On Wed, Apr 15, 2020 at 08:02:04PM -0500, Eric Sandeen wrote:
>
>
> On 4/15/20 7:56 PM, Luis Chamberlain wrote:
> > On Wed, Apr 15, 2020 at 12:38:26PM -0500, Eric Sandeen wrote:
> >> On 4/13/20 11:18 PM, Luis Chamberlain wrote:
> >>> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> >>> merged on v4.12 Omar fixed the original blktrace code for request-based
> >>> drivers (multiqueue). This however left in place a possible crash, if you
> >>> happen to abuse blktrace in a way it was not intended.
> >>>
> >>> Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
> >>> forget to BLKTRACETEARDOWN, and then just remove the device you end up
> >>> with a panic:
> >>
> >> I think this patch makes this all cleaner anyway, but - without the apparent
> >> loop bug mentioned by Bart which allows removal of the loop device while blktrace
> >> is active (if I read that right), can this still happen?
> >
> > I have not tested that, but some modifications of the break-blktrace
> > program could enable us to test that
>
> FWIW, I modified it to modprobe & rmmod scsi_debug instead of the loop ioctls,
> and the module can't be unloaded after the blktrace is started since it's busy.
>
> Not sure that's equivalent tho.
Given what Bart mentioned about sd_open, that might be the saving grace
for blocking async behaviour on scsi. If it only refcounted the
request_queue as the loop driver it would have been exposed to the
same bug.
> > however I don't think the race
> > would be possible after patch 3/5 "blktrace: refcount the request_queue
> > during ioctl" is merged, as removal then a pending blktrace would
> > refcount the request_queue and the removal would have to wait until
> > the refcount is decremeneted, until after the blktrace ioctl.
>
> I'm out of my depth in the block layer, not sure who's supposed to take
> refs on what. ;)
I'm new to all this code, only been a few weeks looking into it, but am
trying to do my best ot make sense of it. So the above is what I can
tell would be needed.
Luis
