On Tue, Nov 19, 2019 at 05:32:14PM +0100, Jan Kara wrote: > On Wed 13-11-19 10:44:03, Darrick J. Wong wrote: > > On Wed, Nov 13, 2019 at 07:00:32PM +0100, Jan Kara wrote: > > > Hello, > > > > > > I've spent today tracking down the syzkaller report of a WARN_ON hit in > > > iov_iter_pipe() [1]. The immediate problem is that syzkaller reproducer > > > (calling sendfile(2) from different threads at the same time a file to the > > > same file in rather evil way) results in splice code leaking pipe pages > > > (nrbufs doesn't return to 0 after read+write in the splice) and eventually > > > we run out of pipe pages and hit the warning in iov_iter_pipe(). The > > > problem is not specific to ext4, I can see in my tracing that when the > > > underlying filesystem is XFS, we can leak the pipe pages in the same way > > > (but for XFS somehow the problem doesn't happen as often). Rather the > > > problem seems to be in how iomap direct IO code, pipe iter code, and splice > > > code interact. > > > > > > So the problematic situation is when we do direct IO read into pipe pages > > > and the read hits EOF which is not on page boundary. Say the file has 4608 > > > (4096+512) bytes, block size == page size == 4096. What happens is that iomap > > > code maps the extent, gets that the extent size is 8192 (mapping ignores > > > > I wonder, would this work properly if the read side returns a 4608-byte > > mapping instead of an 8192-byte mapping? It doesn't make a lot of sense > > (to me, anyway) for a read mapping to go beyond EOF. > > The slight concern I have with this is that that would change e.g. the > behavior of IOMAP_REPORT. We could specialcase IOMAP_REPORT but then it > gets kind of ugly. And it seems kind of fuzzy when do we truncate the > extent with i_size and when not... Generally i_size is kind of a side-band > thing for block mapping operations so if we could leave it out of > ->iomap_begin I'd find that nicer. <nod> > > > i_size). Then we call iomap_dio_bio_actor(), which creates its private > > > iter, truncates it to 8192, and calls bio_iov_iter_get_pages(). That > > > eventually results in preparing two pipe buffers with length 4096 to accept > > > the read. Then read completes, in iomap_dio_complete() we truncate the return > > > value from 8192 (which was the real amount of IO we performed) to 4608. Now > > > this amount (4608) gets passed through splice code to > > > iter_file_splice_write(), we write out that amount, but then when cleaning > > > up pipe buffers, the last pipe buffer has still 3584 unused so we leave > > > the pipe buffer allocated and effectively leak it. > > > > > > Now I was also investigating why the old direct IO code doesn't leak pipe > > > buffers like this and the trick is done by the iov_iter_revert() call > > > generic_file_read_iter(). This results in setting iter position right to > > > the position where direct IO read reported it ended (4608) and truncating > > > pipe buffers after this point. So splice code then sees the second pipe > > > buffer has length only 512 which matches the amount it was asked to write > > > and so the pipe buffer gets freed after the write in > > > iter_file_splice_write(). > > > > > > The question is how to best fix this. The quick fix is to add > > > iov_iter_revert() call to iomap_dio_rw() so that in case of sync IO (we > > > always do only sync IO to pipes), we properly set iter position in case of > > > short read / write. But it looks somewhat hacky to me and this whole > > > interaction of iter and pipes looks fragile to me. > > > > > > Another option I can see is to truncate the iter to min(i_size-pos, length) in > > > iomap_dio_bio_actor() which *should* do the trick AFAICT. But I'm not sure > > > if it won't break something else. > > > > Do the truncation in ->iomap_begin on the read side, as I suggested above? > > Yes, that would be equivalent for this case. > > > > Any other ideas? > > > > > > As a side note the logic copying iter in iomap_dio_bio_actor() looks > > > suspicious. We copy 'dio->submit.iter' to 'iter' but then in the loop we call > > > iov_iter_advance() on dio->submit.iter. So if bio_iov_iter_get_pages() > > > didn't return enough pages and we loop again, 'iter' will have stale > > > contents and things go sideways from there? What am I missing? And why do > > > we do that strange copying of iter instead of using iov_iter_truncate() and > > > iov_iter_reexpand() on the 'dio->submit.iter' directly? > > > > I'm similarly puzzled; I would've thought that we'd need to advance the > > private @iter too. Or just truncate and reexpand the dio->submit.iter > > and not have the private one. > > > > With any luck hch will have some ideas? :/ > > Christoph seems to be busy with something else. So I'll just write patches, > run them through fstests and see if something blows up. Heheh. Ok, sounds good! --D > Honza > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
