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

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Date:   Sun, 5 Aug 2018 11:14:15 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Sun, Aug 5, 2018 at 3:38 AM, Joe Perches <joe@...ches.com> wrote:
> 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.

Ah, good catch! The test was added in the v1->v2 meantime. Will do, thanks!

By the way, the file is a mess... Some other tests go inside the big
"#if GCC_VERSION >= 40000", others go at the end. We should clean it
up and sort it. I might just do it...

Cheers,
Miguel

>
>> +#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 */