| Message ID | 20180413221146.28476-1-labbott@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, Apr 13, 2018 at 03:11:46PM -0700, Laura Abbott wrote: >There's an ongoing effort to remove VLAs[1] from the kernel to eventually >turn on -Wvla. The few VLAs in use have an upper bound based on a size >of 64K. This doesn't produce an excessively large stack so just switch >the upper bound. > >[1] https://lkml.org/lkml/2018/3/7/621 This comment is more in regards to many of these patches, and not as much this one specifically. How confident are we in the upper bounds we're setting, and how obvious is it in the resulting code so that something does later change to overflow these bounds. The danger here is that we're converting something a little easier to detect (a stack overflow), with something harder to detect (overflowing an array on the stack). I guess the question is twofold: how did you determine that 64K was the largest 'size' value, and how should reviewers verify this as well. Perhaps this should at least be in the commit text so someone tracking down something with this code can find it later. David
On 04/13/2018 07:55 PM, David Brown wrote: > On Fri, Apr 13, 2018 at 03:11:46PM -0700, Laura Abbott wrote: > >> There's an ongoing effort to remove VLAs[1] from the kernel to eventually >> turn on -Wvla. The few VLAs in use have an upper bound based on a size >> of 64K. This doesn't produce an excessively large stack so just switch >> the upper bound. >> >> [1] https://lkml.org/lkml/2018/3/7/621 > > This comment is more in regards to many of these patches, and not as > much this one specifically. > > How confident are we in the upper bounds we're setting, and how > obvious is it in the resulting code so that something does later > change to overflow these bounds. > > The danger here is that we're converting something a little easier to > detect (a stack overflow), with something harder to detect > (overflowing an array on the stack). > Several people have remarked on that and the solution has been to put in some kind of WARN and/or error check to make it obvious something needs to be adjusted. > I guess the question is twofold: how did you determine that 64K was > the largest 'size' value, and how should reviewers verify this as > well. Perhaps this should at least be in the commit text so someone > tracking down something with this code can find it later. > It's not in the patch context but there's a large comment below: /* * A GDT can be up to 64k in size, which corresponds to 8192 * 8-byte entries, or 16 4k pages.. */ BUG_ON(size > 65536); Given the frames was calculated based off the size, that seemed sufficient. > David Thanks, Laura
On 14/04/18 00:11, Laura Abbott wrote: > There's an ongoing effort to remove VLAs[1] from the kernel to eventually > turn on -Wvla. The few VLAs in use have an upper bound based on a size > of 64K. This doesn't produce an excessively large stack so just switch > the upper bound. > > [1] https://lkml.org/lkml/2018/3/7/621 > > Signed-off-by: Laura Abbott <labbott@redhat.com> Reviewed-by: Juergen Gross <jgross@suse.com> Juergen
* Laura Abbott <labbott@redhat.com> wrote: > There's an ongoing effort to remove VLAs[1] from the kernel to eventually > turn on -Wvla. The few VLAs in use have an upper bound based on a size > of 64K. This doesn't produce an excessively large stack so just switch > the upper bound. > > [1] https://lkml.org/lkml/2018/3/7/621 > > Signed-off-by: Laura Abbott <labbott@redhat.com> > --- > arch/x86/xen/enlighten_pv.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c > index c36d23aa6c35..d96a5a535cbb 100644 > --- a/arch/x86/xen/enlighten_pv.c > +++ b/arch/x86/xen/enlighten_pv.c > @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr) > { > unsigned long va = dtr->address; > unsigned int size = dtr->size + 1; > - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); > - unsigned long frames[pages]; > + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; > int f; > > /* > @@ -470,8 +469,7 @@ static void __init xen_load_gdt_boot(const struct desc_ptr *dtr) > { > unsigned long va = dtr->address; > unsigned int size = dtr->size + 1; > - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); > - unsigned long frames[pages]; > + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; > int f; Reviewed-by: Ingo Molnar <mingo@kernel.org> Thanks, Ingo
On 04/13/2018 06:11 PM, Laura Abbott wrote: > There's an ongoing effort to remove VLAs[1] from the kernel to eventually > turn on -Wvla. The few VLAs in use have an upper bound based on a size > of 64K. This doesn't produce an excessively large stack so just switch > the upper bound. > > [1] https://lkml.org/lkml/2018/3/7/621 > > Signed-off-by: Laura Abbott <labbott@redhat.com> > --- > arch/x86/xen/enlighten_pv.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c > index c36d23aa6c35..d96a5a535cbb 100644 > --- a/arch/x86/xen/enlighten_pv.c > +++ b/arch/x86/xen/enlighten_pv.c > @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr) > { > unsigned long va = dtr->address; > unsigned int size = dtr->size + 1; > - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); Isn't dtr->size always either GDT_SIZE or 0? -boris > - unsigned long frames[pages]; > + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; > int f; > > /* > @@ -470,8 +469,7 @@ static void __init xen_load_gdt_boot(const struct desc_ptr *dtr) > { > unsigned long va = dtr->address; > unsigned int size = dtr->size + 1; > - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); > - unsigned long frames[pages]; > + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; > int f; > > /*
On 16/04/18 15:27, Boris Ostrovsky wrote: > On 04/13/2018 06:11 PM, Laura Abbott wrote: >> There's an ongoing effort to remove VLAs[1] from the kernel to eventually >> turn on -Wvla. The few VLAs in use have an upper bound based on a size >> of 64K. This doesn't produce an excessively large stack so just switch >> the upper bound. >> >> [1] https://lkml.org/lkml/2018/3/7/621 >> >> Signed-off-by: Laura Abbott <labbott@redhat.com> >> --- >> arch/x86/xen/enlighten_pv.c | 6 ++---- >> 1 file changed, 2 insertions(+), 4 deletions(-) >> >> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c >> index c36d23aa6c35..d96a5a535cbb 100644 >> --- a/arch/x86/xen/enlighten_pv.c >> +++ b/arch/x86/xen/enlighten_pv.c >> @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr) >> { >> unsigned long va = dtr->address; >> unsigned int size = dtr->size + 1; >> - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); > > > > Isn't dtr->size always either GDT_SIZE or 0? GDT_SIZE - 1 :-) >> - unsigned long frames[pages]; >> + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; So we could just go with one frame and modify the BUG_ON() further below accordingly. Juergen
On 04/17/2018 12:16 AM, Juergen Gross wrote: > On 16/04/18 15:27, Boris Ostrovsky wrote: >> On 04/13/2018 06:11 PM, Laura Abbott wrote: >>> There's an ongoing effort to remove VLAs[1] from the kernel to eventually >>> turn on -Wvla. The few VLAs in use have an upper bound based on a size >>> of 64K. This doesn't produce an excessively large stack so just switch >>> the upper bound. >>> >>> [1] https://lkml.org/lkml/2018/3/7/621 >>> >>> Signed-off-by: Laura Abbott <labbott@redhat.com> >>> --- >>> arch/x86/xen/enlighten_pv.c | 6 ++---- >>> 1 file changed, 2 insertions(+), 4 deletions(-) >>> >>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c >>> index c36d23aa6c35..d96a5a535cbb 100644 >>> --- a/arch/x86/xen/enlighten_pv.c >>> +++ b/arch/x86/xen/enlighten_pv.c >>> @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr) >>> { >>> unsigned long va = dtr->address; >>> unsigned int size = dtr->size + 1; >>> - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); >> >> >> >> Isn't dtr->size always either GDT_SIZE or 0? > > GDT_SIZE - 1 :-) > >>> - unsigned long frames[pages]; >>> + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; > > So we could just go with one frame and modify the BUG_ON() further below > accordingly. > Do you want to just remove the loop as well since we're never going to do more than one frame? We end up with net code deletion. Thanks, Laura > > Juergen >
On 04/17/2018 07:33 PM, Laura Abbott wrote: > On 04/17/2018 12:16 AM, Juergen Gross wrote: >> On 16/04/18 15:27, Boris Ostrovsky wrote: >>> On 04/13/2018 06:11 PM, Laura Abbott wrote: >>>> There's an ongoing effort to remove VLAs[1] from the kernel to >>>> eventually >>>> turn on -Wvla. The few VLAs in use have an upper bound based on a size >>>> of 64K. This doesn't produce an excessively large stack so just switch >>>> the upper bound. >>>> >>>> [1] https://lkml.org/lkml/2018/3/7/621 >>>> >>>> Signed-off-by: Laura Abbott <labbott@redhat.com> >>>> --- >>>> arch/x86/xen/enlighten_pv.c | 6 ++---- >>>> 1 file changed, 2 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c >>>> index c36d23aa6c35..d96a5a535cbb 100644 >>>> --- a/arch/x86/xen/enlighten_pv.c >>>> +++ b/arch/x86/xen/enlighten_pv.c >>>> @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr >>>> *dtr) >>>> { >>>> unsigned long va = dtr->address; >>>> unsigned int size = dtr->size + 1; >>>> - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); >>> >>> >>> >>> Isn't dtr->size always either GDT_SIZE or 0? >> >> GDT_SIZE - 1 :-) >> >>>> - unsigned long frames[pages]; >>>> + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; >> >> So we could just go with one frame and modify the BUG_ON() further below >> accordingly. >> > > Do you want to just remove the loop as well since we're never going > to do more than one frame? We end up with net code deletion. > Yes, the loop, as well as the comment about max size being 64K can all be removed. -boris
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c index c36d23aa6c35..d96a5a535cbb 100644 --- a/arch/x86/xen/enlighten_pv.c +++ b/arch/x86/xen/enlighten_pv.c @@ -421,8 +421,7 @@ static void xen_load_gdt(const struct desc_ptr *dtr) { unsigned long va = dtr->address; unsigned int size = dtr->size + 1; - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); - unsigned long frames[pages]; + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; int f; /* @@ -470,8 +469,7 @@ static void __init xen_load_gdt_boot(const struct desc_ptr *dtr) { unsigned long va = dtr->address; unsigned int size = dtr->size + 1; - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE); - unsigned long frames[pages]; + unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)]; int f; /*
There's an ongoing effort to remove VLAs[1] from the kernel to eventually turn on -Wvla. The few VLAs in use have an upper bound based on a size of 64K. This doesn't produce an excessively large stack so just switch the upper bound. [1] https://lkml.org/lkml/2018/3/7/621 Signed-off-by: Laura Abbott <labbott@redhat.com> --- arch/x86/xen/enlighten_pv.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)