Re: [PATCH] block: fix 32 bit overflow in __blkdev_issue_discard()

Dave Chinner
2018-11-14T08:09:35Z

On Wed, Nov 14, 2018 at 10:53:11AM +0800, Ming Lei wrote:
> On Wed, Nov 14, 2018 at 5:44 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > A discard cleanup merged into 4.20-rc2 causes fstests xfs/259 to
> > fall into an endless loop in the discard code. The test is creating
> > a device that is exactly 2^32 sectors in size to test mkfs boundary
> > conditions around the 32 bit sector overflow region.
> >
> > mkfs issues a discard for the entire device size by default, and
> > hence this throws a sector count of 2^32 into
> > blkdev_issue_discard(). It takes the number of sectors to discard as
> > a sector_t - a 64 bit value.
> >
> > The commit ba5d73851e71 ("block: cleanup __blkdev_issue_discard")
> > takes this sector count and casts it to a 32 bit value before
> > comapring it against the maximum allowed discard size the device
> > has. This truncates away the upper 32 bits, and so if the lower 32
> > bits of the sector count is zero, it starts issuing discards of
> > length 0. This causes the code to fall into an endless loop, issuing
> > a zero length discards over and over again on the same sector.
> >
> > Fixes: ba5d73851e71 ("block: cleanup __blkdev_issue_discard")
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> > block/blk-lib.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > index e8b3bb9bf375..144e156ed341 100644
> > --- a/block/blk-lib.c
> > +++ b/block/blk-lib.c
> > @@ -55,9 +55,12 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> > return -EINVAL;
> >
> > while (nr_sects) {
> > - unsigned int req_sects = min_t(unsigned int, nr_sects,
> > + sector_t req_sects = min_t(sector_t, nr_sects,
> > bio_allowed_max_sectors(q));
>
> bio_allowed_max_sectors(q) is always < UINT_MAX, and 'sector_t' is only
> required during the comparison, so another simpler fix might be the following,
> could you test if it is workable?
>
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index e8b3bb9bf375..6ef44f99e83f 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -55,7 +55,7 @@ int __blkdev_issue_discard(struct block_device
> *bdev, sector_t sector,
> return -EINVAL;
>
> while (nr_sects) {
> - unsigned int req_sects = min_t(unsigned int, nr_sects,
> + unsigned int req_sects = min_t(sector_t, nr_sects,
> bio_allowed_max_sectors(q));

Rearrange the deck chairs all you like, just make sure you fix your
regression test suite to exercise obvious boundary conditions like
this so the next cleanup doesn't break the code again.

> >
> > + WARN_ON_ONCE(req_sects == 0);
>
> The above line isn't necessary given 'nr_sects' can't be zero.

Except it was 0 and it caused the bug I had to fix. So it should
have a warning on it.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com