linux-kernel - Re: [PATCH v2 1/2] Support the nonstring variable attribute (gcc >= 8)

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Date:   Sat, 04 Aug 2018 18:38:55 -0700
From:   Joe Perches <joe@...ches.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Will Deacon <will.deacon@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Martin Sebor <msebor@...il.com>, Arnd Bergmann <arnd@...db.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] Support the nonstring variable attribute (gcc >=
 8)

On Wed, 2018-08-01 at 19:54 +0200, Miguel Ojeda wrote:
> From the GCC manual:
> 
> The nonstring variable attribute specifies that an object or member
> declaration with type array of char or pointer to char is intended to
> store character arrays that do not necessarily contain a terminating NUL
> character. This is useful in detecting uses of such arrays or pointers
> with functions that expect NUL-terminated strings, and to avoid warnings
> when such an array or pointer is used as an argument to a bounded string
> manipulation function such as strncpy.
> 
>   https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
> 
> Some reports are already coming to the LKML regarding these
> warnings. When they are false positives, like this one
> 
>   https://lkml.org/lkml/2018/1/16/135
> 
> we can use __nonstring to let gcc know a NUL character is not required.
> 
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Martin Sebor <msebor@...il.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
> ---
> Re-sending this since a few months have passed, Martin has improved
> GCC's feature and warnings are appearing in Geert's build bot.
> Added an example in the second patch as requested by David.
> 
>  include/linux/compiler-gcc.h   | 14 ++++++++++++++
>  include/linux/compiler_types.h |  4 ++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index 573f5a7d42d4..fab4e904f1fe 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -343,6 +343,20 @@
>  #define __designated_init __attribute__((designated_init))
>  #endif
>  
> +#if GCC_VERSION >= 80000
> +/*
> + * The nonstring variable attribute specifies that an object or member
> + * declaration with type array of char or pointer to char is intended
> + * to store character arrays that do not necessarily contain a terminating
> + * NUL character. This is useful in detecting uses of such arrays or pointers
> + * with functions that expect NUL-terminated strings, and to avoid warnings
> + * when such an array or pointer is used as an argument to a bounded string
> + * manipulation function such as strncpy.
> + * https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
> + */

Please move this down to the already existing test
for GCC_VERSION >= 80000 near the bottom of the
file so that version number tests are always in
increasing order in the file.

> +#define __nonstring __attribute__((nonstring))
> +#endif
> +
>  #endif	/* gcc version >= 40000 specific checks */
>  
>  #if !defined(__noclone)
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index a8ba6b04152c..9c07be36e86a 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -289,4 +289,8 @@ struct ftrace_likely_data {
>  #define __diag_error(compiler, version, option, comment) \
>  	__diag_ ## compiler(version, error, option)
>  
> +#ifndef __nonstring
> +# define __nonstring
> +#endif
> +
>  #endif /* __LINUX_COMPILER_TYPES_H */