linux-kernel - Re: dm: fix dax_dev NULL dereference

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Date:   Tue, 30 Jul 2019 15:15:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        device-mapper development <dm-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alasdair Kergon <agk@...hat.com>
Subject: Re: dm: fix dax_dev NULL dereference

On Tue, Jul 30, 2019 at 3:01 PM Mike Snitzer <snitzer@...hat.com> wrote:
>
> On Tue, Jul 30 2019 at  5:38pm -0400,
> Dan Williams <dan.j.williams@...el.com> wrote:
>
> > On Tue, Jul 30, 2019 at 12:07 PM Mike Snitzer <snitzer@...hat.com> wrote:
> > >
> > > I staged the fix (which I tweaked) here:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.3&id=95b9ebb78c4c733f8912a195fbd0bc19960e726e
> >
> > Thanks for picking this up Mike, but I'd prefer to just teach
> > dax_synchronous() to return false if the passed in dax_dev is NULL.
> > Thoughts?
>
> I considered that too but I moved away from it because I'm so used to
> the various block interfaces requiring the caller pass a non-NULL
> pointer (e.g. request_queue):
>
> $ grep -ri return drivers/md/dm-table.c | grep \&\&
> drivers/md/dm-table.c:        return dev->dax_dev && dax_synchronous(dev->dax_dev);
> drivers/md/dm-table.c:        return q && blk_queue_zoned_model(q) == *zoned_model;
> drivers/md/dm-table.c:        return q && blk_queue_zone_sectors(q) == *zone_sectors;
> drivers/md/dm-table.c:        return q && (q->queue_flags & flush);
> drivers/md/dm-table.c:        return q && blk_queue_nonrot(q);
> drivers/md/dm-table.c:        return q && !blk_queue_add_random(q);
> drivers/md/dm-table.c:        return q && !q->limits.max_write_same_sectors;
> drivers/md/dm-table.c:        return q && !q->limits.max_write_zeroes_sectors;
> drivers/md/dm-table.c:        return q && !blk_queue_discard(q);
> drivers/md/dm-table.c:        return q && !blk_queue_secure_erase(q);
> drivers/md/dm-table.c:        return q && bdi_cap_stable_pages_required(q->backing_dev_info);
>
> I'm fine with however you'd like to skin this cat though.
>
> Just let me know and I'll keep/drop this patch accordingly.

Ok, since you've already got it queued, and there are no other
required "if (!dax_dev)" fixups go ahead with what you have.

    Acked-by: Dan Williams <dan.j.williams@...el.com>

I just reserve the right to go push it down a level if the kernel ever
grows more dax_synchronous() users that do that safety check.