Re: [PATCH v3 00/25] fs: fixes for serious clone/dedupe problems

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Oct 11, 2018 at 11:33:57AM +0300, Amir Goldstein wrote:
> On Thu, Oct 11, 2018 at 7:12 AM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> >
> > Hi all,
> >
> > Dave, Eric, and I have been chasing a stale data exposure bug in the XFS
> > reflink implementation, and tracked it down to reflink forgetting to do
> > some of the file-extending activities that must happen for regular
> > writes.
> >
> > We then started auditing the clone, dedupe, and copyfile code and
> > realized that from a file contents perspective, clonerange isn't any
> > different from a regular file write.  Unfortunately, we also noticed
> > that *unlike* a regular write, clonerange skips a ton of overflow
> > checks, such as validating the ranges against s_maxbytes, MAX_NON_LFS,
> > and RLIMIT_FSIZE.  We also observed that cloning into a file did not
> > strip security privileges (suid, capabilities) like a regular write
> > would.  I also noticed that xfs and ocfs2 need to dump the page cache
> > before remapping blocks, not after.
> >
> > In fixing the range checking problems I also realized that both dedupe
> > and copyfile tell userspace how much of the requested operation was
> > acted upon.  Since the range validation can shorten a clone request (or
> > we can ENOSPC midway through), we might as well plumb the short
> > operation reporting back through the VFS indirection code to userspace.
> >
> > So, here's the whole giant pile of patches[1] that fix all the problems.
> > This branch is against 4.19-rc7 with Dave Chinner's XFS for-next branch.
> > The patch "generic: test reflink side effects" recently sent to fstests
> > exercises the fixes in this series.  Tests are in [2].
> >
> > --D
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=djwong-devel
> > [2] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfstests-dev.git/log/?h=djwong-devel
> 
> I tested your branch with overlayfs over xfs.
> I did not observe any failures with -g clone except for test generic/937
> which also failed on xfs in my test.

Ok, matches what I saw overnight.  Good, that means I (at least
theoretically) know how to test overlayfs now. :)

> I though that you forgot to mention I needed to grab xfsprogs from djwong-devel
> for commit e84a9e93 ("xfs_io: dedupe command should only complain
> if we don't dedupe anything"), but even with this change the test still fails:
> 
> generic/937     - output mismatch (see
> /old/home/amir/src/fstests/xfstests-dev/results//generic/937.out.bad)
>     --- tests/generic/937.out   2018-10-11 08:23:00.630938364 +0300
>     +++ /old/home/amir/src/fstests/xfstests-dev/results//generic/937.out.bad
>    2018-10-11 10:54:40.448134832 +0300
>     @@ -4,8 +4,7 @@
>      39578c21e2cb9f6049b1cf7fc7be12a6  TEST_DIR/test-937/file2
>      Files 1-2 do not match (intentional)
>      (partial) dedupe the middle blocks together
>     -deduped XXXX/XXXX bytes at offset XXXX
>     -XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>     +XFS_IOC_FILE_EXTENT_SAME: Extents did not match.

Ohhh, right, g/937 is the test to see if the dedupe implementation will
return a short bytes_deduped if a single byte at the end of the range
doesn't match.  I'll have to update that because...

I reverted the FIDEDUPERANGE behavior to set ->info[x].bytes_deduped =
->src_length even if we rounded the length down to the nearest block
boundary to avoid incorrect sharing of blocks on files with
non-block-aligned EOF.  It turned out that the existing FIDEDUPERANGE
users will hang in infinite loops if the kernel returns ->info[x].status
== FILE_DEDUPE_RANGE_SAME but ->info[x].bytes_deduped < ->src_length.

It seems really stupid to me that the kernel now lies to userspace to
avoid breaking it, but that's what btrfs does so we're stuck with that.
For now.

>      Compare sections
> 
> One thing that *is* different with overlayfs test is that filefrag crashes
> on this same test:
> 
>     QA output created by 937
>     Create the original files
>     35ac8d7917305c385c30f3d82c30a8f6  TEST_DIR/test-937/file1
>     39578c21e2cb9f6049b1cf7fc7be12a6  TEST_DIR/test-937/file2
>     Files 1-2 do not match (intentional)
>     (partial) dedupe the middle blocks together
>     XFS_IOC_FILE_EXTENT_SAME: Extents did not match.
>     ./tests/generic/937: line 59: 19242 Floating point exception(core
> dumped) ${FILEFRAG_PROG} -v $testdir/file1 >> $seqres.full
>     ./tests/generic/937: line 60: 19244 Floating point exception(core
> dumped) ${FILEFRAG_PROG} -v $testdir/file2 >> $seqres.full
> 
> It looks like an overlayfs v4.19-rc1 regression - FIGETBSZ returns zero.
> I never noticed this regression before, because none of the generic tests
> are using filefrag.

Funny, I was wondering just the other day if there were any filesystems
that set s_blocksize == 0... :)

--D

> Thanks,
> Amir.