Received-SPF: pass (google.com: domain of linux-kernel+bounces-23877-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.9.0-alpha0-1374-gc37f3abe3d-fm-20240102.001-gc37f3abe
Precedence: bulk
MIME-Version: 1.0
Message-Id: <bb175a67-462b-41a7-804a-ec990291a00e@app.fastmail.com>
In-Reply-To: <908325ed-08af-4b0c-926e-da9afba25772@app.fastmail.com>
References: 
 <CA+G9fYvDNksfKNvtfERaBa9t2MJNucfD_s3LgKGw_z2otW+nyw@mail.gmail.com>
 <628bf675-77fc-4ccc-be2f-9c3ec8a7b0b8@moroto.mountain>
 <908325ed-08af-4b0c-926e-da9afba25772@app.fastmail.com>
Date: Thu, 11 Jan 2024 17:37:12 +0100
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Dan Carpenter" <dan.carpenter@linaro.org>,
 "Naresh Kamboju" <naresh.kamboju@linaro.org>,
 "Uladzislau Rezki" <urezki@gmail.com>
Cc: linux-next <linux-next@vger.kernel.org>,
 "open list" <linux-kernel@vger.kernel.org>,
 "Linux Regressions" <regressions@lists.linux.dev>,
 clang-built-linux <llvm@lists.linux.dev>, lkft-triage@lists.linaro.org,
 "Andrew Morton" <akpm@linux-foundation.org>
Subject: Re: mm/vmalloc.c:4691:25: error: variable 'addr' is uninitialized when used
 here [-Werror,-Wuninitialized]
Content-Type: text/plain

On Thu, Jan 11, 2024, at 13:55, Arnd Bergmann wrote:
> On Thu, Jan 11, 2024, at 12:16, Dan Carpenter wrote:
>> On Thu, Jan 11, 2024 at 04:23:09PM +0530, Naresh Kamboju wrote:
>>> Following build failures noticed on i386 and x86 with clang builds on the
>>> Linux next-20240111 tag.
>>> 
>>> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
>>> 
>>> Build error:
>>> ----------
>>> mm/vmalloc.c:4691:25: error: variable 'addr' is uninitialized when
>>> used here [-Werror,-Wuninitialized]
>>>  4691 |                 va = __find_vmap_area(addr, &vn->busy.root);
>>>       |                                       ^~~~
>>> mm/vmalloc.c:4684:20: note: initialize the variable 'addr' to silence
>>> this warning
>>>  4684 |         unsigned long addr;
>>>       |                           ^
>>>       |                            = 0
>>> 1 error generated.
>>
>> We turned off uninitialized variable warnings for GCC a long time ago...
>> :/ I don't know if we'll be able to re-enable it in a -Werror world
>> although Clang seems to be managing alright so perhaps there is hope.
>
> The problem with gcc's warning is that it is non-deterministic and
> in recent versions actually got more false-positives even without
> -Os or -fsanitize=. Clang does not catch all that gcc does because
> it doesn't track state across inline functions, but at least its
> output is always the same regardless of optimization and other
> options.
>
> At least this particular one is an obvious bug and easily gets
> caught by lkft and lkp even if gcc's -Wuninitilized doesn't
> flag it.

As it turns out, gcc did find this one in the default -Wuninitialized
regardless of -Wmaybe-uninitialized:

mm/vmalloc.c: In function 'vmalloc_dump_obj':
mm/vmalloc.c:4691:22: error: 'addr' is used uninitialized [-Werror=uninitialized]
 4691 |                 va = __find_vmap_area(addr, &vn->busy.root);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/vmalloc.c:4684:23: note: 'addr' was declared here
 4684 |         unsigned long addr;
      |                       ^~~~

and I see that Uladzislau Rezki already sent a fix, which
is the same that I tried out in my randconfig tree:
https://lore.kernel.org/lkml/ZaARXdbigD1hWuOS@pc638.lan/

    Arnd