Re: [PATCH v2] security: disable FORTIFY_SOURCE on clang

[Nathan Chancellor <natechancellor@gmail.com>]

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 <arnd@arndb.de>
> > Cc: LKML <linux-kernel@vger.kernel.org>
> > Cc: clang-built-linux <clang-built-linux@googlegroups.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: George Burgess <gbiv@google.com>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Link: https://bugs.llvm.org/show_bug.cgi?id=45802
> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> 
> Grudgingly,
> 
> Reviewed-by: Kees Cook <keescook@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 <natechancellor@gmail.com>