| Message ID | alpine.LSU.2.11.1902222222570.1594@eggly.anvils (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | tmpfs: fix uninitialized return value in shmem_link | expand |
On Fri, Feb 22, 2019 at 10:35 PM Hugh Dickins <hughd@google.com> wrote: > > When we made the shmem_reserve_inode call in shmem_link conditional, we > forgot to update the declaration for ret so that it always has a known > value. Dan Carpenter pointed out this deficiency in the original patch. Applied. Side note: how come gcc didn't warn about this? Yes, we disable that warning for some cases because of lots of false positives, but I thought the *default* setup still had it. Is it just that the goto ends up confusing gcc enough that it never notices? Linus
On Mon, 25 Feb 2019, Linus Torvalds wrote: > On Fri, Feb 22, 2019 at 10:35 PM Hugh Dickins <hughd@google.com> wrote: > > > > When we made the shmem_reserve_inode call in shmem_link conditional, we > > forgot to update the declaration for ret so that it always has a known > > value. Dan Carpenter pointed out this deficiency in the original patch. > > Applied. Thanks. And I apologize for letting that slip through: Darrick sent the patch fragment, I dressed it up, and more or less tricked him into taking ownership of the bug, when it's I who should have been more careful. But I'm glad it confirmed your rc8 instinct, rather than messing final :) > > Side note: how come gcc didn't warn about this? Yes, we disable that > warning for some cases because of lots of false positives, but I > thought the *default* setup still had it. I thought so too, and have been puzzled by it. If I try removing the initialization of inode from the next function, shmem_unlink(), I do get the expected warning for that. > > Is it just that the goto ends up confusing gcc enough that it never notices? Since the goto route did have ret properly initialized, I don't see why it might have been confusing, but what do I know... I thought it might be because outside the goto route, ret was used for nothing but the return value. But that's disproved: I tried a very silly "inode->i_flags = ret;" just after d_instantiate(), and still no warning when ret is uninitialized. Seems like a gcc bug? But I don't have a decent recent gcc to hand to submit a proper report, hope someone else can shed light on it. Hugh
On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote: > > Seems like a gcc bug? But I don't have a decent recent gcc to hand > to submit a proper report, hope someone else can shed light on it. I don't have a _very_ recent gcc either, but with gcc-8.2.1 the attached test-case gives me: [torvalds@i7 ~]$ gcc -O2 -S -Wall test.c with no warning, and then [torvalds@i7 ~]$ gcc -O2 -S -Wall -DHIDE_PROBLEM test.c test.c: In function ‘shmem_link’: test.c:60:9: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized] return ret; ^~~ *does* show the expected warning. So it is the presence of that if (ret) return ret; that suppresses the warning. What I *suspect* happens is (a) gcc sees that there is only one assignment to "ret" (b) in the same basic block as the assignment, there is a test against "ret" being nonzero that goes out. and what I think happens is that (a) causes gcc to consider that assignment to be the defining assignment (which makes all kinds of sense in an SSA world), and then (b) means that gcc decides that clearly "ret" has to be zero in any case that doesn't go out due to the if-test. In fact, if I then look at the code generation, gcc will actually do this (edited to be more legible): movl (%rbx), %eax <- load inode->i_nlink testl %eax, %eax je .L1 ... ... call d_instantiate xorl %eax, %eax <- explicitly zero 'ret'! .L1: popq %rbx popq %rbp popq %r12 ret so at least with my compiler, it *effectively* zeroed ret (in %rax) anyway, and it all just _happened_ to get the right result even though 'ret' wasn't actually initialized. Which is why it all worked just fine. And depending on how gcc works internally, it really may not just be a random mistake of register allocation, but really because gcc kind of _thought_ that 'ret' was zero-initialized due to the combination of the one single assigment and test for zero. So it turns out that the patch to initialize to zero doesn't do anything, probably for the same reason that gcc didn't warn about the missing initialization. Gcc kind of added an initialization of its own there. I'm not entirely sure if any gcc developer would be interested in this as a test-case, but I guess I can try to do a bugzilla. Adding a few gcc people who have been on previous kernel gcc bugzilla discussions, just in case they have something to add. The gcc bugzilla is this: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89501 and I tried to make it be self-explanatory, but I wrote the bugzilla in parallel with this email, and maybe there's some missing context either there (or here). Linus /* * Minimal fake declarations of "kernel" data types */ struct superblock; struct inode { int i_nlink; int i_size; int i_ctime; int i_mtime; struct superblock *i_sb; }; struct dentry { struct inode *d_inode; }; #define d_inode(dentry) ((dentry)->d_inode) extern int current_time(struct inode *); extern void inc_nlink(struct inode *); extern void ihold(struct inode *); extern void dget(struct dentry *); extern void d_instantiate(struct dentry *, struct inode *); extern int shmem_reserve_inode(struct superblock *); #define BOGO_DIRENT_SIZE 20 /* * The actual function where I'd have expected a warning * about "ret might be used uninitialized" */ int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry) { struct inode *inode = d_inode(old_dentry); int ret; /* * No ordinary (disk based) filesystem counts links as inodes; * but each new link needs a new dentry, pinning lowmem, and * tmpfs dentries cannot be pruned until they are unlinked. * But if an O_TMPFILE file is linked into the tmpfs, the * first link must skip that, to get the accounting right. */ if (inode->i_nlink) { ret = shmem_reserve_inode(inode->i_sb); #ifndef HIDE_PROBLEM if (ret) return ret; #endif } dir->i_size += BOGO_DIRENT_SIZE; inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode); inc_nlink(inode); ihold(inode); /* New dentry reference */ dget(dentry); /* Extra pinning count for the created dentry */ d_instantiate(dentry, inode); return ret; }
On Mon, Feb 25, 2019 at 2:34 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote: > > > > Seems like a gcc bug? But I don't have a decent recent gcc to hand > > to submit a proper report, hope someone else can shed light on it. > > I don't have a _very_ recent gcc either [..] Well, that was quick. Yup, it's considered a gcc bug. Sadly, it's just a different version of a really old bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501 which goes back to 2004. Which I guess means we should not expect this to be fixed in gcc any time soon. The *good* news (I guess) is that if we have other situations with that pattern, and that lack of warning, it really is because gcc will have generated code as if it was initialized (to the value that we tested it must have been in the one basic block where it *was* initialized). So it won't leak random kernel data, and with the common error condition case (like in this example - checking that we didn't have an error) it will actually end up doing the right thing. Entirely by mistake, and without a warniing, but still.. It could have been much worse. Basically at least for this pattern, "lack of warning" ends up meaning "it got initialized to the expected value". Of course, that's just gcc. I have no idea what llvm ends up doing. Linus
On 2/25/19 6:58 PM, Linus Torvalds wrote: > On Mon, Feb 25, 2019 at 2:34 PM Linus Torvalds > <torvalds@linux-foundation.org> wrote: >> >> On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote: >>> >>> Seems like a gcc bug? But I don't have a decent recent gcc to hand >>> to submit a proper report, hope someone else can shed light on it. >> >> I don't have a _very_ recent gcc either [..] > > Well, that was quick. Yup, it's considered a gcc bug. > > Sadly, it's just a different version of a really old bug: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501 > > which goes back to 2004. > > Which I guess means we should not expect this to be fixed in gcc any time soon. > > The *good* news (I guess) is that if we have other situations with > that pattern, and that lack of warning, it really is because gcc will > have generated code as if it was initialized (to the value that we > tested it must have been in the one basic block where it *was* > initialized). > > So it won't leak random kernel data, and with the common error > condition case (like in this example - checking that we didn't have an > error) it will actually end up doing the right thing. > > Entirely by mistake, and without a warniing, but still.. It could have > been much worse. Basically at least for this pattern, "lack of > warning" ends up meaning "it got initialized to the expected value". > > Of course, that's just gcc. I have no idea what llvm ends up doing. > Clang 7.0: # clang -O2 -S -Wall /tmp/test.c /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized] if (inode->i_nlink) { ^~~~~~~~~~~~~~ /tmp/test.c:60:9: note: uninitialized use occurs here return ret; ^~~ /tmp/test.c:46:2: note: remove the 'if' if its condition is always true if (inode->i_nlink) { ^~~~~~~~~~~~~~~~~~~~ /tmp/test.c:37:9: note: initialize the variable 'ret' to silence this warning int ret; ^ = 0 1 warning generated.
On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote: > > > > Of course, that's just gcc. I have no idea what llvm ends up doing. > > Clang 7.0: > > # clang -O2 -S -Wall /tmp/test.c > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if' > condition is false [-Wsometimes-uninitialized] Ok, good. Do we have any clang builds in any of the zero-day robot infrastructure or something? Should we? And maybe this was how Dan noticed the problem in the first place? Or is it just because of his eagle-eyes? Linus
On Mon, Feb 25, 2019 at 04:07:12PM -0800, Linus Torvalds wrote: > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote: > > > > > > Of course, that's just gcc. I have no idea what llvm ends up doing. > > > > Clang 7.0: > > > > # clang -O2 -S -Wall /tmp/test.c > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if' > > condition is false [-Wsometimes-uninitialized] > > Ok, good. > > Do we have any clang builds in any of the zero-day robot > infrastructure or something? Should we? > > And maybe this was how Dan noticed the problem in the first place? Or > is it just because of his eagle-eyes? He didn't say specifically how he found it, but I would guess he was running smatch...? --D > Linus
On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote: > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote: > > > > > > Of course, that's just gcc. I have no idea what llvm ends up doing. > > > > Clang 7.0: > > > > # clang -O2 -S -Wall /tmp/test.c > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever > > 'if' > > condition is false [-Wsometimes-uninitialized] > > Ok, good. > > Do we have any clang builds in any of the zero-day robot > infrastructure or something? Should we? > > And maybe this was how Dan noticed the problem in the first place? Or > is it just because of his eagle-eyes? > BTW, even clang is able to generate warnings in your sample code, it does not generate any warnings when compiling the buggy shmem.o via "make CC=clang". Here is the objdump for arm64 (with KASAN_SW_TAGS inline). 000000000000effc <shmem_link>: { effc: f81c0ff7 str x23, [sp, #-64]! f000: a90157f6 stp x22, x21, [sp, #16] f004: a9024ff4 stp x20, x19, [sp, #32] f008: a9037bfd stp x29, x30, [sp, #48] f00c: 9100c3fd add x29, sp, #0x30 f010: aa0203f3 mov x19, x2 f014: aa0103f5 mov x21, x1 f018: aa0003f4 mov x20, x0 f01c: 94000000 bl 0 <_mcount> f020: 91016280 add x0, x20, #0x58 f024: d2c20017 mov x23, #0x100000000000 // #17592186044416 f028: b2481c08 orr x8, x0, #0xff00000000000000 f02c: f2fdfff7 movk x23, #0xefff, lsl #48 f030: d344fd08 lsr x8, x8, #4 f034: 38776909 ldrb w9, [x8, x23] f038: 940017d5 bl 14f8c <OUTLINED_FUNCTION_11> f03c: 54000060 b.eq f048 <shmem_link+0x4c> // b.none f040: 7103fd1f cmp w8, #0xff f044: 54000981 b.ne f174 <shmem_link+0x178> // b.any f048: f9400014 ldr x20, [x0] if (inode->i_nlink) { f04c: 91010280 add x0, x20, #0x40 f050: b2481c08 orr x8, x0, #0xff00000000000000 f054: d344fd08 lsr x8, x8, #4 f058: 38776909 ldrb w9, [x8, x23] f05c: 940017cc bl 14f8c <OUTLINED_FUNCTION_11> f060: 54000060 b.eq f06c <shmem_link+0x70> // b.none f064: 7103fd1f cmp w8, #0xff f068: 540008a1 b.ne f17c <shmem_link+0x180> // b.any f06c: b9400008 ldr w8, [x0] f070: 34000148 cbz w8, f098 <shmem_link+0x9c> f074: 940018fd bl 15468 <OUTLINED_FUNCTION_1124> ret = shmem_reserve_inode(inode->i_sb); f078: 38776909 ldrb w9, [x8, x23] f07c: 940017c4 bl 14f8c <OUTLINED_FUNCTION_11> f080: 54000060 b.eq f08c <shmem_link+0x90> // b.none f084: 7103fd1f cmp w8, #0xff f088: 540007e1 b.ne f184 <shmem_link+0x188> // b.any f08c: f9400000 ldr x0, [x0] f090: 97fffcf6 bl e468 <shmem_reserve_inode> if (ret) f094: 35000660 cbnz w0, f160 <shmem_link+0x164> dir->i_size += BOGO_DIRENT_SIZE; f098: 910122a0 add x0, x21, #0x48 f09c: b2481c08 orr x8, x0, #0xff00000000000000 f0a0: d344fd09 lsr x9, x8, #4 f0a4: 3877692a ldrb w10, [x9, x23] f0a8: 94001828 bl 15148 <OUTLINED_FUNCTION_193> f0ac: 54000060 b.eq f0b8 <shmem_link+0xbc> // b.none f0b0: 7103fd1f cmp w8, #0xff f0b4: 540006c1 b.ne f18c <shmem_link+0x190> // b.any f0b8: 38776929 ldrb w9, [x9, x23] f0bc: 94001a4a bl 159e4 <OUTLINED_FUNCTION_1131> f0c0: 54000060 b.eq f0cc <shmem_link+0xd0> // b.none f0c4: 7103fd1f cmp w8, #0xff f0c8: 54000661 b.ne f194 <shmem_link+0x198> // b.any f0cc: f9000009 str x9, [x0] inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode); f0d0: aa1403e0 mov x0, x20 f0d4: 910182b6 add x22, x21, #0x60 f0d8: 94000000 bl 0 <current_time> f0dc: b2481ec9 orr x9, x22, #0xff00000000000000 f0e0: d344fd29 lsr x9, x9, #4
On Wed, 2019-02-27 at 09:09 -0500, Qian Cai wrote: > On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote: > > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote: > > > > > > > > Of course, that's just gcc. I have no idea what llvm ends up doing. > > > > > > Clang 7.0: > > > > > > # clang -O2 -S -Wall /tmp/test.c > > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever > > > 'if' > > > condition is false [-Wsometimes-uninitialized] > > > > Ok, good. > > > > Do we have any clang builds in any of the zero-day robot > > infrastructure or something? Should we? > > > > And maybe this was how Dan noticed the problem in the first place? Or > > is it just because of his eagle-eyes? > > > > BTW, even clang is able to generate warnings in your sample code, it does not > generate any warnings when compiling the buggy shmem.o via "make CC=clang". > Here is the objdump for arm64 (with KASAN_SW_TAGS inline). > Ah, thanks to the commit 6e8d666e9253 ("Disable "maybe-uninitialized" warning globally"), it will no longer generate this type of warnings until using "make W=1" due to the commit a76bcf557ef4 ("Kbuild: enable -Wmaybe-uninitialized warning for 'make W=1'"). Anyway, the generated code is the same using clang with and without this patch. d_instantiate(dentry, inode); 4eec: 94000000 bl 0 <d_instantiate> ret = shmem_reserve_inode(inode->i_sb); 4ef0: 2a1f03e0 mov w0, wzr <---- ret = 0 return ret;
On Wed, Feb 27, 2019 at 09:09:40AM -0500, Qian Cai wrote: > On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote: > > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote: > > > > > > > > Of course, that's just gcc. I have no idea what llvm ends up doing. > > > > > > Clang 7.0: > > > > > > # clang -O2 -S -Wall /tmp/test.c > > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever > > > 'if' > > > condition is false [-Wsometimes-uninitialized] > > > > Ok, good. > > > > Do we have any clang builds in any of the zero-day robot > > infrastructure or something? Should we? > > > > And maybe this was how Dan noticed the problem in the first place? Or > > is it just because of his eagle-eyes? > > > > BTW, even clang is able to generate warnings in your sample code, it does not > generate any warnings when compiling the buggy shmem.o via "make CC=clang". Here Unfortunately, scripts/Kbuild.extrawarn disables -Wuninitialized for Clang, which also disables -Wsometimes-uninitialized: https://github.com/ClangBuiltLinux/linux/issues/381 https://clang.llvm.org/docs/DiagnosticsReference.html#wuninitialized I'm going to be sending out patches to fix the warnings found with it then enable it going forward so that things like this get caught. Nathan > is the objdump for arm64 (with KASAN_SW_TAGS inline). > > 000000000000effc <shmem_link>: > { > effc: f81c0ff7 str x23, [sp, #-64]! > f000: a90157f6 stp x22, x21, [sp, #16] > f004: a9024ff4 stp x20, x19, [sp, #32] > f008: a9037bfd stp x29, x30, [sp, #48] > f00c: 9100c3fd add x29, sp, #0x30 > f010: aa0203f3 mov x19, x2 > f014: aa0103f5 mov x21, x1 > f018: aa0003f4 mov x20, x0 > f01c: 94000000 bl 0 <_mcount> > f020: 91016280 add x0, x20, #0x58 > f024: d2c20017 mov x23, #0x100000000000 // > #17592186044416 > f028: b2481c08 orr x8, x0, #0xff00000000000000 > f02c: f2fdfff7 movk x23, #0xefff, lsl #48 > f030: d344fd08 lsr x8, x8, #4 > f034: 38776909 ldrb w9, [x8, x23] > f038: 940017d5 bl 14f8c <OUTLINED_FUNCTION_11> > f03c: 54000060 b.eq f048 <shmem_link+0x4c> // b.none > f040: 7103fd1f cmp w8, #0xff > f044: 54000981 b.ne f174 <shmem_link+0x178> // b.any > f048: f9400014 ldr x20, [x0] > if (inode->i_nlink) { > f04c: 91010280 add x0, x20, #0x40 > f050: b2481c08 orr x8, x0, #0xff00000000000000 > f054: d344fd08 lsr x8, x8, #4 > f058: 38776909 ldrb w9, [x8, x23] > f05c: 940017cc bl 14f8c <OUTLINED_FUNCTION_11> > f060: 54000060 b.eq f06c <shmem_link+0x70> // b.none > f064: 7103fd1f cmp w8, #0xff > f068: 540008a1 b.ne f17c <shmem_link+0x180> // b.any > f06c: b9400008 ldr w8, [x0] > f070: 34000148 cbz w8, f098 <shmem_link+0x9c> > f074: 940018fd bl 15468 <OUTLINED_FUNCTION_1124> > ret = shmem_reserve_inode(inode->i_sb); > f078: 38776909 ldrb w9, [x8, x23] > f07c: 940017c4 bl 14f8c <OUTLINED_FUNCTION_11> > f080: 54000060 b.eq f08c <shmem_link+0x90> // b.none > f084: 7103fd1f cmp w8, #0xff > f088: 540007e1 b.ne f184 <shmem_link+0x188> // b.any > f08c: f9400000 ldr x0, [x0] > f090: 97fffcf6 bl e468 <shmem_reserve_inode> > if (ret) > f094: 35000660 cbnz w0, f160 <shmem_link+0x164> > dir->i_size += BOGO_DIRENT_SIZE; > f098: 910122a0 add x0, x21, #0x48 > f09c: b2481c08 orr x8, x0, #0xff00000000000000 > f0a0: d344fd09 lsr x9, x8, #4 > f0a4: 3877692a ldrb w10, [x9, x23] > f0a8: 94001828 bl 15148 <OUTLINED_FUNCTION_193> > f0ac: 54000060 b.eq f0b8 <shmem_link+0xbc> // b.none > f0b0: 7103fd1f cmp w8, #0xff > f0b4: 540006c1 b.ne f18c <shmem_link+0x190> // b.any > f0b8: 38776929 ldrb w9, [x9, x23] > f0bc: 94001a4a bl 159e4 <OUTLINED_FUNCTION_1131> > f0c0: 54000060 b.eq f0cc <shmem_link+0xd0> // b.none > f0c4: 7103fd1f cmp w8, #0xff > f0c8: 54000661 b.ne f194 <shmem_link+0x198> // b.any > f0cc: f9000009 str x9, [x0] > inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode); > f0d0: aa1403e0 mov x0, x20 > f0d4: 910182b6 add x22, x21, #0x60 > f0d8: 94000000 bl 0 <current_time> > f0dc: b2481ec9 orr x9, x22, #0xff00000000000000 > f0e0: d344fd29 lsr x9, x9, #4 >
diff --git a/mm/shmem.c b/mm/shmem.c index 0905215fb016..2c012eee133d 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2848,7 +2848,7 @@ static int shmem_create(struct inode *dir, struct dentry *dentry, umode_t mode, static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry) { struct inode *inode = d_inode(old_dentry); - int ret; + int ret = 0; /* * No ordinary (disk based) filesystem counts links as inodes;