On Tue, May 05, 2020 at 07:54:09PM -0700, Kees Cook wrote:
> On Tue, May 05, 2020 at 06:14:53PM -0600, Jason A. Donenfeld wrote:
> > clang-10 has a broken optimization stage that doesn't allow 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.
> >
> > But actually, for versions of clang earlier than 10, fortify source
> > mostly does nothing. So, between being broken and doing nothing, it
> > probably doesn't make sense to pretend to offer this option. So, this
> > commit just disables it entirely when compiling with clang.
> >
> > Cc: Arnd Bergmann <a...@arndb.de>
> > Cc: LKML <linux-kernel@vger.kernel.org>
> > Cc: clang-built-linux <clang-built-li...@googlegroups.com>
> > Cc: Kees Cook <keesc...@chromium.org>
> > Cc: George Burgess <g...@google.com>
> > Cc: Nick Desaulniers <ndesaulni...@google.com>
> > Link: https://bugs.llvm.org/show_bug.cgi?id=45802
> > Signed-off-by: Jason A. Donenfeld <ja...@zx2c4.com>
>
> Grudgingly,
>
> Reviewed-by: Kees Cook <keesc...@chromium.org>
>
> --
> Kees Cook
>
I feel like you should finish your investigation into how broken this
actually is before we give it the hammer like this but if it is going
in regardless...
Reviewed-by: Nathan Chancellor <natechancel...@gmail.com>
