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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Fri, 2 Mar 2018 18:36:35 +0100
Message-ID: <CANiq72mcBGXOw_YgtMFWU8VV5irhuDsb-VRuPjeW9=SbHuyRsw@mail.gmail.com>
Subject: Re: [PATCH] Support the nonstring variable attribute (gcc >= 8)
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     David Rientjes <rientjes@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Lendacky, Thomas" <thomas.lendacky@amd.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Willy Tarreau <w@1wt.eu>,
        Xiongfeng Wang <wangxiongfeng2@huawei.com>,
        Martin Sebor <msebor@gmail.com>
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On Thu, Mar 1, 2018 at 10:57 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Mon, Feb 19, 2018 at 1:01 AM, Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
>> On Mon, Feb 19, 2018 at 12:20 AM, David Rientjes <rientjes@google.com> wrote:
>>> On Sat, 17 Feb 2018, 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, we can use __nonstring to let
>>>> gcc know a NUL character is not required; like in this case:
>>>>
>>>>   https://lkml.org/lkml/2018/1/16/135
>>>>
>>>> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
>>>> Cc: Ingo Molnar <mingo@kernel.org>
>>>> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
>>>> Cc: Kees Cook <keescook@chromium.org>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: David Rientjes <rientjes@google.com>
>>>
>>> I would have expected to have seen __nonstring used somewhere as part of
>>> this patch.
>>
>> Do you mean to expand the commit message with an actual code example
>> instead of the links to the docs and the discussion about the report?
>> Otherwise, if you mean in the actual commit, I think in that case it
>> should be a patch series, not a single commit.
>>
>> In any case, the key point here is to agree on the short-term policy:
>> i.e. whether we want to disable the upcoming warning or try to take
>> advantage of it (which not *necessarily* implies using __nonstring,
>> there are other workarounds; though where applicable, __nonstring is
>> probably the right thing to use).
>
> What David was asking for is to have a couple of users of the
> __nonstring attribute in places for which it is the right solution.
>

I understood :-) My question was regarding where he was asking to see it.

> I would suggest making it a patch series, with patch 1/x introducing
> the attribute (i.e. your patch), and followed by additional patches
> that add the attribute to individual header files or drivers for which
> it is the right solution.

Yep, that is what I suggested too.

>
> When I looked at the warning, I found that we have around 120 file
> for which we warn. The majority of them are actually questionable
> uses of strncpy() that probably should have been strscpy(), but
> most of those do not actually cause undefined behavior.

Then it looks like enabling the warning by default is useful and not
too noisy (at least for just char).

>
> A smaller number like the example from ext4 are nonstrings
> (i.e. character arrays without nul-termination) that would benefit
> from the nonstring attribute. About half of those are actually
> arrays of u8/__u8/uint8_t/__uint8_t for which the currently
> implemented nonstring attribute is invalid, and it seems odd
> to convert those to 'char', e.g.
>
> struct ext4_super_block {
>         __le32  s_first_error_time;     /* first time an error happened */
>         __le32  s_first_error_ino;      /* inode involved in first error */
>         __le64  s_first_error_block;    /* block involved of first error */
> -       __u8    s_first_error_func[32]; /* function where the error happened */
> +       char    s_first_error_func[32] __nonstring;     /* function
> where the error happened */
>         __le32  s_first_error_line;     /* line number where error happened */
>         __le32  s_last_error_time;      /* most recent time of an error */
>         __le32  s_last_error_ino;       /* inode involved in last error */
>         __le32  s_last_error_line;      /* line number where error happened */
>         __le64  s_last_error_block;     /* block involved of last error */
> -       __u8    s_last_error_func[32];  /* function where the error happened */
> +       char    s_last_error_func[32] __nonstring;      /* function
> where the error happened */
>
> doesn't feel right. Maybe we can extend gcc to also accept
> the attribute on arrays of other 8-bit types.

Hum... On one hand, the warning is meant to protect against misuses of
the typical string handling functions, and those take pointers to
char. Therefore, one could argue that using signed or unsigned char
already marks an array/pointer as "not a string" (for the purposes of
the attribute).

On the other hand, people *will* call string handling functions with
signed and unsigned char, and for those cases, it is useful to have
the warning nevertheless and being able to annotate those arrays with
nonstring, which is also good documentation-wise. On top of that, C
specifies char as equivalent to either signed or unsigned char (even
if it is a distinct type), so one could argue it should work for the
three types anyway.

Given that 1) this is a warning that can disabled just fine and that
2) we already have real life cases using nonstring, non-char arrays
calling typical string handling functions, I would favor supporting
the warning and the attribute for all the three types.

>
>> [By the way, CC'ing Xiongfeng, Willy and Arnd, since they were
>> involved in the example report; sorry guys!].
>
> Martin Sebor also asked me about this, he's the one that worked on
> the gcc code that introduced the warning. Sorry for not replying earlier.
>

Maybe you can pass this to him? (maybe open a bug in gcc's bugzilla?)

> For a complete list of affected files, see https://pastebin.com/eWFQf58i
> this is what I come up with by doing randconfig builds, but I have not
> tried to submit additional patches here, since I'm sure that a lot of
> those are wrong -- they need a much closer inspection to decide which
> ones are actual bugs vs harmless warnings, and which ones should
> use strscpy()/strlcpy() vs a nonstring annotation or a rewrite of that
> function.

Indeed -- nice work anyway finding those. If we agree on getting the
nonstring attribute, maybe you can send that patch as an RFC to ping
the respective maintainers?

Thanks,
Miguel