Re: [PATCH 1/3] nbd: del_gendisk after we cleanup the queue

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Fri, 2018-04-13 at 12:21 -0400, Josef Bacik wrote:
> On Fri, Apr 13, 2018 at 04:16:18PM +0000, Bart Van Assche wrote:
> > On Fri, 2018-04-13 at 12:03 -0400, Josef Bacik wrote:
> > > This fixes a use after free bug, we need to do the del_gendisk after we
> > > cleanup the queue on the device.
> > 
> > Hello Josef,
> > 
> > Which use-after-free bug does this patch fix? Are you aware that most block
> > drivers call blk_cleanup_queue() before calling del_gendisk()? Why do you
> > think that the nbd driver should use the opposite order?
> 
> I'm confused, that's what this patch does.  Before I had del_gendisk() first and
> then the blk_cleanup_queue(), which was bugging out when I was testing stuff
> with a null pointer deref whenever I rmmod'ed the nbd.  Swapping it to the way
> everybody else did it fixed the problem.  Thanks,

Hello Josef,

Oops, I swapped "blk_cleanup_queue()" and "del_gendisk()" in my e-mail.

Can you share the call stack of the NULL pointer deref and also the translation
of the crash address into a source code line?

Thanks,

Bart.