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Subject: Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
To:     Joe Perches <joe@perches.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Cc:     Jonathan Corbet <corbet@lwn.net>, Stephen Kitt <steve@sk2.org>,
        Kees Cook <keescook@chromium.org>,
        Nitin Gote <nitin.r.gote@intel.com>, jannh@google.com,
        kernel-hardening@lists.openwall.com,
        Andrew Morton <akpm@linux-foundation.org>
References: <cover.1563841972.git.joe@perches.com>
 <7ab8957eaf9b0931a59eff6e2bd8c5169f2f6c41.1563841972.git.joe@perches.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <eec901c6-ca51-89e4-1887-1ccab0288bee@rasmusvillemoes.dk>
Date:   Tue, 23 Jul 2019 08:55:31 +0200
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On 23/07/2019 02.38, 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

yes

like u8 or unsigned char.

no. "unsigned char" aka u8, "signed char" aka s8 and plain char are not
__builtin_types_compatible_p to one another.

> A BUILD_BUG is emitted when the type of to is not compatible.
> 
> Signed-off-by: Joe Perches <joe@perches.com>
> ---
>  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.

Well, yes, but more importantly and generally: -E2BIG if the copy
including %NUL didn't fit. [The zero size array thing could be made into
a build bug for these stra* variants if one thinks that might actually
occur in real code.]

> + */
> +#define stracpy(to, from)					\
> +({								\
> +	size_t size = ARRAY_SIZE(to);				\
> +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> +								\

ARRAY_SIZE should ensure to is indeed an array, but it doesn't hurt to
spell the second condition

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

(the gcc docs explicitly mention that "The type 'int[]' and 'int[5]' are
compatible.) - just in case that line gets copy-pasted somewhere that
doesn't have another must-be-array check nearby.

You should use a more "unique" identifier than "size". from could be
some expression that refers to such a variable from the surrounding scope.

Rasmus