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

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Message-ID: <CANpmjNNwzup9o+XNcjQcXEjFrU5QUQEc4qMP1yA07e74eyk6Dw@mail.gmail.com>
Date:   Mon, 24 Apr 2023 14:39:32 +0200
From:   Marco Elver <elver@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, 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, 24 Apr 2023 at 13:23, Alexander Potapenko <glider@...gle.com> 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/

Reviewed-by: Marco Elver <elver@...gle.com>

Looks reasonable.

> ---
>  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;
>  }
> --
> 2.40.0.634.g4ca3ef3211-goog
>