linux-kernel - Re: [PATCH io_uring-5.17] io_uring: Fix build error potential reading uninitialized value

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Date:   Mon, 7 Feb 2022 17:20:46 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        GNU/Weeb Mailing List <gwml@...weeb.org>,
        io-uring Mailing list <io-uring@...r.kernel.org>,
        Tea Inside Mailing List <timl@...r.teainside.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...il.com>,
        kernel test robot <lkp@...el.com>,
        "Chen, Rong A" <rong.a.chen@...el.com>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH io_uring-5.17] io_uring: Fix build error potential
 reading uninitialized value

On Mon, Feb 07, 2022 at 06:45:57AM -0700, Jens Axboe wrote:
> On 2/7/22 4:43 AM, Ammar Faizi wrote:
> > From: Alviro Iskandar Setiawan <alviro.iskandar@...il.com>
> > 
> > In io_recv() if import_single_range() fails, the @flags variable is
> > uninitialized, then it will goto out_free.
> > 
> > After the goto, the compiler doesn't know that (ret < min_ret) is
> > always true, so it thinks the "if ((flags & MSG_WAITALL) ..."  path
> > could be taken.
> > 
> > The complaint comes from gcc-9 (Debian 9.3.0-22) 9.3.0:
> > ```
> >   fs/io_uring.c:5238 io_recvfrom() error: uninitialized symbol 'flags'
> > ```
> > Fix this by bypassing the @ret and @flags check when
> > import_single_range() fails.
> 
> The compiler should be able to deduce this, and I guess newer compilers
> do which is why we haven't seen this warning before.

No, we disabled GCC's uninitialized variable checking a couple years
back.  Linus got sick of the false positives.  You can still see it if
you enable W=2

fs/io_uring.c: In function ‘io_recv’:
fs/io_uring.c:5252:20: warning: ‘flags’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  } else if ((flags & MSG_WAITALL) && (msg.msg_flags & (MSG_TRUNC | MSG_CTRUNC))) {
             ~~~~~~~^~~~~~~~~~~~~~

If you introduce an uninitialized variable bug then likelyhood is the
kbuild-bot will send you a Clang warning or a Smatch warning or both.
I don't think anyone looks at GCC W=2 warnings.

regards,
dan carpenter