Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 5 May 2020 17:57:02 -0700 | Subject | Re: [PATCH v2] security: disable FORTIFY_SOURCE on clang |
| |
On Tue, May 5, 2020 at 5:15 PM Jason A. Donenfeld <Jason@zx2c4.com> 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>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
> --- > security/Kconfig | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/security/Kconfig b/security/Kconfig > index cd3cc7da3a55..76bcfb3eb16f 100644 > --- a/security/Kconfig > +++ b/security/Kconfig > @@ -191,6 +191,7 @@ config HARDENED_USERCOPY_PAGESPAN > config FORTIFY_SOURCE > bool "Harden common str/mem functions against buffer overflows" > depends on ARCH_HAS_FORTIFY_SOURCE > + depends on !CC_IS_CLANG > help > Detect overflows of buffers in common string and memory functions > where the compiler can determine and validate the buffer sizes. > -- > 2.26.2 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200506001453.764332-1-Jason%40zx2c4.com.
-- Thanks, ~Nick Desaulniers
| |