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[PATCH] string: use __builtin_memcpy() in strlcpy/strlcat

Alexander Potapenko posted 1 patch 2 years, 9 months ago
There is a newer version of this series
lib/string.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
[PATCH] string: use __builtin_memcpy() in strlcpy/strlcat
Posted by Alexander Potapenko 2 years, 9 months ago 
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@google.com>
Signed-off-by: Alexander Potapenko <glider@google.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;
 }
-- 
2.40.0.634.g4ca3ef3211-goog
Re: [PATCH] string: use __builtin_memcpy() in strlcpy/strlcat
Posted by Kees Cook 2 years, 9 months ago 
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@google.com>
> Signed-off-by: Alexander Potapenko <glider@google.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
Re: [PATCH] string: use __builtin_memcpy() in strlcpy/strlcat
Posted by Alexander Potapenko 2 years, 9 months ago 
>
> 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?

Is fortify relevant here? Note that the whole file is compiled with
__NO_FORTIFY.
Re: [PATCH] string: use __builtin_memcpy() in strlcpy/strlcat
Posted by Kees Cook 2 years, 9 months ago 
On Fri, Apr 28, 2023 at 03:48:28PM +0200, Alexander Potapenko wrote:
> >
> > 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?
> 
> Is fortify relevant here? Note that the whole file is compiled with
> __NO_FORTIFY.

Yeah, agreed. I think I was just curious if that got verified. I'm good
with this.

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook
Re: [PATCH] string: use __builtin_memcpy() in strlcpy/strlcat
Posted by Marco Elver 2 years, 9 months ago 
On Mon, 24 Apr 2023 at 13:23, Alexander Potapenko <glider@google.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@google.com>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Link: https://lore.kernel.org/all/20230224085942.1791837-1-elver@google.com/

Reviewed-by: Marco Elver <elver@google.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
>

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