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Date:   Tue, 5 May 2020 19:54:09 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        arnd@...db.de, George Burgess <gbiv@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] security: disable FORTIFY_SOURCE on clang

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@...nkpad /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@...nkpad /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@...db.de>
> Cc: LKML <linux-kernel@...r.kernel.org>
> Cc: clang-built-linux <clang-built-linux@...glegroups.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: George Burgess <gbiv@...gle.com>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Link: https://bugs.llvm.org/show_bug.cgi?id=45802
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Grudgingly,

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook

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