[PATCH v2] xfs: add agf freeblocks verify in xfs_agf_verify

[Zheng Bin <zhengbin13@huawei.com>]

We recently used fuzz(hydra) to test XFS and automatically generate
tmp.img(XFS v5 format, but some metadata is wrong)

xfs_repair information(just one AG):
agf_freeblks 0, counted 3224 in ag 0
agf_longest 536874136, counted 3224 in ag 0
sb_fdblocks 613, counted 3228

Test as follows:
mount tmp.img tmpdir
cp file1M tmpdir
sync

In 4.19-stable, sync will stuck, the reason is:
xfs_mountfs
  xfs_check_summary_counts
    if ((!xfs_sb_version_haslazysbcount(&mp->m_sb) ||
       XFS_LAST_UNMOUNT_WAS_CLEAN(mp)) &&
       !xfs_fs_has_sickness(mp, XFS_SICK_FS_COUNTERS))
	return 0;  -->just return, incore sb_fdblocks still be 613
    xfs_initialize_perag_data

cp file1M tmpdir -->ok(write file to pagecache)
sync -->stuck(write pagecache to disk)
xfs_map_blocks
  xfs_iomap_write_allocate
    while (count_fsb != 0) {
      nimaps = 0;
      while (nimaps == 0) { --> endless loop
         nimaps = 1;
         xfs_bmapi_write(..., &nimaps) --> nimaps becomes 0 again
xfs_bmapi_write
  xfs_bmap_alloc
    xfs_bmap_btalloc
      xfs_alloc_vextent
        xfs_alloc_fix_freelist
          xfs_alloc_space_available -->fail(agf_freeblks is 0)

In linux-next, sync not stuck, cause commit c2b3164320b5 ("xfs:
use the latest extent at writeback delalloc conversion time") remove
the above while, dmesg is as follows:
[   55.250114] XFS (loop0): page discard on page ffffea0008bc7380, inode 0x1b0c, offset 0.

Users do not know why this page is discard, the better soultion is:
1. Like xfs_repair, make sure sb_fdblocks is equal to counted
(xfs_initialize_perag_data did this, who is not called at this mount)
2. Add agf verify, if fail, will tell users to repair

This patch use the second soultion.

Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Ren Xudong <renxudong1@huawei.com>
---
v1->v2: modify comment, add more agf verify
 fs/xfs/libxfs/xfs_alloc.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index d8053bc..5faed42 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -2839,6 +2839,7 @@ xfs_agf_verify(
 {
 	struct xfs_mount	*mp = bp->b_mount;
 	struct xfs_agf		*agf = XFS_BUF_TO_AGF(bp);
+	int i;

 	if (xfs_sb_version_hascrc(&mp->m_sb)) {
 		if (!uuid_equal(&agf->agf_uuid, &mp->m_sb.sb_meta_uuid))
@@ -2858,6 +2859,22 @@ xfs_agf_verify(
 	      be32_to_cpu(agf->agf_flcount) <= xfs_agfl_size(mp)))
 		return __this_address;

+	if (be32_to_cpu(agf->agf_length) > mp->m_sb.sb_dblocks ||
+	    be32_to_cpu(agf->agf_btreeblks) > be32_to_cpu(agf->agf_length) ||
+	    be32_to_cpu(agf->agf_rmap_blocks) > be32_to_cpu(agf->agf_length) ||
+	    be32_to_cpu(agf->agf_refcount_blocks) > be32_to_cpu(agf->agf_length) ||
+	    be32_to_cpu(agf->agf_spare2) != 0)
+		return __this_address;
+
+	for (i = 0; i < ARRAY_SIZE(agf->agf_spare64); i++)
+		if (be64_to_cpu(agf->agf_spare64[i]) != 0)
+			return __this_address;
+
+	if (be32_to_cpu(agf->agf_freeblks) < be32_to_cpu(agf->agf_longest) ||
+	    be32_to_cpu(agf->agf_freeblks) > be32_to_cpu(agf->agf_length) ||
+	    be32_to_cpu(agf->agf_freeblks) > mp->m_sb.sb_fdblocks)
+		return __this_address;
+
 	if (be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) < 1 ||
 	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_CNT]) < 1 ||
 	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) > XFS_BTREE_MAXLEVELS ||
--
2.7.4