linux-hardening - Re: [PATCH] fortify: Avoid shadowing previous locals

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Date:   Mon, 25 Oct 2021 12:34:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Qian Cai <quic_qiancai@...cinc.com>
Cc:     linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fortify: Avoid shadowing previous locals

On Mon, Oct 25, 2021 at 02:37:28PM -0400, Qian Cai wrote:
> __compiletime_strlen macro expansion will shadow p_size and p_len local
> variables. Just rename those in __compiletime_strlen.

They don't escape their local context, though, right? i.e. I don't see a
problem with the existing macro. Did you encounter a specific issue that
this patch fixes?

-Kees

> 
> Signed-off-by: Qian Cai <quic_qiancai@...cinc.com>
> ---
>  include/linux/fortify-string.h | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index fdb0a74c9ca2..155c622e4f24 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -10,18 +10,18 @@ void __read_overflow(void) __compiletime_error("detected read beyond size of obj
>  void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
>  void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
>  
> -#define __compiletime_strlen(p)				\
> -({							\
> -	unsigned char *__p = (unsigned char *)(p);      \
> -	size_t ret = (size_t)-1;			\
> -	size_t p_size = __builtin_object_size(p, 1);	\
> -	if (p_size != (size_t)-1) {			\
> -		size_t p_len = p_size - 1;		\
> -		if (__builtin_constant_p(__p[p_len]) &&	\
> -		    __p[p_len] == '\0')			\
> -			ret = __builtin_strlen(__p);	\
> -	}						\
> -	ret;						\
> +#define __compiletime_strlen(ptr)				\
> +({								\
> +	unsigned char *__ptr = (unsigned char *)(ptr);		\
> +	size_t ret = (size_t)-1;				\
> +	size_t ptr_size = __builtin_object_size(ptr, 1);	\
> +	if (ptr_size != (size_t)-1) {				\
> +		size_t ptr_len = ptr_size - 1;			\
> +		if (__builtin_constant_p(__ptr[ptr_len]) &&	\
> +		    __ptr[ptr_len] == '\0')			\
> +			ret = __builtin_strlen(__ptr);		\
> +	}							\
> +	ret;							\
>  })
>  
>  #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
> -- 
> 2.30.2
> 

-- 
Kees Cook