On Tue, May 5, 2020 at 8:54 PM Kees Cook <[email protected]> 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 <[email protected]> > > Cc: LKML <[email protected]> > > Cc: clang-built-linux <[email protected]> > > Cc: Kees Cook <[email protected]> > > Cc: George Burgess <[email protected]> > > Cc: Nick Desaulniers <[email protected]> > > Link: https://bugs.llvm.org/show_bug.cgi?id=45802 > > Signed-off-by: Jason A. Donenfeld <[email protected]> > > Grudgingly, > > Reviewed-by: Kees Cook <[email protected]>
Do you want to take this into your tree to send to Linus? Seems like security kconfig switches is in line with your usual submissions.
