linux-kernel - Re: WARNING: Support for this device (Terratec Grabster AV400) is experimental.

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Date:   Tue, 30 Apr 2019 17:14:28 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     syzbot <syzbot+af8f8d2ac0d39b0ed3a0@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING: Support for this device (Terratec Grabster AV400) is experimental.

On Tue, Apr 30, 2019 at 5:00 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Tue, 30 Apr 2019, syzbot wrote:
>
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    9a33b369 usb-fuzzer: main usb gadget fuzzer driver
> > git tree:       https://github.com/google/kasan.git usb-fuzzer
> > console output: https://syzkaller.appspot.com/x/log.txt?x=141ca62d200000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=23e37f59d94ddd15
> > dashboard link: https://syzkaller.appspot.com/bug?extid=af8f8d2ac0d39b0ed3a0
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1405bedd200000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13ce3bbb200000
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+af8f8d2ac0d39b0ed3a0@...kaller.appspotmail.com
> >
> > usb 1-1: New USB device found, idVendor=0ccd, idProduct=0039, bcdDevice=
> > d.3c
> > usb 1-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
> > usb 1-1: config 0 descriptor??
> > pvrusb2: Hardware description: Terratec Grabster AV400
> > pvrusb2: **********
> > pvrusb2: WARNING: Support for this device (Terratec Grabster AV400) is
> > experimental.
> > pvrusb2: Important functionality might not be entirely working.
> > pvrusb2: Please consider contacting the driver author to help with further
> > stabilization of the driver.
> > pvrusb2: **********
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> This does seem like a bug in syzbot.  Why does it think this pr_info()
> output indicates a crash?  Is it fooled by the capitalized "WARNING" at
> the start of one of the lines?

Well, as of [1] WARN*() should only be used to indicate a kernel bug.
Normally, WARN*() prints a line that start with a warning, which is
followed by a stack trace. Unfortunately the stack trace is not always
present (kernel memory is badly corrupted, console deadlocked, etc.),
so syzbot detects the "WARNING:" line as a beginning of a WARN*()
reported bug. In this case the driver does something like
`pr_info("WARNING: ...", ...)`, which confuses syzbot. I'd say it's
the kernel that needs to be changed here to use some other kind of
prefix for printing warning messages.

[1] https://github.com/torvalds/linux/commit/96c6a32ccb55a366054fd82cc63523bb7f7493d3