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

Jens Axboe
2018-11-14T15:18:29Z

On 11/13/18 2:43 PM, Dave Chinner 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.

Applied, thanks. Ming, can you please add a blktests test for
this case? This is the 2nd time it's been broken.

--
Jens Axboe