linux-kernel - Re: mm/vmalloc.c:4691:25: error: variable 'addr' is uninitialized when used here [-Werror,-Wuninitialized]

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Date: Thu, 11 Jan 2024 17:37:12 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Dan Carpenter" <dan.carpenter@...aro.org>,
 "Naresh Kamboju" <naresh.kamboju@...aro.org>,
 "Uladzislau Rezki" <urezki@...il.com>
Cc: linux-next <linux-next@...r.kernel.org>,
 "open list" <linux-kernel@...r.kernel.org>,
 "Linux Regressions" <regressions@...ts.linux.dev>,
 clang-built-linux <llvm@...ts.linux.dev>, lkft-triage@...ts.linaro.org,
 "Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: mm/vmalloc.c:4691:25: error: variable 'addr' is uninitialized when used
 here [-Werror,-Wuninitialized]

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@...aro.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