| Message ID | 20200214054706.33870-1-natechancellor@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | drm/i915: Cast remain to unsigned long in eb_relocate_vma | expand |
Quoting Jani Nikula (2020-02-14 06:36:15) > On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: > > A recent commit in clang added -Wtautological-compare to -Wall, which is > > enabled for i915 after -Wtautological-compare is disabled for the rest > > of the kernel so we see the following warning on x86_64: > > > > ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: > > result of comparison of constant 576460752303423487 with expression of > > type 'unsigned int' is always false > > [-Wtautological-constant-out-of-range-compare] > > if (unlikely(remain > N_RELOC(ULONG_MAX))) > > ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ > > ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' > > # define unlikely(x) __builtin_expect(!!(x), 0) > > ^ > > 1 warning generated. > > > > It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not > > account for the case where this file is built for 32-bit x86, where > > ULONG_MAX == UINT_MAX and this check is still relevant. > > > > Cast remain to unsigned long, which keeps the generated code the same > > (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and > > the warning is silenced so we can catch more potential issues in the > > future. > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/778 > > Suggested-by: Michel Dänzer <michel@daenzer.net> > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> > > Works for me as a workaround, But the whole point was that the compiler could see that it was impossible and not emit the code. Doesn't this break that? -Chris
On Fri, 14 Feb 2020, Chris Wilson <chris@chris-wilson.co.uk> wrote: > Quoting Jani Nikula (2020-02-14 06:36:15) >> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: >> > A recent commit in clang added -Wtautological-compare to -Wall, which is >> > enabled for i915 after -Wtautological-compare is disabled for the rest >> > of the kernel so we see the following warning on x86_64: >> > >> > ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: >> > result of comparison of constant 576460752303423487 with expression of >> > type 'unsigned int' is always false >> > [-Wtautological-constant-out-of-range-compare] >> > if (unlikely(remain > N_RELOC(ULONG_MAX))) >> > ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ >> > ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' >> > # define unlikely(x) __builtin_expect(!!(x), 0) >> > ^ >> > 1 warning generated. >> > >> > It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not >> > account for the case where this file is built for 32-bit x86, where >> > ULONG_MAX == UINT_MAX and this check is still relevant. >> > >> > Cast remain to unsigned long, which keeps the generated code the same >> > (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and >> > the warning is silenced so we can catch more potential issues in the >> > future. >> > >> > Link: https://github.com/ClangBuiltLinux/linux/issues/778 >> > Suggested-by: Michel Dänzer <michel@daenzer.net> >> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> >> >> Works for me as a workaround, > > But the whole point was that the compiler could see that it was > impossible and not emit the code. Doesn't this break that? It seems that goal and the warning are fundamentally incompatible. Back to the original patch? BR, Jani.
On Fri, Feb 14, 2020 at 08:32:19AM +0000, Chris Wilson wrote: > Quoting Jani Nikula (2020-02-14 06:36:15) > > On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: > > > A recent commit in clang added -Wtautological-compare to -Wall, which is > > > enabled for i915 after -Wtautological-compare is disabled for the rest > > > of the kernel so we see the following warning on x86_64: > > > > > > ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: > > > result of comparison of constant 576460752303423487 with expression of > > > type 'unsigned int' is always false > > > [-Wtautological-constant-out-of-range-compare] > > > if (unlikely(remain > N_RELOC(ULONG_MAX))) > > > ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ > > > ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' > > > # define unlikely(x) __builtin_expect(!!(x), 0) > > > ^ > > > 1 warning generated. > > > > > > It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not > > > account for the case where this file is built for 32-bit x86, where > > > ULONG_MAX == UINT_MAX and this check is still relevant. > > > > > > Cast remain to unsigned long, which keeps the generated code the same > > > (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and > > > the warning is silenced so we can catch more potential issues in the > > > future. > > > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/778 > > > Suggested-by: Michel Dänzer <michel@daenzer.net> > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> > > > > Works for me as a workaround, > > But the whole point was that the compiler could see that it was > impossible and not emit the code. Doesn't this break that? > -Chris As noted in the commit message, I ran diff <(objdump -Dr) <(objdump -Dr) on objects files compiled with and without the patch with clang and gcc for x86_64 and gcc for i386 (i386 does not build with clang) and there was zero difference aside from the file names. At the end of the day, I do not really care how the warning get fixed, just that it does since it is the only one on x86_64 defconfig. Cheers, Nathan
On 2020-02-14 12:49 p.m., Jani Nikula wrote: > On Fri, 14 Feb 2020, Chris Wilson <chris@chris-wilson.co.uk> wrote: >> Quoting Jani Nikula (2020-02-14 06:36:15) >>> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: >>>> A recent commit in clang added -Wtautological-compare to -Wall, which is >>>> enabled for i915 after -Wtautological-compare is disabled for the rest >>>> of the kernel so we see the following warning on x86_64: >>>> >>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: >>>> result of comparison of constant 576460752303423487 with expression of >>>> type 'unsigned int' is always false >>>> [-Wtautological-constant-out-of-range-compare] >>>> if (unlikely(remain > N_RELOC(ULONG_MAX))) >>>> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ >>>> ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' >>>> # define unlikely(x) __builtin_expect(!!(x), 0) >>>> ^ >>>> 1 warning generated. >>>> >>>> It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not >>>> account for the case where this file is built for 32-bit x86, where >>>> ULONG_MAX == UINT_MAX and this check is still relevant. >>>> >>>> Cast remain to unsigned long, which keeps the generated code the same >>>> (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and >>>> the warning is silenced so we can catch more potential issues in the >>>> future. >>>> >>>> Link: https://github.com/ClangBuiltLinux/linux/issues/778 >>>> Suggested-by: Michel Dänzer <michel@daenzer.net> >>>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> >>> >>> Works for me as a workaround, >> >> But the whole point was that the compiler could see that it was >> impossible and not emit the code. Doesn't this break that? > > It seems that goal and the warning are fundamentally incompatible. Not really: if (sizeof(remain) >= sizeof(unsigned long) && unlikely(remain > N_RELOC(ULONG_MAX))) return -EINVAL; In contrast to the cast, this doesn't generate any machine code on 64-bit: https://godbolt.org/z/GmUE4S but still generates the same code on 32-bit: https://godbolt.org/z/hAoz8L
On Fri, Feb 14, 2020 at 7:36 AM Michel Dänzer <michel@daenzer.net> wrote: > > On 2020-02-14 12:49 p.m., Jani Nikula wrote: > > On Fri, 14 Feb 2020, Chris Wilson <chris@chris-wilson.co.uk> wrote: > >> Quoting Jani Nikula (2020-02-14 06:36:15) > >>> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: > >>>> A recent commit in clang added -Wtautological-compare to -Wall, which is > >>>> enabled for i915 after -Wtautological-compare is disabled for the rest > >>>> of the kernel so we see the following warning on x86_64: > >>>> > >>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: > >>>> result of comparison of constant 576460752303423487 with expression of > >>>> type 'unsigned int' is always false > >>>> [-Wtautological-constant-out-of-range-compare] > >>>> if (unlikely(remain > N_RELOC(ULONG_MAX))) > >>>> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ > >>>> ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' > >>>> # define unlikely(x) __builtin_expect(!!(x), 0) > >>>> ^ > >>>> 1 warning generated. > >>>> > >>>> It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not > >>>> account for the case where this file is built for 32-bit x86, where > >>>> ULONG_MAX == UINT_MAX and this check is still relevant. > >>>> > >>>> Cast remain to unsigned long, which keeps the generated code the same > >>>> (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and > >>>> the warning is silenced so we can catch more potential issues in the > >>>> future. > >>>> > >>>> Link: https://github.com/ClangBuiltLinux/linux/issues/778 > >>>> Suggested-by: Michel Dänzer <michel@daenzer.net> > >>>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> > >>> > >>> Works for me as a workaround, > >> > >> But the whole point was that the compiler could see that it was > >> impossible and not emit the code. Doesn't this break that? > > > > It seems that goal and the warning are fundamentally incompatible. > > Not really: > > if (sizeof(remain) >= sizeof(unsigned long) && > unlikely(remain > N_RELOC(ULONG_MAX))) > return -EINVAL; > > In contrast to the cast, this doesn't generate any machine code on 64-bit: > > https://godbolt.org/z/GmUE4S > > but still generates the same code on 32-bit: > > https://godbolt.org/z/hAoz8L Exactly. This check is only a tautology when `sizeof(long) == sizeof(int)` (ie. ILP32 platforms, like 32b x86), notice how BOTH GCC AND Clang generate exactly the same code: https://godbolt.org/z/6ShrDM Both compilers eliminate the check when `-m32` is not set, and generate the exact same check otherwise. How about: ``` diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c index d3f4f28e9468..25b9d3f3ad57 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c @@ -1415,8 +1415,10 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct eb_vma *ev) urelocs = u64_to_user_ptr(entry->relocs_ptr); remain = entry->relocation_count; +#ifndef CONFIG_64BIT if (unlikely(remain > N_RELOC(ULONG_MAX))) return -EINVAL; +#endif /* * We must check that the entire relocation array is safe ``` We now have 4 proposed solutions: 1. https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/ 2. https://lore.kernel.org/lkml/20200211050808.29463-1-natechancellor@gmail.com/ 3. https://lore.kernel.org/lkml/20200214054706.33870-1-natechancellor@gmail.com/ 4. my diff above Let's please come to a resolution on this.
On Mon, Mar 16, 2020 at 02:41:23PM -0700, Nick Desaulniers wrote: > On Fri, Feb 14, 2020 at 7:36 AM Michel Dänzer <michel@daenzer.net> wrote: > > > > On 2020-02-14 12:49 p.m., Jani Nikula wrote: > > > On Fri, 14 Feb 2020, Chris Wilson <chris@chris-wilson.co.uk> wrote: > > >> Quoting Jani Nikula (2020-02-14 06:36:15) > > >>> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: > > >>>> A recent commit in clang added -Wtautological-compare to -Wall, which is > > >>>> enabled for i915 after -Wtautological-compare is disabled for the rest > > >>>> of the kernel so we see the following warning on x86_64: > > >>>> > > >>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: > > >>>> result of comparison of constant 576460752303423487 with expression of > > >>>> type 'unsigned int' is always false > > >>>> [-Wtautological-constant-out-of-range-compare] > > >>>> if (unlikely(remain > N_RELOC(ULONG_MAX))) > > >>>> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ > > >>>> ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' > > >>>> # define unlikely(x) __builtin_expect(!!(x), 0) > > >>>> ^ > > >>>> 1 warning generated. > > >>>> > > >>>> It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not > > >>>> account for the case where this file is built for 32-bit x86, where > > >>>> ULONG_MAX == UINT_MAX and this check is still relevant. > > >>>> > > >>>> Cast remain to unsigned long, which keeps the generated code the same > > >>>> (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and > > >>>> the warning is silenced so we can catch more potential issues in the > > >>>> future. > > >>>> > > >>>> Link: https://github.com/ClangBuiltLinux/linux/issues/778 > > >>>> Suggested-by: Michel Dänzer <michel@daenzer.net> > > >>>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> > > >>> > > >>> Works for me as a workaround, > > >> > > >> But the whole point was that the compiler could see that it was > > >> impossible and not emit the code. Doesn't this break that? > > > > > > It seems that goal and the warning are fundamentally incompatible. > > > > Not really: > > > > if (sizeof(remain) >= sizeof(unsigned long) && > > unlikely(remain > N_RELOC(ULONG_MAX))) > > return -EINVAL; > > > > In contrast to the cast, this doesn't generate any machine code on 64-bit: > > > > https://godbolt.org/z/GmUE4S > > > > but still generates the same code on 32-bit: > > > > https://godbolt.org/z/hAoz8L > > Exactly. > > This check is only a tautology when `sizeof(long) == sizeof(int)` (ie. > ILP32 platforms, like 32b x86), notice how BOTH GCC AND Clang generate > exactly the same code: https://godbolt.org/z/6ShrDM > > Both compilers eliminate the check when `-m32` is not set, and > generate the exact same check otherwise. How about: > ``` > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c > b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c > index d3f4f28e9468..25b9d3f3ad57 100644 > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c > @@ -1415,8 +1415,10 @@ static int eb_relocate_vma(struct > i915_execbuffer *eb, struct eb_vma *ev) > > urelocs = u64_to_user_ptr(entry->relocs_ptr); > remain = entry->relocation_count; > +#ifndef CONFIG_64BIT > if (unlikely(remain > N_RELOC(ULONG_MAX))) > return -EINVAL; > +#endif > > /* > * We must check that the entire relocation array is safe > ``` > > We now have 4 proposed solutions: > 1. https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/ > 2. https://lore.kernel.org/lkml/20200211050808.29463-1-natechancellor@gmail.com/ > 3. https://lore.kernel.org/lkml/20200214054706.33870-1-natechancellor@gmail.com/ > 4. my diff above > Let's please come to a resolution on this. This is the only warning on an x86_64 defconfig build. Apologies if we are being too persistent or nagging but we need guidance from the i915 maintainers on which solution they would prefer so it can be picked up. I understand you all are busy and I appreciate the work you all do but I do not want this to fall between the cracks because it is annoying to constantly see this warning. Cheers, Nathan
On Thu, 26 Mar 2020, Nathan Chancellor <natechancellor@gmail.com> wrote: > This is the only warning on an x86_64 defconfig build. Apologies if we > are being too persistent or nagging but we need guidance from the i915 > maintainers on which solution they would prefer so it can be picked up. > I understand you all are busy and I appreciate the work you all do but > I do not want this to fall between the cracks because it is annoying to > constantly see this warning. Apologies for the delay. As I replied first thing in this thread, this works for me. Thanks for the patch, pushed to drm-intel-next-queued. BR, Jani.
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c index 60c984e10c4a..47f4d8ab281e 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c @@ -1430,7 +1430,7 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct i915_vma *vma) urelocs = u64_to_user_ptr(entry->relocs_ptr); remain = entry->relocation_count; - if (unlikely(remain > N_RELOC(ULONG_MAX))) + if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX))) return -EINVAL; /*
A recent commit in clang added -Wtautological-compare to -Wall, which is enabled for i915 after -Wtautological-compare is disabled for the rest of the kernel so we see the following warning on x86_64: ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning: result of comparison of constant 576460752303423487 with expression of type 'unsigned int' is always false [-Wtautological-constant-out-of-range-compare] if (unlikely(remain > N_RELOC(ULONG_MAX))) ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely' # define unlikely(x) __builtin_expect(!!(x), 0) ^ 1 warning generated. It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not account for the case where this file is built for 32-bit x86, where ULONG_MAX == UINT_MAX and this check is still relevant. Cast remain to unsigned long, which keeps the generated code the same (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and the warning is silenced so we can catch more potential issues in the future. Link: https://github.com/ClangBuiltLinux/linux/issues/778 Suggested-by: Michel Dänzer <michel@daenzer.net> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> --- Round 3 :) Previous threads/patches: https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/ https://lore.kernel.org/lkml/20200211050808.29463-1-natechancellor@gmail.com/ drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)