[v3,00/25] fs: fixes for serious clone/dedupe problems
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Message ID 153923113649.5546.9840926895953408273.stgit@magnolia
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  • fs: fixes for serious clone/dedupe problems
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Darrick J. Wong Oct. 11, 2018, 4:12 a.m. UTC
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

Comments

Darrick J. Wong Oct. 11, 2018, 3:55 p.m. UTC | #1
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.