Re: Block device direct read EIO handling broken?

Damien Le Moal
2019-08-07T09:43:02Z

On 2019/08/06 22:34, Jens Axboe wrote:
> On 8/6/19 12:05 AM, Damien Le Moal wrote:
>> On 2019/08/06 13:09, Jens Axboe wrote:
>>> On 8/5/19 5:05 PM, Damien Le Moal wrote:
>>>> On 2019/08/06 7:05, Damien Le Moal wrote:
>>>>> On 2019/08/06 6:59, Damien Le Moal wrote:
>>>>>> On 2019/08/06 6:28, Jens Axboe wrote:
>>>>>>> On 8/5/19 2:27 PM, Damien Le Moal wrote:
>>>>>>>> On 2019/08/06 6:26, Jens Axboe wrote:
>>>>>>>>>> In any case, looking again at this code, it looks like there is a
>>>>>>>>>> problem with dio->size being incremented early, even for fragments
>>>>>>>>>> that get BLK_QC_T_EAGAIN, because dio->size is being used in
>>>>>>>>>> blkdev_bio_end_io(). So an incorrect size can be reported to user
>>>>>>>>>> space in that case on completion (e.g. large asynchronous no-wait dio
>>>>>>>>>> that cannot be issued in one go).
>>>>>>>>>>
>>>>>>>>>> So maybe something like this ? (completely untested)
>>>>>>>>>
>>>>>>>>> I think that looks pretty good, I like not double accounting with
>>>>>>>>> this_size and dio->size, and we retain the old style ordering for the
>>>>>>>>> ret value.
>>>>>>>>
>>>>>>>> Do you want a proper patch with real testing backup ? I can send that
>>>>>>>> later today.
>>>>>>>
>>>>>>> Yeah that'd be great, I like your approach better.
>>>>>>>
>>>>>>
>>>>>> Looking again, I think this is not it yet: dio->size is being referenced after
>>>>>> submit_bio(), so blkdev_bio_end_io() may see the old value if the bio completes
>>>>>> before dio->size increment. So the use-after-free is still there. And since
>>>>>> blkdev_bio_end_io() processes completion to user space only when dio->ref
>>>>>> becomes 0, adding an atomic_inc/dec(&dio->ref) over the loop would not help and
>>>>>> does not cover the single BIO case. Any idea how to address this one ?
>>>>>>
>>>>>
>>>>> May be add a bio_get/put() over the 2 places that do submit_bio() would work,
>>>>> for all cases (single/multi BIO, sync & async). E.g.:
>>>>>
>>>>> + bio_get(bio);
>>>>> qc = submit_bio(bio);
>>>>> if (qc == BLK_QC_T_EAGAIN) {
>>>>> if (!dio->size)
>>>>> ret = -EAGAIN;
>>>>> + bio_put(bio);
>>>>> goto error;
>>>>> }
>>>>> dio->size += bio_size;
>>>>> + bio_put(bio);
>>>>>
>>>>> Thoughts ?
>>>>>
>>>>
>>>> That does not work since the reference to dio->size in
>>>> blkdev_bio_end_io() depends on atomic_dec_and_test(&dio->ref) which
>>>> counts the BIO fragments for the dio (+1 for async multi-bio case). So
>>>> completion of the last bio can still reference the old value of
>>>> dio->size.
>>>>
>>>> Adding a bio_get/put() on dio->bio ensures that dio stays around, but
>>>> does not prevent the use of the wrong dio->size. Adding an additional
>>>> atomic_inc/dec(&dio->ref) would prevent that, but we would need to
>>>> handle dio completion at the end of __blkdev_direct_IO() if all BIO
>>>> fragments already completed at that point. That is a lot more plumbing
>>>> needed, relying completely on dio->ref for all cases, thus removing
>>>> the dio->multi_bio management.
>>>>
>>>> Something like this:
>>>
>>> Don't like this, as it adds unnecessary atomics for the sync case.
>>> What's wrong with just adjusting dio->size if we get BLK_QC_T_EAGAIN?
>>> It's safe to do so, since we're doing the final put later. We just can't
>>> do it for the normal case of submit_bio() succeeding. Kill the new 'ret'
>>> usage and return to what we had as well, it's more readable too imho.
>>>
>>> Totally untested...
>>>
>>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>>> index a6f7c892cb4a..131e2e0582a6 100644
>>> --- a/fs/block_dev.c
>>> +++ b/fs/block_dev.c
>>> @@ -349,7 +349,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>> loff_t pos = iocb->ki_pos;
>>> blk_qc_t qc = BLK_QC_T_NONE;
>>> gfp_t gfp;
>>> - ssize_t ret;
>>> + int ret;
>>>
>>> if ((pos | iov_iter_alignment(iter)) &
>>> (bdev_logical_block_size(bdev) - 1))
>>> @@ -386,8 +386,6 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>>
>>> ret = 0;
>>> for (;;) {
>>> - int err;
>>> -
>>> bio_set_dev(bio, bdev);
>>> bio->bi_iter.bi_sector = pos >> 9;
>>> bio->bi_write_hint = iocb->ki_hint;
>>> @@ -395,10 +393,8 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>> bio->bi_end_io = blkdev_bio_end_io;
>>> bio->bi_ioprio = iocb->ki_ioprio;
>>>
>>> - err = bio_iov_iter_get_pages(bio, iter);
>>> - if (unlikely(err)) {
>>> - if (!ret)
>>> - ret = err;
>>> + ret = bio_iov_iter_get_pages(bio, iter);
>>> + if (unlikely(ret)) {
>>> bio->bi_status = BLK_STS_IOERR;
>>> bio_endio(bio);
>>> break;
>>> @@ -421,7 +417,6 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>> if (nowait)
>>> bio->bi_opf |= (REQ_NOWAIT | REQ_NOWAIT_INLINE);
>>>
>>> - dio->size += bio->bi_iter.bi_size;
>>> pos += bio->bi_iter.bi_size;
>>>
>>> nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
>>> @@ -433,13 +428,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>> polled = true;
>>> }
>>>
>>> + dio->size += bio->bi_iter.bi_size;
>>> qc = submit_bio(bio);
>>> if (qc == BLK_QC_T_EAGAIN) {
>>> - if (!ret)
>>> - ret = -EAGAIN;
>>> + dio->size -= bio->bi_iter.bi_size;
>>
>> ref after free of bio here. Easy to fix though. Also, with this, the
>> bio_endio() call within submit_bio() for the EAGAIN failure will see a
>> dio->size too large, including this failed bio. So this does not work.
>
> There's no ref after free here - if BLK_QC_T_EAGAIN is being returned,
> the bio has not been freed. There's no calling bio_endio() for that
> case.
>
> For dio->size, it doesn't matter. If we get the error here, bio_endio()
> was never called. And if the submission is successful, we use dio->size
> for the success case.
>

OK. Got it (I checked REQ_NOWAIT_INLINE, I should have done this earlier !).

But, unless I am missing something again, there are 2 other problems though:
1) I do not see a bio_put() for the bio submission failure with BLK_QC_T_EAGAIN.
2) If the submission failure is for a bio fragment of a multi-bio dio, dio->ref
for this failed bio will never be decremented, so the dio will never complete.

I am still trying to get a reliable way to generate BLK_QC_T_EAGAIN submit_bio()
failures for testing. Any hint on how to do this ? I tried setting qd to 1 and
nr_requests to the minimum 4 and generate a lot of load on the test disk, but
never got BLK_QC_T_EAGAIN.

Best regards.