linux-kernel - Re: [PATCH] string: use __builtin

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Date:   Mon, 24 Apr 2023 09:24:52 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, elver@...gle.com, dvyukov@...gle.com,
        kasan-dev@...glegroups.com, andy@...nel.org,
        ndesaulniers@...gle.com, nathan@...nel.org
Subject: Re: [PATCH] string: use __builtin_memcpy() in strlcpy/strlcat

On Mon, Apr 24, 2023 at 01:23:13PM +0200, Alexander Potapenko wrote:
> lib/string.c is built with -ffreestanding, which prevents the compiler
> from replacing certain functions with calls to their library versions.
> 
> On the other hand, this also prevents Clang and GCC from instrumenting
> calls to memcpy() when building with KASAN, KCSAN or KMSAN:
>  - KASAN normally replaces memcpy() with __asan_memcpy() with the
>    additional cc-param,asan-kernel-mem-intrinsic-prefix=1;
>  - KCSAN and KMSAN replace memcpy() with __tsan_memcpy() and
>    __msan_memcpy() by default.
> 
> To let the tools catch memory accesses from strlcpy/strlcat, replace
> the calls to memcpy() with __builtin_memcpy(), which KASAN, KCSAN and
> KMSAN are able to replace even in -ffreestanding mode.
> 
> This preserves the behavior in normal builds (__builtin_memcpy() ends up
> being replaced with memcpy()), and does not introduce new instrumentation
> in unwanted places, as strlcpy/strlcat are already instrumented.
> 
> Suggested-by: Marco Elver <elver@...gle.com>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> Link: https://lore.kernel.org/all/20230224085942.1791837-1-elver@google.com/
> ---
>  lib/string.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/string.c b/lib/string.c
> index 3d55ef8901068..be26623953d2e 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -110,7 +110,7 @@ size_t strlcpy(char *dest, const char *src, size_t size)
>  
>  	if (size) {
>  		size_t len = (ret >= size) ? size - 1 : ret;
> -		memcpy(dest, src, len);
> +		__builtin_memcpy(dest, src, len);
>  		dest[len] = '\0';
>  	}
>  	return ret;
> @@ -260,7 +260,7 @@ size_t strlcat(char *dest, const char *src, size_t count)
>  	count -= dsize;
>  	if (len >= count)
>  		len = count-1;
> -	memcpy(dest, src, len);
> +	__builtin_memcpy(dest, src, len);
>  	dest[len] = 0;
>  	return res;

I *think* this isn't a problem for CONFIG_FORTIFY, since these will be
replaced and checked separately -- but it still seems strange that you
need to explicitly use __builtin_memcpy.

Does this end up changing fortify coverage?

-- 
Kees Cook