", and then you'll actually *use* the HEAD that has the wrong type cast associated with it. See what my worry is? With the "don't use iterator outside the loop" approach, the exact same code works in both the old world order and the new world order, and you don't have the semantic confusion. And *if* you try to use the iterator outside the loop, you'll _mostly_ (*) get a compiler warning about it not being initialized. Linus (*) Unless somebody initializes the iterator pointer pointlessly. Which clearly does happen. Thus the "mostly". It's not perfect, and that's most definitely not nice - but it should at least hopefully make it that much harder to mess up.
On 02/03/2022 00.55, Linus Torvalds wrote: > On Tue, Mar 1, 2022 at 3:19 PM David Laight <David.Laight@aculab.com> wrote: >> > With the "don't use iterator outside the loop" approach, the exact > same code works in both the old world order and the new world order, > and you don't have the semantic confusion. And *if* you try to use the > iterator outside the loop, you'll _mostly_ (*) get a compiler warning > about it not being initialized. > > Linus > > (*) Unless somebody initializes the iterator pointer pointlessly. > Which clearly does happen. Thus the "mostly". It's not perfect, and > that's most definitely not nice - but it should at least hopefully > make it that much harder to mess up. This won't help the current issue (because it doesn't exist and might never), but just in case some compiler people are listening, I'd like to have some sort of way to tell the compiler "treat this variable as uninitialized from here on". So one could do #define kfree(p) do { __kfree(p); __magic_uninit(p); } while (0) with __magic_uninit being a magic no-op that doesn't affect the semantics of the code, but could be used by the compiler's "[is/may be] used uninitialized" machinery to flag e.g. double frees on some odd error path etc. It would probably only work for local automatic variables, but it should be possible to just ignore the hint if p is some expression like foo->bar or has side effects. If we had that, the end-of-loop test could include that to "uninitialize" the iterator. Maybe sparse/smatch or some other static analyzer could implement such a magic thing? Maybe it's better as a function attribute [__attribute__((uninitializes(1)))] to avoid having to macrofy all functions that release resources. Rasmus
On Mon, 28 Feb 2022 16:41:04 -0800, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > But basically to _me_, the important part is that the end result is > maintainable longer-term. I couldn't agree more. And because of that, I stick with the following approach because it's maintainable longer-term than "type(pos) pos" one: Implements a new macro for each list_for_each_entry* with _inside suffix. #define list_for_each_entry_inside(pos, type, head, member) I have posted a patch series here to demonstrate this approach: https://lore.kernel.org/lkml/20220301075839.4156-3-xiam0nd.tong@gmail.com/ Although we need replace all the use of list_for_each_entry* (15000+) with list_for_each_entry*_inside, the work can be done gradually rather than all at once. We can incrementally replace these callers until all these in the kernel are completely updated with *_inside* one. At that time, we can just remove the implements of origin macros and rename the *_inside* macro back to the origin name just in one single patch. And the "type(pos) pos" approach need teach developers to "not initialize the iterator variable, otherwise the use-after-loop will not be reported by compiler", which is unreasonable and impossible for all developers. And it will mess up the following code logic and no warnning reported by compiler, even without initializing "ext" at the beginning: void foo(struct mem_extent *arg) { struct mem_extent *ext; // used both for iterator and normal ptr ... ext = arg; // this assignment can alse be done in another bar() func ... list_for_each_entry(ext, head, member) { if (found(ext)) break; } ... // use ext after the loop ret = ext; } If the loop hit the break, the last "ret" will be the found ext iterator. However, if the "type(pos) pos" approach applied, the last "ret" will be "arg" which is not the intention of the developers, because the "ext" is two different variables inside and outside the loop. Thus, my idea is *better a finger off than always aching*, let's choose the "list_for_each_entry_inside(pos, type, head, member)" approach. > It turns out that just syntactically, it's really nice to give the > type of the iterator from outside the way we do now. Yeah, it may be a > bit odd, and maybe it's partly because I'm so used to the > "list_for_each_list_entry()" syntax, but moving the type into the loop > construct really made it nasty - either one very complex line, or > having to split it over two lines which was even worse. > > Maybe the place I looked at just happened to have a long typename, but > it's basically always going to be a struct, so it's never a _simple_ > type. And it just looked very odd adn unnatural to have the type as > one of the "arguments" to that list_for_each_entry() macro. we can pass a shorter type name to list_for_each_entry_inside, thus no need to split it over two lines. Actually it is not a big problem. + #define t struct sram_bank_info - list_for_each_entry(pos, head, member) { + list_for_each_entry_inside(pos, t, head, member) { I put the type at the second argument not the first to avoid messing up the pattern match in some coccinelle scripts. > (b) gives us a nice warning for any normal use-after-loop case > (unless you explicitly initialized it like that > sgx_mmu_notifier_release() function did for no good reason sometimes developers can be confused by the reported warnning: "used without having been initialized", and can not figure out immediately that "oh, now i am using another different variable but with the same name of the loop iterator variable", which has changed the programming habits of developers. > (c) also guarantees that even if you don't get a warning, > non-converted (or newly written) bad code won't actually _work_ > > so you end up getting the new rules without any ambiguity or mistaken It will lead to a wrong/NULL pointer dereference if the pointer is used anywhere else, depend on which value is used to initialized with. Best regard, -- Xiaomeng Tong
From: Xiaomeng Tong > Sent: 02 March 2022 09:31 > > On Mon, 28 Feb 2022 16:41:04 -0800, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > But basically to _me_, the important part is that the end result is > > maintainable longer-term. > > I couldn't agree more. And because of that, I stick with the following > approach because it's maintainable longer-term than "type(pos) pos" one: > Implements a new macro for each list_for_each_entry* with _inside suffix. > #define list_for_each_entry_inside(pos, type, head, member) I think that it would be better to make any alternate loop macro just set the variable to NULL on the loop exit. That is easier to code for and the compiler might be persuaded to not redo the test. It also doesn't need an extra variable defined in the for() statement so can be back-ported to older kernels without required declaration in the middle of blocks. OTOH there may be alternative definitions that can be used to get the compiler (or other compiler-like tools) to detect broken code. Even if the definition can't possibly generate a working kerrnel. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Wed, Mar 02, 2022 at 10:29:31AM +0100, Rasmus Villemoes wrote: > This won't help the current issue (because it doesn't exist and might > never), but just in case some compiler people are listening, I'd like to > have some sort of way to tell the compiler "treat this variable as > uninitialized from here on". So one could do > > #define kfree(p) do { __kfree(p); __magic_uninit(p); } while (0) > > with __magic_uninit being a magic no-op that doesn't affect the > semantics of the code, but could be used by the compiler's "[is/may be] > used uninitialized" machinery to flag e.g. double frees on some odd > error path etc. It would probably only work for local automatic > variables, but it should be possible to just ignore the hint if p is > some expression like foo->bar or has side effects. If we had that, the > end-of-loop test could include that to "uninitialize" the iterator. I've long wanted to change kfree() to explicitly set pointers to NULL on free. https://github.com/KSPP/linux/issues/87 The thing stopping a trivial transformation of kfree() is: kfree(get_some_pointer()); I would argue, though, that the above is poor form: the thing holding the pointer should be the thing freeing it, so these cases should be refactored and kfree() could do the NULLing by default. Quoting myself in the above issue: Without doing massive tree-wide changes, I think we need compiler support. If we had something like __builtin_is_lvalue(), we could distinguish function returns from lvalues. For example, right now a common case are things like: kfree(get_some_ptr()); But if we could at least gain coverage of the lvalue cases, and detect them statically at compile-time, we could do: #define __kfree_and_null(x) do { __kfree(*x); *x = NULL; } while (0) #define kfree(x) __builtin_choose_expr(__builtin_is_lvalue(x), __kfree_and_null(&(x)), __kfree(x)) Alternatively, we could do a tree-wide change of the former case (findable with Coccinelle) and change them into something like kfree_no_null() and redefine kfree() itself: #define kfree_no_null(x) do { void *__ptr = (x); __kfree(__ptr); } while (0) #define kfree(x) do { __kfree(x); x = NULL; } while (0)
On Wed, Mar 2, 2022 at 12:07 PM Kees Cook <keescook@chromium.org> wrote: > > I've long wanted to change kfree() to explicitly set pointers to NULL on > free. https://github.com/KSPP/linux/issues/87 We've had this discussion with the gcc people in the past, and gcc actually has some support for it, but it's sadly tied to the actual function name (ie gcc has some special-casing for "free()") See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94527 for some of that discussion. Oh, and I see some patch actually got merged since I looked there last so that you can mark "deallocator" functions, but I think it's only for the context matching, not for actually killing accesses to the pointer afterwards. Linus
On Wed, Mar 02, 2022 at 12:18:45PM -0800, Linus Torvalds wrote: > On Wed, Mar 2, 2022 at 12:07 PM Kees Cook <keescook@chromium.org> wrote: > > > > I've long wanted to change kfree() to explicitly set pointers to NULL on > > free. https://github.com/KSPP/linux/issues/87 > > We've had this discussion with the gcc people in the past, and gcc > actually has some support for it, but it's sadly tied to the actual > function name (ie gcc has some special-casing for "free()") > > See > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94527 > > for some of that discussion. > > Oh, and I see some patch actually got merged since I looked there last > so that you can mark "deallocator" functions, but I think it's only > for the context matching, not for actually killing accesses to the > pointer afterwards. Ah! I missed that getting added in GCC 11. But yes, there it is: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-malloc-function-attribute Hah, now we may need to split __malloc from __alloc_size. ;) I'd still like the NULL assignment behavior, though, since some things can easily avoid static analysis.
On Wed, 2 Mar 2022 14:04:06 +0000, David Laight <David.Laight@ACULAB.COM> wrote: > I think that it would be better to make any alternate loop macro > just set the variable to NULL on the loop exit. > That is easier to code for and the compiler might be persuaded to > not redo the test. No, that would lead to a NULL dereference. The problem is the mis-use of iterator outside the loop on exit, and the iterator will be the HEAD's container_of pointer which pointers to a type-confused struct. Sidenote: The *mis-use* here refers to mistakely access to other members of the struct, instead of the list_head member which acutally is the valid HEAD. IOW, you would dereference a (NULL + offset_of_member) address here. Please remind me if i missed something, thanks. > OTOH there may be alternative definitions that can be used to get > the compiler (or other compiler-like tools) to detect broken code. > Even if the definition can't possibly generate a working kerrnel. The "list_for_each_entry_inside(pos, type, head, member)" way makes the iterator invisiable outside the loop, and would be catched by compiler if use-after-loop things happened. Can you share your "alternative definitions" details? thanks! -- Xiaomeng Tong
From: Xiaomeng Tong > Sent: 03 March 2022 02:27 > > On Wed, 2 Mar 2022 14:04:06 +0000, David Laight > <David.Laight@ACULAB.COM> wrote: > > I think that it would be better to make any alternate loop macro > > just set the variable to NULL on the loop exit. > > That is easier to code for and the compiler might be persuaded to > > not redo the test. > > No, that would lead to a NULL dereference. Why, it would make it b ethe same as the 'easy to use': for (item = head; item; item = item->next) { ... if (...) break; ... } if (!item) return; > The problem is the mis-use of iterator outside the loop on exit, and > the iterator will be the HEAD's container_of pointer which pointers > to a type-confused struct. Sidenote: The *mis-use* here refers to > mistakely access to other members of the struct, instead of the > list_head member which acutally is the valid HEAD. The problem is that the HEAD's container_of pointer should never be calculated at all. This is what is fundamentally broken about the current definition. > IOW, you would dereference a (NULL + offset_of_member) address here. Where? > Please remind me if i missed something, thanks. > > Can you share your "alternative definitions" details? thanks! The loop should probably use as extra variable that points to the 'list node' in the next structure. Something like: for (xxx *iter = head->next; iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1)); iter = item->member->next) { ... With a bit of casting you can use 'item' to hold 'iter'. > > > OTOH there may be alternative definitions that can be used to get > > the compiler (or other compiler-like tools) to detect broken code. > > Even if the definition can't possibly generate a working kerrnel. > > The "list_for_each_entry_inside(pos, type, head, member)" way makes > the iterator invisiable outside the loop, and would be catched by > compiler if use-after-loop things happened. It is also a compete PITA for anything doing a search. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Thu, 3 Mar 2022 04:58:23 +0000, David Laight wrote: > on 3 Mar 2022 10:27:29 +0800, Xiaomeng Tong wrote: > > The problem is the mis-use of iterator outside the loop on exit, and > > the iterator will be the HEAD's container_of pointer which pointers > > to a type-confused struct. Sidenote: The *mis-use* here refers to > > mistakely access to other members of the struct, instead of the > > list_head member which acutally is the valid HEAD. > > The problem is that the HEAD's container_of pointer should never > be calculated at all. > This is what is fundamentally broken about the current definition. Yes, the rule is "the HEAD's container_of pointer should never be calculated at all outside the loop", but how do you make sure everyone follows this rule? Everyone makes mistakes, but we can eliminate them all from the beginning with the help of compiler which can catch such use-after-loop things. > > IOW, you would dereference a (NULL + offset_of_member) address here. > >Where? In the case where a developer do not follows the above rule, and mistakely access a non-list-head member of the HEAD's container_of pointer outside the loop. For example: struct req{ int a; struct list_head h; } struct req *r; list_for_each_entry(r, HEAD, h) { if (r->a == 0x10) break; } // the developer made a mistake: he didn't take this situation into // account where all entries in the list are *r->a != 0x10*, and now // the r is the HEAD's container_of pointer. r->a = 0x20; Thus the "r->a = 0x20" would dereference a (NULL + offset_of_member) address here. > > Please remind me if i missed something, thanks. > > > > Can you share your "alternative definitions" details? thanks! > > The loop should probably use as extra variable that points > to the 'list node' in the next structure. > Something like: > for (xxx *iter = head->next; > iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1)); > iter = item->member->next) { > ... > With a bit of casting you can use 'item' to hold 'iter'. you still can not make sure everyone follows this rule: "do not use iterator outside the loop" without the help of compiler, because item is declared outside the loop. BTW, to avoid ambiguity，the "alternative definitions" here i asked is something from you in this context: "OTOH there may be alternative definitions that can be used to get the compiler (or other compiler-like tools) to detect broken code. Even if the definition can't possibly generate a working kerrnel." > > > > > OTOH there may be alternative definitions that can be used to get > > > the compiler (or other compiler-like tools) to detect broken code. > > > Even if the definition can't possibly generate a working kerrnel. > > > > The "list_for_each_entry_inside(pos, type, head, member)" way makes > > the iterator invisiable outside the loop, and would be catched by > > compiler if use-after-loop things happened. > It is also a compete PITA for anything doing a search. You mean it would be a burden on search? can you show me some examples? Or you mean it is too long the list_for_each_entry_inside* string to live in one single line, and should spilt into two line? If it is the case, there are 2 way to mitigate it. 1. pass a shorter t as struct type to the macro 2. after all list_for_each_entry macros be replaced with list_for_each_entry_inside, remove the list_for_each_entry implementations and rename all list_for_each_entry_inside use back to the origin list_for_each_entry in a single patch. For me, it is acceptable and not a such big problem. -- Xiaomeng Tong
> On 3. Mar 2022, at 05:58, David Laight <David.Laight@ACULAB.COM> wrote: > > From: Xiaomeng Tong >> Sent: 03 March 2022 02:27 >> >> On Wed, 2 Mar 2022 14:04:06 +0000, David Laight >> <David.Laight@ACULAB.COM> wrote: >>> I think that it would be better to make any alternate loop macro >>> just set the variable to NULL on the loop exit. >>> That is easier to code for and the compiler might be persuaded to >>> not redo the test. >> >> No, that would lead to a NULL dereference. > > Why, it would make it b ethe same as the 'easy to use': > for (item = head; item; item = item->next) { > ... > if (...) > break; > ... > } > if (!item) > return; > >> The problem is the mis-use of iterator outside the loop on exit, and >> the iterator will be the HEAD's container_of pointer which pointers >> to a type-confused struct. Sidenote: The *mis-use* here refers to >> mistakely access to other members of the struct, instead of the >> list_head member which acutally is the valid HEAD. > > The problem is that the HEAD's container_of pointer should never > be calculated at all. > This is what is fundamentally broken about the current definition. > >> IOW, you would dereference a (NULL + offset_of_member) address here. > > Where? > >> Please remind me if i missed something, thanks. >> >> Can you share your "alternative definitions" details? thanks! > > The loop should probably use as extra variable that points > to the 'list node' in the next structure. > Something like: > for (xxx *iter = head->next; > iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1)); > iter = item->member->next) { > ... > With a bit of casting you can use 'item' to hold 'iter'. I think this would make sense, it would mean you only assign the containing element on valid elements. I was thinking something along the lines of: #define list_for_each_entry(pos, head, member) \ for (struct list_head *list = head->next, typeof(pos) pos; \ list == head ? 0 : (( pos = list_entry(pos, list, member), 1)); \ list = list->next) Although the initialization block of the for loop is not valid C, I'm not sure there is any way to declare two variables of a different type in the initialization part of the loop. I believe all this does is get rid of the &pos->member == (head) check to terminate the list. It alone will not fix any of the other issues that using the iterator variable after the loop currently has. AFAIK Adrián Moreno is working on doing something along those lines for the list iterator in openvswitch (that was similar to the kernel one before) [1]. I *think* they don't declare 'pos' within the loop which we *do want* to avoid any uses of it after the loop. (If pos is not declared in the initialization block, shadowing the *outer* pos, it would just point to the last element of the list or stay uninitialized if the list is empty). [1] https://www.mail-archive.com/ovs-dev@openvswitch.org/msg63497.html > >> >>> OTOH there may be alternative definitions that can be used to get >>> the compiler (or other compiler-like tools) to detect broken code. >>> Even if the definition can't possibly generate a working kerrnel. >> >> The "list_for_each_entry_inside(pos, type, head, member)" way makes >> the iterator invisiable outside the loop, and would be catched by >> compiler if use-after-loop things happened. > > It is also a compete PITA for anything doing a search. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > - Jakob
On Wed, Mar 02, 2022 at 12:07:04PM -0800, Kees Cook wrote: > On Wed, Mar 02, 2022 at 10:29:31AM +0100, Rasmus Villemoes wrote: > > This won't help the current issue (because it doesn't exist and might > > never), but just in case some compiler people are listening, I'd like to > > have some sort of way to tell the compiler "treat this variable as > > uninitialized from here on". So one could do > > > > #define kfree(p) do { __kfree(p); __magic_uninit(p); } while (0) > > > > with __magic_uninit being a magic no-op that doesn't affect the > > semantics of the code, but could be used by the compiler's "[is/may be] > > used uninitialized" machinery to flag e.g. double frees on some odd > > error path etc. It would probably only work for local automatic > > variables, but it should be possible to just ignore the hint if p is > > some expression like foo->bar or has side effects. If we had that, the > > end-of-loop test could include that to "uninitialize" the iterator. > > I've long wanted to change kfree() to explicitly set pointers to NULL on > free. https://github.com/KSPP/linux/issues/87 You also need to be a bit careful with existing code because there are places which do things like: drivers/usb/host/r8a66597-hcd.c 424 kfree(dev); ^^^ 425 426 for (port = 0; port < r8a66597->max_root_hub; port++) { 427 if (r8a66597->root_hub[port].dev == dev) { ^^^ 428 r8a66597->root_hub[port].dev = NULL; 429 break; 430 } 431 } Printing the freed pointer in debug code is another thing people do. regards, dan carpenter
From: Xiaomeng Tong > Sent: 03 March 2022 07:27 > > On Thu, 3 Mar 2022 04:58:23 +0000, David Laight wrote: > > on 3 Mar 2022 10:27:29 +0800, Xiaomeng Tong wrote: > > > The problem is the mis-use of iterator outside the loop on exit, and > > > the iterator will be the HEAD's container_of pointer which pointers > > > to a type-confused struct. Sidenote: The *mis-use* here refers to > > > mistakely access to other members of the struct, instead of the > > > list_head member which acutally is the valid HEAD. > > > > The problem is that the HEAD's container_of pointer should never > > be calculated at all. > > This is what is fundamentally broken about the current definition. > > Yes, the rule is "the HEAD's container_of pointer should never be > calculated at all outside the loop", but how do you make sure everyone > follows this rule? > Everyone makes mistakes, but we can eliminate them all from the beginning > with the help of compiler which can catch such use-after-loop things. > > > > IOW, you would dereference a (NULL + offset_of_member) address here. > > > >Where? > > In the case where a developer do not follows the above rule, and mistakely > access a non-list-head member of the HEAD's container_of pointer outside > the loop. For example: > struct req{ > int a; > struct list_head h; > } > struct req *r; > list_for_each_entry(r, HEAD, h) { > if (r->a == 0x10) > break; > } > // the developer made a mistake: he didn't take this situation into > // account where all entries in the list are *r->a != 0x10*, and now > // the r is the HEAD's container_of pointer. > r->a = 0x20; > Thus the "r->a = 0x20" would dereference a (NULL + offset_of_member) > address here. That is just a bug. No different to failing to check anything else might 'return' a NULL pointer. Because it is a NULL dereference you find out pretty quickly. The existing loop leaves you with a valid pointer to something that isn't a list item. > > > Please remind me if i missed something, thanks. > > > > > > Can you share your "alternative definitions" details? thanks! > > > > The loop should probably use as extra variable that points > > to the 'list node' in the next structure. > > Something like: > > for (xxx *iter = head->next; > > iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1)); > > iter = item->member->next) { > > ... > > With a bit of casting you can use 'item' to hold 'iter'. > > you still can not make sure everyone follows this rule: > "do not use iterator outside the loop" without the help of compiler, > because item is declared outside the loop. That one has 'iter' defined in the loop. > BTW, to avoid ambiguity，the "alternative definitions" here i asked is > something from you in this context: > "OTOH there may be alternative definitions that can be used to get > the compiler (or other compiler-like tools) to detect broken code. > Even if the definition can't possibly generate a working kerrnel." I was thinking of something like: if ((pos = list_first)), 1) pos = NULL else so that unchecked dereferences after the loop will be detectable as NULL pointer offsets - but that in itself isn't enough to avoid other warnings. > > > The "list_for_each_entry_inside(pos, type, head, member)" way makes > > > the iterator invisiable outside the loop, and would be catched by > > > compiler if use-after-loop things happened. > > > It is also a compete PITA for anything doing a search. > > You mean it would be a burden on search? can you show me some examples? The whole business of having to save the pointer to the located item before breaking the loop, remembering to have set it to NULL earlier etc. It is so much better if you can just do: if (found) break; David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Wed, Mar 02, 2022 at 10:29:31AM +0100, Rasmus Villemoes wrote: > This won't help the current issue (because it doesn't exist and might > never), but just in case some compiler people are listening, I'd like to > have some sort of way to tell the compiler "treat this variable as > uninitialized from here on". So one could do > > #define kfree(p) do { __kfree(p); __magic_uninit(p); } while (0) > I think this is a good idea. Smatch can already find all the iterator used outside the loop bugs that Jakob did with a manageably small number of false positives. The problems are that: 1) It would be better to find it in the compile stage instead of later. 2) I hadn't published that check. Will do shortly. 3) A couple weeks back I noticed that the list_for_each_entry() check was no longer working. Fixed now. 4) Smatch was only looking at cases which dereferenced the iterator and not checks for NULL. I will test the fix for that tonight. 5) Smatch is broken on PowerPC. Coccinelle also has checks for iterator used outside the loop. Coccinelle had these checks before Smatch did. I copied Julia's idea. If your annotation was added to GCC it would solve all those problems. But it's kind of awkward that we can't annotate kfree() directly instead of creating the kfree() macro. And there are lots of other functions which free things so you'd have to create a ton of macros like: #define gr_free_dma_desc(a, b) do { __gr_free_dma_desc(a, b); __magic_uninit(b); } while (0) And then there are functions which free a struct member: void free_bar(struct foo *p) { kfree(p->bar); } Or functions which free a container_of(). Smatch is more evolved than designed but what I do these days is use $0, $1, $2 to represent the parameters. So you can say a function frees $0->bar. For container_of() then is "(168<~$0)->bar" which means 168 bytes from $0. Returns are parameter -1 so I guess it would be $(-1), but as I said Smatch evolved so right now places that talk about returned values use a different format. What you could do is just make a parseable table next to the function definition with all the information. Then you would use a Perl script to automatically generate a Coccinelle check to warn about use after frees. diff --git a/mm/slab.c b/mm/slab.c index ddf5737c63d9..c9dffa5c40a2 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -3771,6 +3771,9 @@ EXPORT_SYMBOL(kmem_cache_free_bulk); * * Don't free memory not originally allocated by kmalloc() * or you will run into trouble. + * + * CHECKER information + * frees: $0 */ void kfree(const void *objp) { regards, dan carpenter
On Thu, Mar 03, 2022 at 03:26:57PM +0800, Xiaomeng Tong wrote: > On Thu, 3 Mar 2022 04:58:23 +0000, David Laight wrote: > > on 3 Mar 2022 10:27:29 +0800, Xiaomeng Tong wrote: > > > The problem is the mis-use of iterator outside the loop on exit, and > > > the iterator will be the HEAD's container_of pointer which pointers > > > to a type-confused struct. Sidenote: The *mis-use* here refers to > > > mistakely access to other members of the struct, instead of the > > > list_head member which acutally is the valid HEAD. > > > > The problem is that the HEAD's container_of pointer should never > > be calculated at all. > > This is what is fundamentally broken about the current definition. > > Yes, the rule is "the HEAD's container_of pointer should never be > calculated at all outside the loop", but how do you make sure everyone > follows this rule? Your formulation of the rule is correct: never run container_of() on HEAD pointer. However the rule that is introduced by list_for_each_entry_inside() is *not* this rule. The rule it introduces is: never access the iterator variable outside the loop. Making the iterator NULL on loop exit does follow the rule you proposed but using a different technique: do not allow HEAD to be stored in the iterator variable after loop exit. This also makes it impossible to run container_of() on the HEAD pointer. > Everyone makes mistakes, but we can eliminate them all from the beginning > with the help of compiler which can catch such use-after-loop things. Indeed but if we introduce new interfaces then we don't have to worry about existing usages and silent regressions. Code will have been written knowing the loop can exit with the iterator set to NULL. Sure it is still possible for programmers to make mistakes and dereference the NULL pointer but C programmers are well training w.r.t. NULL pointer checking so such mistakes are much less likely than with the current list_for_each_entry() macro. This risk must be offset against the way a NULLify approach can lead to more elegant code when we are doing a list search. Daniel.
> From: Xiaomeng Tong > > Sent: 03 March 2022 07:27 > > > > On Thu, 3 Mar 2022 04:58:23 +0000, David Laight wrote: > > > on 3 Mar 2022 10:27:29 +0800, Xiaomeng Tong wrote: > > > > The problem is the mis-use of iterator outside the loop on exit, and > > > > the iterator will be the HEAD's container_of pointer which pointers > > > > to a type-confused struct. Sidenote: The *mis-use* here refers to > > > > mistakely access to other members of the struct, instead of the > > > > list_head member which acutally is the valid HEAD. > > > > > > The problem is that the HEAD's container_of pointer should never > > > be calculated at all. > > > This is what is fundamentally broken about the current definition. > > > > Yes, the rule is "the HEAD's container_of pointer should never be > > calculated at all outside the loop", but how do you make sure everyone > > follows this rule? > > Everyone makes mistakes, but we can eliminate them all from the beginning > > with the help of compiler which can catch such use-after-loop things. > > > > > > IOW, you would dereference a (NULL + offset_of_member) address here. > > > > > >Where? > > > > In the case where a developer do not follows the above rule, and mistakely > > access a non-list-head member of the HEAD's container_of pointer outside > > the loop. For example: > > struct req{ > > int a; > > struct list_head h; > > } > > struct req *r; > > list_for_each_entry(r, HEAD, h) { > > if (r->a == 0x10) > > break; > > } > > // the developer made a mistake: he didn't take this situation into > > // account where all entries in the list are *r->a != 0x10*, and now > > // the r is the HEAD's container_of pointer. > > r->a = 0x20; > > Thus the "r->a = 0x20" would dereference a (NULL + offset_of_member) > > address here. > > That is just a bug. > No different to failing to check anything else might 'return' > a NULL pointer. Yes, but it‘s a mistake everyone has made and will make, we should avoid this at the beginning with the help of compiler. > Because it is a NULL dereference you find out pretty quickly. AFAIK，NULL dereference is a undefined bahavior, can compiler quickly catch it? Or it can only be applied to some simple/restricted cases. > The existing loop leaves you with a valid pointer to something > that isn't a list item. > > > > > Please remind me if i missed something, thanks. > > > > > > > > Can you share your "alternative definitions" details? thanks! > > > > > > The loop should probably use as extra variable that points > > > to the 'list node' in the next structure. > > > Something like: > > > for (xxx *iter = head->next; > > > iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1)); > > > iter = item->member->next) { > > > ... > > > With a bit of casting you can use 'item' to hold 'iter'. > > > > you still can not make sure everyone follows this rule: > > "do not use iterator outside the loop" without the help of compiler, > > because item is declared outside the loop. > > That one has 'iter' defined in the loop. Oh, sorry, I misunderstood. Then this is the same way with my way of list_for_each_entry_inside(pos, type, head, member), which declare the iterator inside the loop. You go further and make things like "&pos->member == (head)" goes away to avoid calculate the HEAD's container_of pointer, although the calculation is harmless. > > > BTW, to avoid ambiguity，the "alternative definitions" here i asked is > > something from you in this context: > > "OTOH there may be alternative definitions that can be used to get > > the compiler (or other compiler-like tools) to detect broken code. > > Even if the definition can't possibly generate a working kerrnel." > > I was thinking of something like: > if ((pos = list_first)), 1) pos = NULL else > so that unchecked dereferences after the loop will be detectable > as NULL pointer offsets - but that in itself isn't enough to avoid > other warnings. > Do you mean put this right after the loop (I changed somthing that i do not understand, please correct me if i am worng, thanks): if (pos == list_first) pos = NULL; else and compiler can detect the following NULL derefernce? But if the NULL derefernce is just right after the loop originally, it will be masked by the *else* keyword。 > > > > The "list_for_each_entry_inside(pos, type, head, member)" way makes > > > > the iterator invisiable outside the loop, and would be catched by > > > > compiler if use-after-loop things happened. > > > > > It is also a compete PITA for anything doing a search. > > > > You mean it would be a burden on search? can you show me some examples? > > The whole business of having to save the pointer to the located item > before breaking the loop, remembering to have set it to NULL earlier etc. Ok, i see. And then you need pass a "item" to the list_for_each_entry macro as a new argument. > > It is so much better if you can just do: > if (found) > break; > > David this confused me. this way is better or the "save the pointer to the located item before breaking the loop" one? -- Xiaomeng Tong
On Thu, 3 Mar 2022 12:18:24 +0000, Daniel Thompson wrote: > On Thu, Mar 03, 2022 at 03:26:57PM +0800, Xiaomeng Tong wrote: > > On Thu, 3 Mar 2022 04:58:23 +0000, David Laight wrote: > > > on 3 Mar 2022 10:27:29 +0800, Xiaomeng Tong wrote: > > > > The problem is the mis-use of iterator outside the loop on exit, and > > > > the iterator will be the HEAD's container_of pointer which pointers > > > > to a type-confused struct. Sidenote: The *mis-use* here refers to > > > > mistakely access to other members of the struct, instead of the > > > > list_head member which acutally is the valid HEAD. > > > > > > The problem is that the HEAD's container_of pointer should never > > > be calculated at all. > > > This is what is fundamentally broken about the current definition. > > > > Yes, the rule is "the HEAD's container_of pointer should never be > > calculated at all outside the loop", but how do you make sure everyone > > follows this rule? > > Your formulation of the rule is correct: never run container_of() on HEAD > pointer. Actually, it is not my rule. My rule is that never access other members of the struct except for the list_head member after the loop, because this is a invalid member after loop exit, but valid for the list_head member which just is HEAD and the lately caculation (&pos->head) seems harmless. I have considered the case that the HEAD's container "pos" is layouted across the max and the min address boundary, which means the address of HEAD is likely 0x60, and the address of pos is likely 0xffffffe0. It seems ok to caculate pos with: ((type *)(__mptr - offsetof(type, member))); and it seems ok to caculate head outside the loop with: if (&pos->head == &HEAD) return NULL; The only case I can think of with the rule "never run container_of() on HEAD" must be followed is when the first argument (which is &HEAD) passing to container_of() is NULL + some offset, it may lead to the resulting "pos->member" access being a NULL dereference. But maybe the caller can take the responsibility to check if it is NULL, not container_of() itself. Please remind me if i missed somthing, thanks. > > However the rule that is introduced by list_for_each_entry_inside() is > *not* this rule. The rule it introduces is: never access the iterator > variable outside the loop. Sorry for the confusion, indeed, that is two *different* rule. > > Making the iterator NULL on loop exit does follow the rule you proposed > but using a different technique: do not allow HEAD to be stored in the > iterator variable after loop exit. This also makes it impossible to run > container_of() on the HEAD pointer. > It does not. My rule is: never access the iterator variable outside the loop. The "Making the iterator NULL on loop exit" way still leak the pos with NULL outside the loop, may lead to a NULL deference. > > > Everyone makes mistakes, but we can eliminate them all from the beginning > > with the help of compiler which can catch such use-after-loop things. > > Indeed but if we introduce new interfaces then we don't have to worry > about existing usages and silent regressions. Code will have been > written knowing the loop can exit with the iterator set to NULL. Yes, it is more simple and compatible with existing interfaces. Howerver, you should make every developers to remember that "pos will be set NULL on loop exit", which is unreasonable and impossible for *every* single person. Otherwise the mis-use-after-loop will lead to a NULL dereference. But we can kill this problem by declaring iterator inside the loop and the complier will catch it if somebody mis-use-after-loop. > > Sure it is still possible for programmers to make mistakes and > dereference the NULL pointer but C programmers are well training w.r.t. > NULL pointer checking so such mistakes are much less likely than with > the current list_for_each_entry() macro. This risk must be offset > against the way a NULLify approach can lead to more elegant code when we > are doing a list search. > Yes, the NULLify approach is better than the current list_for_each_entry() macro, but i stick with that the list_for_each_entry_inside() way is best and perfect _technically_. Thus, my idea is *better a finger off than always aching*, let's settle this damn problem once and for all, with list_for_each_entry_inside(). -- Xiaomeng Tong
diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c index 48afe96ae0f0..6c916416decc 100644 --- a/arch/x86/kernel/cpu/sgx/encl.c +++ b/arch/x86/kernel/cpu/sgx/encl.c @@ -450,7 +450,8 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn, struct mm_struct *mm) { struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier); - struct sgx_encl_mm *tmp = NULL; + struct sgx_encl_mm *found_encl_mm = NULL; + struct sgx_encl_mm *tmp; /* * The enclave itself can remove encl_mm. Note, objects can't be moved @@ -460,12 +461,13 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn, list_for_each_entry(tmp, &encl_mm->encl->mm_list, list) { if (tmp == encl_mm) { list_del_rcu(&encl_mm->list); + found_encl_mm = tmp; break; } } spin_unlock(&encl_mm->encl->mm_lock); - if (tmp == encl_mm) { + if (found_encl_mm) { synchronize_srcu(&encl_mm->encl->srcu); mmu_notifier_put(mn); } diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c index 4ee578b181da..a8cbd90db9d2 100644 --- a/drivers/scsi/scsi_transport_sas.c +++ b/drivers/scsi/scsi_transport_sas.c @@ -1060,26 +1060,29 @@ EXPORT_SYMBOL(sas_port_get_phy); * connected to a remote device is a port, so ports must be formed on * all devices with phys if they're connected to anything. */ -void sas_port_add_phy(struct sas_port *port, struct sas_phy *phy) +void sas_port_add_phy(struct sas_port *port, struct sas_phy *_phy) { mutex_lock(&port->phy_list_mutex); - if (unlikely(!list_empty(&phy->port_siblings))) { + if (unlikely(!list_empty(&_phy->port_siblings))) { /* make sure we're already on this port */ + struct sas_phy *phy = NULL; struct sas_phy *tmp; list_for_each_entry(tmp, &port->phy_list, port_siblings) - if (tmp == phy) + if (tmp == _phy) { + phy = tmp; break; + } /* If this trips, you added a phy that was already * part of a different port */ - if (unlikely(tmp != phy)) { + if (unlikely(!phy)) { dev_printk(KERN_ERR, &port->dev, "trying to add phy %s fails: it's already part of another port\n", - dev_name(&phy->dev)); + dev_name(&_phy->dev)); BUG(); } } else { - sas_port_create_link(port, phy); - list_add_tail(&phy->port_siblings, &port->phy_list); + sas_port_create_link(port, _phy); + list_add_tail(&_phy->port_siblings, &port->phy_list); port->num_phys++; } mutex_unlock(&port->phy_list_mutex); diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c index 82654dc8382b..97198543448b 100644 --- a/drivers/thermal/thermal_core.c +++ b/drivers/thermal/thermal_core.c @@ -625,24 +625,30 @@ int thermal_zone_bind_cooling_device(struct thermal_zone_device *tz, { struct thermal_instance *dev; struct thermal_instance *pos; - struct thermal_zone_device *pos1; - struct thermal_cooling_device *pos2; + struct thermal_zone_device *pos1 = NULL; + struct thermal_zone_device *tmp1; + struct thermal_cooling_device *pos2 = NULL; + struct thermal_cooling_device *tmp2; unsigned long max_state; int result, ret; if (trip >= tz->trips || trip < 0) return -EINVAL; - list_for_each_entry(pos1, &thermal_tz_list, node) { - if (pos1 == tz) + list_for_each_entry(tmp1, &thermal_tz_list, node) { + if (tmp1 == tz) { + pos1 = tmp1; break; + } } - list_for_each_entry(pos2, &thermal_cdev_list, node) { - if (pos2 == cdev) + list_for_each_entry(tmp2, &thermal_cdev_list, node) { + if (tmp2 == cdev) { + pos2 = tmp2; break; + } } - if (tz != pos1 || cdev != pos2) + if (!pos1 || !pos2) return -EINVAL; ret = cdev->ops->get_max_state(cdev, &max_state); @@ -1074,15 +1080,18 @@ void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev) const struct thermal_zone_params *tzp; struct thermal_zone_device *tz; struct thermal_cooling_device *pos = NULL; + struct thermal_cooling_device *tmp; if (!cdev) return; mutex_lock(&thermal_list_lock); - list_for_each_entry(pos, &thermal_cdev_list, node) - if (pos == cdev) + list_for_each_entry(tmp, &thermal_cdev_list, node) + if (tmp == cdev) { + pos = tmp; break; - if (pos != cdev) { + } + if (!pos) { /* thermal cooling device not found */ mutex_unlock(&thermal_list_lock); return; @@ -1335,6 +1344,7 @@ void thermal_zone_device_unregister(struct thermal_zone_device *tz) const struct thermal_zone_params *tzp; struct thermal_cooling_device *cdev; struct thermal_zone_device *pos = NULL; + struct thermal_zone_device *tmp; if (!tz) return; @@ -1343,10 +1353,12 @@ void thermal_zone_device_unregister(struct thermal_zone_device *tz) tz_id = tz->id; mutex_lock(&thermal_list_lock); - list_for_each_entry(pos, &thermal_tz_list, node) - if (pos == tz) + list_for_each_entry(tmp, &thermal_tz_list, node) + if (tmp == tz) { + pos = tmp; break; - if (pos != tz) { + } + if (!pos) { /* thermal zone device not found */ mutex_unlock(&thermal_list_lock); return; diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c index d4a678c0806e..99f10cbd8878 100644 --- a/drivers/usb/gadget/configfs.c +++ b/drivers/usb/gadget/configfs.c @@ -418,7 +418,8 @@ static int config_usb_cfg_link( struct usb_function_instance *fi = to_usb_function_instance(usb_func_ci); - struct usb_function_instance *a_fi; + struct usb_function_instance *a_fi = NULL; + struct usb_function_instance *tmp; struct usb_function *f; int ret; @@ -428,11 +429,13 @@ static int config_usb_cfg_link( * from another gadget or a random directory. * Also a function instance can only be linked once. */ - list_for_each_entry(a_fi, &gi->available_func, cfs_list) { - if (a_fi == fi) + list_for_each_entry(tmp, &gi->available_func, cfs_list) { + if (tmp == fi) { + a_fi = tmp; break; + } } - if (a_fi != fi) { + if (!a_fi) { ret = -EINVAL; goto out; } @@ -882,15 +885,18 @@ static int os_desc_link(struct config_item *os_desc_ci, struct gadget_info *gi = os_desc_item_to_gadget_info(os_desc_ci); struct usb_composite_dev *cdev = &gi->cdev; struct config_usb_cfg *c_target = to_config_usb_cfg(usb_cfg_ci); - struct usb_configuration *c; + struct usb_configuration *c = NULL; + struct usb_configuration *tmp; int ret; mutex_lock(&gi->lock); - list_for_each_entry(c, &cdev->configs, list) { - if (c == &c_target->c) + list_for_each_entry(tmp, &cdev->configs, list) { + if (tmp == &c_target->c) { + c = tmp; break; + } } - if (c != &c_target->c) { + if (!c) { ret = -EINVAL; goto out; } diff --git a/drivers/usb/gadget/udc/max3420_udc.c b/drivers/usb/gadget/udc/max3420_udc.c index d2a2b20cc1ad..d1b010b5f4a0 100644 --- a/drivers/usb/gadget/udc/max3420_udc.c +++ b/drivers/usb/gadget/udc/max3420_udc.c @@ -1044,22 +1044,25 @@ static int max3420_ep_queue(struct usb_ep *_ep, struct usb_request *_req, static int max3420_ep_dequeue(struct usb_ep *_ep, struct usb_request *_req) { - struct max3420_req *t, *req = to_max3420_req(_req); + struct max3420_req *t = NULL; + struct max3420_req *req = to_max3420_req(_req); + struct max3420_req *tmp; struct max3420_ep *ep = to_max3420_ep(_ep); unsigned long flags; spin_lock_irqsave(&ep->lock, flags); /* Pluck the descriptor from queue */ - list_for_each_entry(t, &ep->queue, queue) - if (t == req) { + list_for_each_entry(tmp, &ep->queue, queue) + if (tmp == req) { list_del_init(&req->queue); + t = tmp; break; } spin_unlock_irqrestore(&ep->lock, flags); - if (t == req) + if (t) max3420_req_done(req, -ECONNRESET); return 0; diff --git a/drivers/usb/gadget/udc/tegra-xudc.c b/drivers/usb/gadget/udc/tegra-xudc.c index 43f1b0d461c1..c37e3148a208 100644 --- a/drivers/usb/gadget/udc/tegra-xudc.c +++ b/drivers/usb/gadget/udc/tegra-xudc.c @@ -1413,18 +1413,21 @@ __tegra_xudc_ep_dequeue(struct tegra_xudc_ep *ep, struct tegra_xudc_request *req) { struct tegra_xudc *xudc = ep->xudc; - struct tegra_xudc_request *r; + struct tegra_xudc_request *r = NULL; + struct tegra_xudc_request *tmp; struct tegra_xudc_trb *deq_trb; bool busy, kick_queue = false; int ret = 0; /* Make sure the request is actually queued to this endpoint. */ - list_for_each_entry(r, &ep->queue, list) { - if (r == req) + list_for_each_entry(tmp, &ep->queue, list) { + if (tmp == req) { + r = tmp; break; + } } - if (r != req) + if (!r) return -EINVAL; /* Request hasn't been queued in the transfer ring yet. */ diff --git a/drivers/usb/mtu3/mtu3_gadget.c b/drivers/usb/mtu3/mtu3_gadget.c index 9977600616d7..2e4daaa081a0 100644 --- a/drivers/usb/mtu3/mtu3_gadget.c +++ b/drivers/usb/mtu3/mtu3_gadget.c @@ -323,7 +323,8 @@ static int mtu3_gadget_dequeue(struct usb_ep *ep, struct usb_request *req) { struct mtu3_ep *mep = to_mtu3_ep(ep); struct mtu3_request *mreq = to_mtu3_request(req); - struct mtu3_request *r; + struct mtu3_request *r = NULL; + struct mtu3_request *tmp; struct mtu3 *mtu = mep->mtu; unsigned long flags; int ret = 0; @@ -336,11 +337,13 @@ static int mtu3_gadget_dequeue(struct usb_ep *ep, struct usb_request *req) spin_lock_irqsave(&mtu->lock, flags); - list_for_each_entry(r, &mep->req_list, list) { - if (r == mreq) + list_for_each_entry(tmp, &mep->req_list, list) { + if (tmp == mreq) { + r = tmp; break; + } } - if (r != mreq) { + if (!r) { dev_dbg(mtu->dev, "req=%p not queued to %s\n", req, ep->name); ret = -EINVAL; goto done; diff --git a/drivers/usb/musb/musb_gadget.c b/drivers/usb/musb/musb_gadget.c index 51274b87f46c..26b61ad7ab1b 100644 --- a/drivers/usb/musb/musb_gadget.c +++ b/drivers/usb/musb/musb_gadget.c @@ -1266,7 +1266,8 @@ static int musb_gadget_dequeue(struct usb_ep *ep, struct usb_request *request) { struct musb_ep *musb_ep = to_musb_ep(ep); struct musb_request *req = to_musb_request(request); - struct musb_request *r; + struct musb_request *r = NULL; + struct musb_request *tmp; unsigned long flags; int status = 0; struct musb *musb = musb_ep->musb; @@ -1278,11 +1279,13 @@ static int musb_gadget_dequeue(struct usb_ep *ep, struct usb_request *request) spin_lock_irqsave(&musb->lock, flags); - list_for_each_entry(r, &musb_ep->req_list, list) { - if (r == req) + list_for_each_entry(tmp, &musb_ep->req_list, list) { + if (tmp == req) { + r = tmp; break; + } } - if (r != req) { + if (!r) { dev_err(musb->controller, "request %p not queued to %s\n", request, ep->name); status = -EINVAL; diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c index b314101237fe..52cfa44c24a7 100644 --- a/drivers/vfio/mdev/mdev_core.c +++ b/drivers/vfio/mdev/mdev_core.c @@ -337,16 +337,19 @@ int mdev_device_create(struct mdev_type *type, const guid_t *uuid) int mdev_device_remove(struct mdev_device *mdev) { - struct mdev_device *tmp; + struct mdev_device *tmp = NULL; + struct mdev_device *iter; struct mdev_parent *parent = mdev->type->parent; mutex_lock(&mdev_list_lock); - list_for_each_entry(tmp, &mdev_list, next) { - if (tmp == mdev) + list_for_each_entry(iter, &mdev_list, next) { + if (iter == mdev) { + tmp = iter; break; + } } - if (tmp != mdev) { + if (!tmp) { mutex_unlock(&mdev_list_lock); return -ENODEV; }
If the list does not contain the expected element, the value of list_for_each_entry() iterator will not point to a valid structure. To avoid type confusion in such case, the list iterator scope will be limited to list_for_each_entry() loop. In preparation to limiting scope of a list iterator to the list traversal loop, use a dedicated pointer to point to the found element. Determining if an element was found is then simply checking if the pointer is != NULL. Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com> --- arch/x86/kernel/cpu/sgx/encl.c | 6 +++-- drivers/scsi/scsi_transport_sas.c | 17 ++++++++----- drivers/thermal/thermal_core.c | 38 ++++++++++++++++++---------- drivers/usb/gadget/configfs.c | 22 ++++++++++------ drivers/usb/gadget/udc/max3420_udc.c | 11 +++++--- drivers/usb/gadget/udc/tegra-xudc.c | 11 +++++--- drivers/usb/mtu3/mtu3_gadget.c | 11 +++++--- drivers/usb/musb/musb_gadget.c | 11 +++++--- drivers/vfio/mdev/mdev_core.c | 11 +++++--- 9 files changed, 88 insertions(+), 50 deletions(-) -- 2.25.1