Re: [PATCH] usb: core: urb: change a dev_WARN() to dev_err() for syzbot

From: Greg KH <gregkh@linuxfoundation.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Dan Carpenter <dan.carpenter@oracle.com>,
	syzbot <syzbot+1bc2c2afd44f820a669f@syzkaller.appspotmail.com>,
	andreyknvl@google.com, ingrassia@epigenesys.com,
	linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
	syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] usb: core: urb: change a dev_WARN() to dev_err() for syzbot
Date: Mon, 10 Feb 2020 13:50:06 -0800	[thread overview]
Message-ID: <20200210215006.GA1628323@kroah.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.2002101609090.13988-100000@netrider.rowland.org>

On Mon, Feb 10, 2020 at 04:11:10PM -0500, Alan Stern wrote:
> On Mon, 10 Feb 2020, Greg KH wrote:
> 
> > On Fri, Jan 31, 2020 at 08:06:52AM +0300, Dan Carpenter wrote:
> > > We changed this from dev_err() to dev_WARN() in commit 0cb54a3e47cb
> > > ("USB: debugging code shouldn't alter control flow").
> > > 
> > > The difference between dev_WARN() and dev_err() is that dev_WARN()
> > > prints a stack trace and if you have panic on OOPS enabled then it leads
> > > to a panic.  The dev_err() function just prints the error message.
> > > 
> > > Back in the day we didn't have usb emulators fuzz testing the kernel
> > > so dev_WARN() didn't cause a problem for anyone, but these days the
> > > dev_WARN() interferes with syzbot so let's change this to a dev_err().
> > > 
> > > Reported-by: syzbot+1bc2c2afd44f820a669f@syzkaller.appspotmail.com
> > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > ---
> > > 
> > >  drivers/usb/core/urb.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
> > > index da923ec17612..0980c1d2253d 100644
> > > --- a/drivers/usb/core/urb.c
> > > +++ b/drivers/usb/core/urb.c
> > > @@ -475,7 +475,7 @@ int usb_submit_urb(struct urb *urb, gfp_t mem_flags)
> > >  
> > >  	/* Check that the pipe's type matches the endpoint's type */
> > >  	if (usb_urb_ep_type_check(urb))
> > > -		dev_WARN(&dev->dev, "BOGUS urb xfer, pipe %x != type %x\n",
> > > +		dev_err(&dev->dev, "BOGUS urb xfer, pipe %x != type %x\n",
> > >  			usb_pipetype(urb->pipe), pipetypes[xfertype]);
> > 
> > Like others said, we should have the stack trace here.  So can you
> > change this to dev_warn() and a stacktrace?
> 
> In fact we want both a stack trace and a syzbot notification, because 
> this particular error indicates a bug in a kernel driver.  Therefore 
> dev_WARN is appropriate.

Ok, nevermind, you are right we should fix up the driver if that
happens.

greg k-h