linux-ext4 - Re: [PATCH v5 12/15] Compiler Attributes: add support for _

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Date:   Thu, 20 Sep 2018 13:08:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Olof Johansson <olof@...m.net>,
        Konstantin Ryabitsev <konstantin@...uxfoundation.org>,
        "David S . Miller" <davem@...emloft.net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Sandipan Das <sandipan@...ux.vnet.ibm.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Paul Burton <paul.burton@...s.com>,
        David Rientjes <rientjes@...gle.com>, Willy Tarreau <w@....eu>,
        Martin Sebor <msebor@...il.com>,
        Christopher Li <sparse@...isli.org>,
        Jonathan Corbet <corbet@....net>,
        "Theodore Ts'o" <tytso@....edu>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Joe Perches <joe@...ches.com>, Arnd Bergmann <arnd@...db.de>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Stefan Agner <stefan@...er.ch>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH v5 12/15] Compiler Attributes: add support for __nonstring
 (gcc >= 8)

On Thu, Sep 20, 2018 at 10:22 AM, Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
> From the GCC manual:
>
>   nonstring
>
>     The nonstring variable attribute specifies that an object or member
>     declaration with type array of char, signed char, or unsigned char,
>     or pointer to such a type is intended to store character arrays that
>     do not necessarily contain a terminating NUL. 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
>
> This attribute can be used for documentation purposes (i.e. replacing
> comments), but it is most helpful when the following warnings are enabled:
>
>   -Wstringop-overflow
>
>     Warn for calls to string manipulation functions such as memcpy and
>     strcpy that are determined to overflow the destination buffer.
>
>     [...]
>
>   -Wstringop-truncation
>
>     Warn for calls to bounded string manipulation functions such as
>     strncat, strncpy, and stpncpy that may either truncate the copied
>     string or leave the destination unchanged.
>
>     [...]
>
>     In situations where a character array is intended to store a sequence
>     of bytes with no terminating NUL such an array may be annotated with
>     attribute nonstring to avoid this warning. Such arrays, however,
>     are not suitable arguments to functions that expect NUL-terminated
>     strings. To help detect accidental misuses of such arrays GCC issues
>     warnings unless it can prove that the use is safe.
>
>   https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
>
> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>

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

-Kees

-- 
Kees Cook
Pixel Security