Brian Foster <bfoster@xxxxxxxxxx> 于2024年9月9日周一 21:27写道： > > On Mon, Sep 09, 2024 at 08:15:43PM +0800, Julian Sun wrote: > > Hi Brian, > > > > Brian Foster <bfoster@xxxxxxxxxx> 于2024年9月7日周六 03:11写道： > > > > > > On Thu, Sep 05, 2024 at 06:24:24PM +0800, Julian Sun wrote: > > > > Attempting to unshare extents beyond EOF will trigger > > > > the need zeroing case, which in turn triggers a warning. > > > > Therefore, let's skip the unshare process if extents are > > > > beyond EOF. > > > > > > > > Reported-and-tested-by: syzbot+296b1c84b9cbf306e5a0@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > Closes: https://syzkaller.appspot.com/bug?extid=296b1c84b9cbf306e5a0 > > > > Fixes: 32a38a499104 ("iomap: use write_begin to read pages to unshare") > > > > Inspired-by: Dave Chinner <david@xxxxxxxxxxxxx> > > > > Signed-off-by: Julian Sun <sunjunchao2870@xxxxxxxxx> > > > > --- > > > > fs/iomap/buffered-io.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > > > > index f420c53d86ac..8898d5ec606f 100644 > > > > --- a/fs/iomap/buffered-io.c > > > > +++ b/fs/iomap/buffered-io.c > > > > @@ -1340,6 +1340,9 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter) > > > > /* don't bother with holes or unwritten extents */ > > > > if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN) > > > > return length; > > > > + /* don't try to unshare any extents beyond EOF. */ > > > > + if (pos > i_size_read(iter->inode)) > > > > + return length; > > > > > > Hi Julian, > > > > > > > > > > What about if pos starts within EOF and the operation extends beyond it? > > > > Extents within EOF will be unshared as usual. Details are below. > > > > > > > I ask because I think I've reproduced this scenario, though it is a bit > > > > tricky and has dependencies... > > > > > > > > For one, it seems to depend on the cowblocks patch I recently posted > > > > here [1] (though I don't think this is necessarily a problem with the > > > > patch, it just depends on keeping COW fork blocks around after the > > > > unshare). With that, I reproduce via fsx with unshare range support [2] > > > > using the ops file appended below [3] on a -bsize=1k XFS fs. > > > > > > > > I haven't quite characterized the full sequence other than it looks like > > > > the unshare walks across EOF with a shared data fork block and COW fork > > > > delalloc, presumably finds the post-eof part of the folio !uptodate (so > > > > iomap_adjust_read_range() doesn't skip it), and then trips over the > > > > warning and error return associated with the folio zeroing in > > > > __iomap_write_begin(). > > > > The scenario has already been reproduced by syzbot[1]. The reproducer > > provided by syzbot constructed the following extent maps for a file of > > size 0xE00 before fallocate unshare: > > > > 0 - 4k: shared between two files > > 4k - 6k: hole beyond EOF, not shared > > 6k - 8k: delalloc extends > > > > Then the reproducer attempted to unshare the extent between 0 and > > 0x2000 bytes, but the file size is 0xE00. This is likely the scenario > > you were referring to? > > > > Yes, sort of.. > > > Eventually the unshare code does: > > first map: 0 - 4k - unshare successfully. > > second map: 4k - 6k - hole, skip. Beyond EOF. > > third map: 6k - 8k - delalloc, beyond EOF so needs zeroing. > > Fires warnings because UNSHARE. > > > > During the first call to iomap_unshare_iter(), iomap_length() returned > > 4k, so 4k bytes were unshared. > > See discuss here[2] for more details. > > > > > > This all kind of has me wondering.. do we know the purpose of this > > > warning/error in the first place? It seems like it's more of an > > > "unexpected situation" than a specific problem. E.g., assuming the same > > > page were mmap'd, I _think_ the read fault path would do the same sort > > > of zeroing such that the unshare would see a fully uptodate folio and > > > carry on as normal. I added the mapread op to the opsfile below to give > > > that a quick test (remove the "skip" text to enable it), and it seems to > > > prevent the error, but I've not confirmed whether that theory is > > > actually what's happening. > > > > > > > > > > FWIW, I also wonder if another way to handle this would be to just > > > > restrict the range of iomap_file_unshare() to within EOF. IOW if a > > > > caller passes a range beyond EOF, just process whatever part of the > > > > range falls within EOF. It seems iomap isn't responsible for the file > > > > extending aspect of the fallocate unshare command anyways. > > > > It already does 'just process whatever part of the range falls within EOF'. > > Check the above case. > > > > I'm not sure if I fully understand what you mean. This patch does not > > prevent unsharing extents within the EOF. This patch checks if pos is > > beyond EOF, instead of checking if pos + length is beyond EOF. So the > > extents within EOF should be unshared as usual. > > > > I'm not concerned about preventing unsharing. I'm concerned that this > patch doesn't always prevent attempts to unshare post-eof ranges. I > think the difference here is that in the variant I was hitting, we end > > > up with a mapping that starts within EOF and ends beyond EOF, whereas > > the syzbot variant produces a scenario where the problematic mapping > > always starts beyond EOF. This is not true. In the case above, the syzbot did indeed unshare the extents between 0-4k, which were started within EOF and ended beyond EOF. The specific variants here are: pos:0 len:0x1000 EOF: 0xE00. And the unshare code successfully unshared extents between 0 and 4k. During the next loop in iomap_file_unshare(), the pos became 0x1000, which is beyond EOF. What this patch does is to skip the unshare during the second loop. Is there anything I misunderstand？ > > So IOW, this patch works for the syzbot variant because it happens to > reproduce a situation where pos will be beyond EOF, but that is an > > > assumption that might not always be true. The fsx generated variant runs > > a sequence that produces a mapping that spans across EOF, which means > > that pos is within EOF at the start of unshare, so unshare proceeds to > > walk across the EOF boundary, the corresponding EOF folio is not fully > > uptodate, and thus write begin wants to do partial zeroing and > > fails/warns. Yeah, it's exactly what the syzbot does. > > I suspect that if the higher level range were trimmed to be within EOF > in iomap_file_unshare(), that would prevent this problem in either case. > Note that this was on a -bsize=1k fs, so what I'm not totally sure about > is whether skipping zeroing as such would be a problem with larger FSBs. > My initial thinking was this might not be possible since the EOF folio > should be fully uptodate in that case, but that probably requires some > thought/review/testing. > > Brian > > > BTW, maybe the check here should be > > if (pos >= i_size_read(iter->inode)) > > > > If there is any misunderstanding, please let me know, thanks. > > > > [1]: https://lore.kernel.org/all/0000000000008964f1061f8c32b6@xxxxxxxxxx/T/ > > [2]: https://lore.kernel.org/all/20240903054808.126799-1-sunjunchao2870@xxxxxxxxx/ > > > > > > > > Thoughts? > > > > > > Brian > > > > > > [1] https://lore.kernel.org/linux-xfs/20240906114051.120743-1-bfoster@xxxxxxxxxx/ > > > [2] https://lore.kernel.org/fstests/20240906185606.136402-1-bfoster@xxxxxxxxxx/ > > > [3] fsx ops file: > > > > > > fallocate 0x3bc00 0x400 0 > > > write 0x3bc00 0x800 0x3c000 > > > clone_range 0x3bc00 0x400 0x0 0x3c400 > > > skip mapread 0x3c000 0x400 0x3c400 > > > fallocate 0x3bc00 0xc00 0x3c400 unshare > > > > > > > > > Thanks, > > -- > > Julian Sun <sunjunchao2870@xxxxxxxxx> > > > Thanks, -- Julian Sun <sunjunchao2870@xxxxxxxxx>
