Re: Block device direct read EIO handling broken?

Jens Axboe
2019-08-05T20:31:51Z

On 8/5/19 11:31 AM, Jens Axboe wrote:
> On 8/5/19 11:15 AM, Darrick J. Wong wrote:
>> Hi Damien,
>>
>> I noticed a regression in xfs/747 (an unreleased xfstest for the
>> xfs_scrub media scanning feature) on 5.3-rc3. I'll condense that down
>> to a simpler reproducer:
>>
>> # dmsetup table
>> error-test: 0 209 linear 8:48 0
>> error-test: 209 1 error
>> error-test: 210 6446894 linear 8:48 210
>>
>> Basically we have a ~3G /dev/sdd and we set up device mapper to fail IO
>> for sector 209 and to pass the io to the scsi device everywhere else.
>>
>> On 5.3-rc3, performing a directio pread of this range with a < 1M buffer
>> (in other words, a request for fewer than MAX_BIO_PAGES bytes) yields
>> EIO like you'd expect:
>>
>> # strace -e pread64 xfs_io -d -c 'pread -b 1024k 0k 1120k' /dev/mapper/error-test
>> pread64(3, 0x7f880e1c7000, 1048576, 0) = -1 EIO (Input/output error)
>> pread: Input/output error
>> +++ exited with 0 +++
>>
>> But doing it with a larger buffer succeeds(!):
>>
>> # strace -e pread64 xfs_io -d -c 'pread -b 2048k 0k 1120k' /dev/mapper/error-test
>> pread64(3, "XFSB\0\0\20\0\0\0\0\0\0\fL\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1146880, 0) = 1146880
>> read 1146880/1146880 bytes at offset 0
>> 1 MiB, 1 ops; 0.0009 sec (1.124 GiB/sec and 1052.6316 ops/sec)
>> +++ exited with 0 +++
>>
>> (Note that the part of the buffer corresponding to the dm-error area is
>> uninitialized)
>>
>> On 5.3-rc2, both commands would fail with EIO like you'd expect. The
>> only change between rc2 and rc3 is commit 0eb6ddfb865c ("block: Fix
>> __blkdev_direct_IO() for bio fragments").
>>
>> AFAICT we end up in __blkdev_direct_IO with a 1120K buffer, which gets
>> split into two bios: one for the first BIO_MAX_PAGES worth of data (1MB)
>> and a second one for the 96k after that.
>>
>> I think the problem is that every time we submit a bio, we increase ret
>> by the size of that bio, but at the time we do that we have no idea if
>> the bio is going to succeed or not. At the end of the function we do:
>>
>> if (!ret)
>> ret = blk_status_to_errno(dio->bio.bi_status);
>>
>> Which means that we only pick up the IO error if we haven't already set
>> ret. I suppose that was useful for being able to return a short read,
>> but now that we always increment ret by the size of the bio, we act like
>> the whole buffer was read. I tried a -rc2 kernel and found that 40% of
>> the time I'd get an EIO and the rest of the time I got a short read.
>>
>> Not sure where to go from here, but something's not right...
>
> I'll take a look.

How about this? The old code did:

if (!ret)
ret = blk_status_to_errno(dio->bio.bi_status);
if (likely(!ret))
ret = dio->size;

where 'ret' was just tracking the error. With 'ret' now being the
positive IO size, we should overwrite it if ret is >= 0, not just if
it's zero.

Also kill a use-after-free.

diff --git a/fs/block_dev.c b/fs/block_dev.c
index a6f7c892cb4a..67c8e87c9481 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -386,6 +386,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)

ret = 0;
for (;;) {
+ ssize_t this_size;
int err;

bio_set_dev(bio, bdev);
@@ -433,13 +434,14 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
polled = true;
}

+ this_size = bio->bi_iter.bi_size;
qc = submit_bio(bio);
if (qc == BLK_QC_T_EAGAIN) {
if (!ret)
ret = -EAGAIN;
goto error;
}
- ret = dio->size;
+ ret += this_size;

if (polled)
WRITE_ONCE(iocb->ki_cookie, qc);
@@ -460,13 +462,14 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
atomic_inc(&dio->ref);
}

+ this_size = bio->bi_iter.bi_size;
qc = submit_bio(bio);
if (qc == BLK_QC_T_EAGAIN) {
if (!ret)
ret = -EAGAIN;
goto error;
}
- ret = dio->size;
+ ret += this_size;

bio = bio_alloc(gfp, nr_pages);
if (!bio) {
@@ -494,7 +497,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
__set_current_state(TASK_RUNNING);

out:
- if (!ret)
+ if (ret >= 0)
ret = blk_status_to_errno(dio->bio.bi_status);

bio_put(&dio->bio);

--
Jens Axboe