[PATCH V4 00/10] Bail out if transaction can cause extent count to overflow

Chandan Babu R
2020-09-18T09:48:19Z

XFS does not check for possible overflow of per-inode extent counter
fields when adding extents to either data or attr fork.

For e.g.
1. Insert 5 million xattrs (each having a value size of 255 bytes) and
then delete 50% of them in an alternating manner.

2. On a 4k block sized XFS filesystem instance, the above causes 98511
extents to be created in the attr fork of the inode.

xfsaild/loop0 2008 [003] 1475.127209: probe:xfs_inode_to_disk: (ffffffffa43fb6b0) if_nextents=98511 i_ino=131

3. The incore inode fork extent counter is a signed 32-bit
quantity. However, the on-disk extent counter is an unsigned 16-bit
quantity and hence cannot hold 98511 extents.

4. The following incorrect value is stored in the xattr extent counter,
# xfs_db -f -c 'inode 131' -c 'print core.naextents' /dev/loop0
core.naextents = -32561

This patchset adds a new helper function
(i.e. xfs_iext_count_may_overflow()) to check for overflow of the
per-inode data and xattr extent counters and invokes it before
starting an fs operation (e.g. creating a new directory entry). With
this patchset applied, XFS detects counter overflows and returns with
an error rather than causing a silent corruption.

The patchset has been tested by executing xfstests with the following
mkfs.xfs options,
1. -m crc=0 -b size=1k
2. -m crc=0 -b size=4k
3. -m crc=0 -b size=512
4. -m rmapbt=1,reflink=1 -b size=1k
5. -m rmapbt=1,reflink=1 -b size=4k

The patches can also be obtained from
https://github.com/chandanr/linux.git at branch xfs-reserve-extent-count-v4.

I have a patch that defines the newly introduced error injection tag
in xfsprogs
(https://github.com/chandanr/xfsprogs-dev/commit/7fd7aeef1cefbcc9abd6dd5887e710c80e48079d).
I have also written tests (https://paste.debian.net/1164261/) for
verifying the checks introduced in the kernel. The tests have to be
edited to make them suitable for merging with xfstests. But they also
depend on the changes introduced in xfs_bmap_add_extent_hole_real()
and xfs_growfs_rt_alloc(). Hence, I am planning to post the changes
for xfsprogs and xfstests if other developers are fine with the
changes made to the previously listed functions.

Changelog:
V3 -> V4:
1. Introduce new patch which lets userspace programs to test "extent
count overflow detection" by injecting an error tag. The new
error tag reduces the maximum allowed extent count to 10.
2. Injecting the newly defined error tag prevents
xfs_bmap_add_extent_hole_real() from merging a new extent with
its neighbours to allow writing deterministic tests for testing
extent count overflow for Directories, Xattr and growing realtime
devices. This is required because the new extent being allocated
can be contiguous with its neighbours (w.r.t both file and disk
offsets).
3. Injecting the newly defined error tag forces block sized extents
to be allocated for summary/bitmap files when growing a realtime
device. This is required because xfs_growfs_rt_alloc() allocates
as large an extent as possible for summary/bitmap files and hence
it would be impossible to write deterministic tests.
4. Rename XFS_IEXT_REMOVE_CNT to XFS_IEXT_PUNCH_HOLE_CNT to reflect
the actual meaning of the fs operation.
5. Fold XFS_IEXT_INSERT_HOLE_CNT code into that associated with
XFS_IEXT_PUNCH_HOLE_CNT since both perform the same job.
6. xfs_swap_extent_rmap(): Check for extent overflow should be made
on the source file only if the donor file extent has a valid
on-disk mapping and vice versa.

V2 -> V3:
1. Move the definition of xfs_iext_count_may_overflow() from
libxfs/xfs_trans_resv.c to libxfs/xfs_inode_fork.c. Also, I tried
to make xfs_iext_count_may_overflow() an inline function by
placing the definition in libxfs/xfs_inode_fork.h. However this
required that the definition of 'struct xfs_inode' be available,
since xfs_iext_count_may_overflow() uses a 'struct xfs_inode *'
type variable.
2. Handle XFS_COW_FORK within xfs_iext_count_may_overflow() by
returning a success value.
3. Rename XFS_IEXT_ADD_CNT to XFS_IEXT_ADD_NOSPLIT_CNT. Thanks to
Darrick for the suggesting the new name.
4. Expand comments to make use of 80 columns.

V1 -> V2:
1. Rename helper function from xfs_trans_resv_ext_cnt() to
xfs_iext_count_may_overflow().
2. Define and use macros to represent fs operations and the
corresponding increase in extent count.
3. Split the patches based on the fs operation being performed.

Chandan Babu R (10):
xfs: Add helper for checking per-inode extent count overflow
xfs: Check for extent overflow when trivally adding a new extent
xfs: Check for extent overflow when punching a hole
xfs: Check for extent overflow when adding/removing xattrs
xfs: Check for extent overflow when adding/removing dir entries
xfs: Check for extent overflow when writing to unwritten extent
xfs: Check for extent overflow when moving extent from cow to data
fork
xfs: Check for extent overflow when remapping an extent
xfs: Check for extent overflow when swapping extents
xfs: Introduce error injection to reduce maximum inode fork extent
count

fs/xfs/libxfs/xfs_attr.c | 13 ++++++
fs/xfs/libxfs/xfs_bmap.c | 33 ++++++++++-----
fs/xfs/libxfs/xfs_bmap.h | 1 +
fs/xfs/libxfs/xfs_errortag.h | 4 +-
fs/xfs/libxfs/xfs_inode_fork.c | 27 ++++++++++++
fs/xfs/libxfs/xfs_inode_fork.h | 77 ++++++++++++++++++++++++++++++++++
fs/xfs/libxfs/xfs_types.h | 1 +
fs/xfs/xfs_bmap_item.c | 5 +++
fs/xfs/xfs_bmap_util.c | 32 ++++++++++++++
fs/xfs/xfs_dquot.c | 8 +++-
fs/xfs/xfs_error.c | 3 ++
fs/xfs/xfs_inode.c | 27 ++++++++++++
fs/xfs/xfs_iomap.c | 10 +++++
fs/xfs/xfs_reflink.c | 10 +++++
fs/xfs/xfs_rtalloc.c | 21 +++++++++-
fs/xfs/xfs_symlink.c | 5 +++
16 files changed, 262 insertions(+), 15 deletions(-)

--
2.28.0