linux-kernel - Re: [RFC PATCH] string.h: Add stracpy/stracpy_pad (was: Re: [PATCH] checkpatch: Added warnings in favor of strscpy().)

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Message-ID: <201907221031.8B87A9DE@keescook>
Date:   Mon, 22 Jul 2019 10:33:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Nitin Gote <nitin.r.gote@...el.com>, akpm@...ux-foundation.org,
        corbet@....net, apw@...onical.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Subject: Re: [RFC PATCH] string.h: Add stracpy/stracpy_pad (was: Re: [PATCH]
 checkpatch: Added warnings in favor of strscpy().)

On Thu, Jul 04, 2019 at 05:15:57PM -0700, Joe Perches wrote:
> On Thu, 2019-07-04 at 13:46 -0700, Joe Perches wrote:
> > On Thu, 2019-07-04 at 11:24 +0530, Nitin Gote wrote:
> > > Added warnings in checkpatch.pl script to :
> > > 
> > > 1. Deprecate strcpy() in favor of strscpy().
> > > 2. Deprecate strlcpy() in favor of strscpy().
> > > 3. Deprecate strncpy() in favor of strscpy() or strscpy_pad().
> > > 
> > > Updated strncpy() section in Documentation/process/deprecated.rst
> > > to cover strscpy_pad() case.
> 
> []
> 
> I sent a patch series for some strscpy/strlcpy misuses.
> 
> How about adding a macro helper to avoid the misuses like:
> ---
>  include/linux/string.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 4deb11f7976b..ef01bd6f19df 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -35,6 +35,22 @@ 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);
>  
> +#define stracpy(to, from)					\
> +({								\
> +	size_t size = ARRAY_SIZE(to);				\
> +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> +								\
> +	strscpy(to, from, size);				\
> +})
> +
> +#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

This seems like a reasonable addition, yes. I think Coccinelle might
actually be able to find all the existing strscpy(dst, src, sizeof(dst))
cases to jump-start this conversion.

Devil's advocate: this adds yet more string handling functions... will
this cause even more confusion?

-- 
Kees Cook