On Tue, May 05, 2020 at 03:02:16PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> On Tue, May 5, 2020 at 2:55 PM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> >
> > clang-10 has a broken optimization stage that doesn't enable the
> > compiler to prove at compile time that certain memcpys are within
> > bounds, and thus the outline memcpy is always called, resulting in
> > horrific performance, and in some cases, excessive stack frame growth.
> > Here's a simple reproducer:
> >
> > typedef unsigned long size_t;
> > void *c(void *dest, const void *src, size_t n) __asm__("memcpy");
> > extern inline __attribute__((gnu_inline)) void *memcpy(void *dest, const void *src, size_t n) { return c(dest, src, n); }
> > void blah(char *a)
> > {
> > unsigned long long b[10], c[10];
> > int i;
> >
> > memcpy(b, a, sizeof(b));
> > for (i = 0; i < 10; ++i)
> > c[i] = b[i] ^ b[9 - i];
> > for (i = 0; i < 10; ++i)
> > b[i] = c[i] ^ a[i];
> > memcpy(a, b, sizeof(b));
> > }
> >
> > Compile this with clang-9 and clang-10 and observe:
> >
> > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-10 -Wframe-larger-than=0 -O3 -c b.c -o c10.o
> > b.c:5:6: warning: stack frame size of 104 bytes in function 'blah' [-Wframe-larger-than=]
> > void blah(char *a)
> > ^
> > 1 warning generated.
> > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-9 -Wframe-larger-than=0 -O3 -c b.c -o c9.o
> >
> > Looking at the disassembly of c10.o and c9.o, one can see that c9.o is
> > properly optimized in the obvious way one would expect, while c10.o has
> > blown up and includes extern calls to memcpy.
> >
> > This is present on the most trivial bits of code. Thus, for clang-10, we
> > just set __NO_FORTIFY globally, so that this issue won't be incurred.
> >
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> > Cc: clang-built-linux <clang-built-linux@xxxxxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: George Burgess <gbiv@xxxxxxxxxx>
> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Link: https://bugs.llvm.org/show_bug.cgi?id=45802
> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
>
> I'm going to request this not be merged until careful comment from
> George and Kees. My hands are quite full at the moment with other
> regressions. I suspect these threads may be relevant, though I
> haven't had time to read through them myself.
> https://github.com/ClangBuiltLinux/linux/issues/979
> https://github.com/ClangBuiltLinux/linux/pull/980
I believe these issues are one in the same. I did a reverse bisect with
Arnd's test case and converged on George's first patch:
https://github.com/llvm/llvm-project/commit/2dd17ff08165e6118e70f00e22b2c36d2d4e0a9a
I think that in lieu of this patch, we should have that patch and its
follow-up fix merged into 10.0.1.
I can file an upstream PR for Tom to take a look out later tonight.
Cheers,
Nathan
> > ---
> > Makefile | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index 49b2709ff44e..f022f077591d 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -768,6 +768,13 @@ KBUILD_CFLAGS += -Wno-gnu
> > # source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > # See modpost pattern 2
> > KBUILD_CFLAGS += -mno-global-merge
> > +
> > +# clang-10 has a broken optimization stage that causes memcpy to always be
> > +# outline, resulting in excessive stack frame growth and poor performance.
> > +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 100000 && test $(CONFIG_CLANG_VERSION) -lt 110000; echo $$?),0)
> > +KBUILD_CFLAGS += -D__NO_FORTIFY
> > +endif
> > +
> > else
> >
> > # These warnings generated too much noise in a regular build.
> > --
> > 2.26.2
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
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