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Date:   Tue, 23 Jul 2019 14:36:25 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Joe Perches <joe@perches.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        Stephen Kitt <steve@sk2.org>,
        Nitin Gote <nitin.r.gote@intel.com>, jannh@google.com,
        kernel-hardening@lists.openwall.com,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
Message-ID: <201907231435.FABB1CC@keescook>
References: <cover.1563841972.git.joe@perches.com>
 <7ab8957eaf9b0931a59eff6e2bd8c5169f2f6c41.1563841972.git.joe@perches.com>
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On Mon, Jul 22, 2019 at 05:38:15PM -0700, Joe Perches wrote:
> Several uses of strlcpy and strscpy have had defects because the
> last argument of each function is misused or typoed.
> 
> Add macro mechanisms to avoid this defect.
> 
> stracpy (copy a string to a string array) must have a string
> array as the first argument (to) and uses sizeof(to) as the
> size.
> 
> These mechanisms verify that the to argument is an array of
> char or other compatible types like u8 or unsigned char.
> 
> A BUILD_BUG is emitted when the type of to is not compatible.
> 
> Signed-off-by: Joe Perches <joe@perches.com>

I think Rasmus's suggestion would make sense:

	BUILD_BUG_ON(!__same_type(typeof(to), char[]))

Either way, I think it should be fine:

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

-Kees

> ---
>  include/linux/string.h | 41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 4deb11f7976b..f80b0973f0e5 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -35,6 +35,47 @@ ssize_t strscpy(char *, const char *, size_t);
>  /* Wraps calls to strscpy()/memset(), no arch specific code required */
>  ssize_t strscpy_pad(char *dest, const char *src, size_t count);
>  
> +/**
> + * stracpy - Copy a C-string into an array of char
> + * @to: Where to copy the string, must be an array of char and not a pointer
> + * @from: String to copy, may be a pointer or const char array
> + *
> + * Helper for strscpy.
> + * Copies a maximum of sizeof(@to) bytes of @from with %NUL termination.
> + *
> + * Returns:
> + * * The number of characters copied (not including the trailing %NUL)
> + * * -E2BIG if @to is a zero size array.
> + */
> +#define stracpy(to, from)					\
> +({								\
> +	size_t size = ARRAY_SIZE(to);				\
> +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> +								\
> +	strscpy(to, from, size);				\
> +})
> +
> +/**
> + * stracpy_pad - Copy a C-string into an array of char with %NUL padding
> + * @to: Where to copy the string, must be an array of char and not a pointer
> + * @from: String to copy, may be a pointer or const char array
> + *
> + * Helper for strscpy_pad.
> + * Copies a maximum of sizeof(@to) bytes of @from with %NUL termination
> + * and zero-pads the remaining size of @to
> + *
> + * Returns:
> + * * The number of characters copied (not including the trailing %NUL)
> + * * -E2BIG if @to is a zero size array.
> + */
> +#define stracpy_pad(to, from)					\
> +({								\
> +	size_t size = ARRAY_SIZE(to);				\
> +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> +								\
> +	strscpy_pad(to, from, size);				\
> +})
> +
>  #ifndef __HAVE_ARCH_STRCAT
>  extern char * strcat(char *, const char *);
>  #endif
> -- 
> 2.15.0
> 

-- 
Kees Cook