Re: [PATCH] block: Bail out iteration functions upon SIGKILL.

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2019/11/08 20:54, Tetsuo Handa wrote:
> syzbot found that a thread can stall for minutes inside fallocate()
> after that thread was killed by SIGKILL [1]. While trying to allocate
> 64TB of disk space using fallocate() is legal, delaying termination of
> killed thread for minutes is bad. Thus, allow iteration functions in
> block/blk-lib.c to be killable.
> 
> [1] https://syzkaller.appspot.com/bug?id=9386d051e11e09973d5a4cf79af5e8cedf79386d
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+b48daca8639150bc5e73@syzkaller.appspotmail.com>
> ---
>  block/blk-lib.c | 44 ++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 40 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 5f2c429..6ca7cae 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -7,9 +7,22 @@
>  #include <linux/bio.h>
>  #include <linux/blkdev.h>
>  #include <linux/scatterlist.h>
> +#include <linux/sched/signal.h>
>  
>  #include "blk.h"
>  
> +static int blk_should_abort(struct bio *bio)
> +{
> +	int ret;
> +
> +	cond_resched();
> +	if (!fatal_signal_pending(current))
> +		return 0;
> +	ret = submit_bio_wait(bio);

This will change the behavior of __blkdev_issue_discard() to a sync IO
execution instead of the current async execution since submit_bio_wait()
call is the responsibility of the caller (e.g. blkdev_issue_discard()).
Have you checked if users of __blkdev_issue_discard() are OK with that ?
f2fs, ext4, xfs, dm and nvme use this function.

Looking at f2fs, this does not look like it is going to work as expected
since the bio setup, including end_io callback, is done after this
function is called and a regular submit_bio() execution is being used.

> +	bio_put(bio);
> +	return ret ? ret : -EINTR;
> +}
> +
>  struct bio *blk_next_bio(struct bio *bio, unsigned int nr_pages, gfp_t gfp)
>  {
>  	struct bio *new = bio_alloc(gfp, nr_pages);
> @@ -55,6 +68,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  		return -EINVAL;
>  
>  	while (nr_sects) {
> +		int ret;

Please add a white line after the declaration similarly to your change
in __blkdev_issue_write_same() and __blkdev_issue_zero_pages().

>  		sector_t req_sects = min_t(sector_t, nr_sects,
>  				bio_allowed_max_sectors(q));
>  
> @@ -75,7 +89,11 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  		 * us to schedule out to avoid softlocking if preempt
>  		 * is disabled.
>  		 */
> -		cond_resched();
> +		ret = blk_should_abort(bio);
> +		if (ret) {
> +			*biop = NULL;
> +			return ret;
> +		}
>  	}
>  
>  	*biop = bio;
> @@ -154,6 +172,8 @@ static int __blkdev_issue_write_same(struct block_device *bdev, sector_t sector,
>  	max_write_same_sectors = bio_allowed_max_sectors(q);
>  
>  	while (nr_sects) {
> +		int ret;
> +
>  		bio = blk_next_bio(bio, 1, gfp_mask);
>  		bio->bi_iter.bi_sector = sector;
>  		bio_set_dev(bio, bdev);
> @@ -171,7 +191,11 @@ static int __blkdev_issue_write_same(struct block_device *bdev, sector_t sector,
>  			bio->bi_iter.bi_size = nr_sects << 9;
>  			nr_sects = 0;
>  		}
> -		cond_resched();
> +		ret = blk_should_abort(bio);
> +		if (ret) {
> +			*biop = NULL;
> +			return ret;
> +		}
>  	}
>  
>  	*biop = bio;
> @@ -230,6 +254,8 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>  		return -EOPNOTSUPP;
>  
>  	while (nr_sects) {
> +		int ret;
> +
>  		bio = blk_next_bio(bio, 0, gfp_mask);
>  		bio->bi_iter.bi_sector = sector;
>  		bio_set_dev(bio, bdev);
> @@ -245,7 +271,11 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>  			bio->bi_iter.bi_size = nr_sects << 9;
>  			nr_sects = 0;
>  		}
> -		cond_resched();
> +		ret = blk_should_abort(bio);
> +		if (ret) {
> +			*biop = NULL;
> +			return ret;
> +		}
>  	}
>  
>  	*biop = bio;
> @@ -281,6 +311,8 @@ static int __blkdev_issue_zero_pages(struct block_device *bdev,
>  		return -EPERM;
>  
>  	while (nr_sects != 0) {
> +		int ret;
> +
>  		bio = blk_next_bio(bio, __blkdev_sectors_to_bio_pages(nr_sects),
>  				   gfp_mask);
>  		bio->bi_iter.bi_sector = sector;
> @@ -295,7 +327,11 @@ static int __blkdev_issue_zero_pages(struct block_device *bdev,
>  			if (bi_size < sz)
>  				break;
>  		}
> -		cond_resched();
> +		ret = blk_should_abort(bio);
> +		if (ret) {
> +			*biop = NULL;
> +			return ret;
> +		}
>  	}
>  
>  	*biop = bio;
> 


-- 
Damien Le Moal
Western Digital Research