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

[Alviro Iskandar Setiawan <alviro.iskandar@gmail.com>]

On Mon, Feb 7, 2022 at 9:21 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> 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@gmail.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.

The compiler can't deduce this because the import_single_range() is
located in a different translation unit (different C file), so it
can't prove that (ret < min_ret) is always true as it can't see the
function definition (in reality, it is always true because it only
returns either 0 or -EFAULT).

>
> 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.
>

This warning is valid, and the compiler should really warn that. But
again, in reality, this is still a false-positive warning, because
that "else if" will never be taken from the "goto out_free" path.

-- Viro