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Subject: Re: [PATCH] media: sq905.c: fix uninitialized variable
From:   Pavel Skripkin <paskripkin@gmail.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Alaa Emad <alaaemadhossney.ae@gmail.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        syzbot+a4e309017a5f3a24c7b3@syzkaller.appspotmail.com
Date:   Fri, 26 Mar 2021 10:53:54 +0300
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Hi!

On Fri, 2021-03-26 at 08:40 +0100, Dmitry Vyukov wrote:
> On Fri, Mar 26, 2021 at 8:24 AM Pavel Skripkin <paskripkin@gmail.com>
> wrote:
> > 
> > Hi!
> > 
> > On Fri, 2021-03-26 at 08:14 +0100, 'Dmitry Vyukov' via syzkaller
> > wrote:
> > > On Fri, Mar 26, 2021 at 8:11 AM Greg KH
> > > <gregkh@linuxfoundation.org>
> > > wrote:
> > > > 
> > > > On Thu, Mar 25, 2021 at 11:22:02PM +0200, Alaa Emad wrote:
> > > > > Reported-by:
> > > > > syzbot+a4e309017a5f3a24c7b3@syzkaller.appspotmail.com
> > > > > Signed-off-by: Alaa Emad <alaaemadhossney.ae@gmail.com>
> > > > > ---
> > > > 
> > > > I know I do not take patches with no changelog text, but other
> > > > maintainers might be more leniant :(
> > > 
> > > I wonder if it's the right fix or not.
> > > Initializing variables will, of course, silence the warning, but
> > > it's
> > > not necessarily the right fix. I suspect there is something wrong
> > > in
> > > how ret/act_len are user/checked.
> > > 
> > 
> > There is a problem in usb_bulk_msg() call. It could return before
> > act_len initialization, so i think, act_len should be intialized with
> > smth wrong like 0 or -1. BTW, I already send patch for that, but it
> > was
> > marked as obsoleted.
> 
> If usb_bulk_msg returns before act_len initialization, it should
> signify that fact with an error code in return value or something,
> right? It does not initialize act_len only in case of errors, right?
> If so, sq905_read_data must check ret and don't use act_for any
> checks. But it does, and that's I think the bug. Or maybe usb_bulk_msg
> does not properly signify that it failed (and did not initialize
> act_len). Either way silencing the warning with pre-initializing
> act_len looks very fishy.
> For example, consider, in some contexts it's OK to transmit 0-length
> packets, I don't know if it's the case for usb_bulk_msg or not, but it
> does not affect the idea. Now, if we pre-initialize act_len to 0, we
> can falsely think that such 0-length transfer has succeeded (act_len
> == size), while it actually failed (I assume so since usb_bulk_msg
> left act_len unitialized).

You are absolutely rigth, and sq905_read_data doesn't use act_len for
checks in case of usb_bulk_msg fail. But it uses it for error printing:

	if (ret < 0 || act_len != size) {
		pr_err("bulk read fail (%d) len %d/%d\n", ret,
act_len, size);
		return -EIO;
	}

So, value like -1 can be a flag for smth went wrong internally. Or
maybe remove this error log and replace it with other, which will rely
on error code, idk how it will be better
--
With regards,
Pavel Skripkin