[PATCH 5/6] ocfs2: do not index before the entry array when dividing an empty bucket

[Joseph Qi <joseph.qi@linux.alibaba.com>]

ocfs2_divide_xattr_bucket() appends an empty bucket carrying a sentinel
hash when the source bucket has no divide position, taking that hash
from xh_entries[start - 1].  ocfs2_xattr_find_divide_pos() returns count
when every entry shares one hash, and its loop does not run at all for
count < 2, so an empty bucket comes back with start == count == 0 and
the read is xh_entries[-1].

That is outside the __counted_by_le(xh_count) bounds of xh_entries[] and
lands on the header, so the value picked up is
xh_count | xh_free_start << 16.  One above it is written into the new
bucket's xh_entries[0] and committed, which leaves a bucket whose first
hash does not follow the bucket before it.  ocfs2_xattr_bucket_find()
binary searches on xh_entries[0], so the broken key hides the range the
sentinel was meant to cover: getxattr() answers -ENODATA for an
attribute that is stored, and an insert can go to the wrong bucket and
leave two copies of a name.  The cross cluster divide hands the same
value back as a tree record's e_cpos, which then claims a hash range it
does not cover.

Read xh_entries[0] instead.  The branch is only reached when every entry
shares one hash, so for a non-empty bucket it holds the same value, and
for an empty one it is the slot that carries the bucket key.

An empty bucket reaches the divide from
ocfs2_check_xattr_bucket_collision(), which reports no collision for
one, through ocfs2_add_new_xattr_bucket() and
ocfs2_extend_xattr_bucket().  It does not take a failed set to get
there: a divide that finds no position is itself what leaves an empty
sentinel bucket behind, and a later extend can pick it as the target.

Fixes: 80bcaf3469b8 ("ocfs2/xattr: Proper hash collision handle in bucket division")
Cc: <stable@vger.kernel.org>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
 fs/ocfs2/xattr.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/xattr.c b/fs/ocfs2/xattr.c
index 3a16b4a36d9a..822b1e9c3ba2 100644
--- a/fs/ocfs2/xattr.c
+++ b/fs/ocfs2/xattr.c
@@ -5228,7 +5228,15 @@ static int ocfs2_divide_xattr_bucket(struct inode *inode,
 	start = ocfs2_xattr_find_divide_pos(xh);
 
 	if (start == count) {
-		xe = &xh->xh_entries[start-1];
+		/*
+		 * start == count means every entry shares one hash, so the
+		 * first of them carries it too.  An empty bucket has no
+		 * entries[start - 1], but its entries[0] still carries the
+		 * bucket key: either the hash ocfs2_xa_remove_entry()
+		 * deliberately leaves behind, or the sentinel a previous
+		 * divide wrote.
+		 */
+		xe = &xh->xh_entries[0];
 
 		/*
 		 * initialized a new empty bucket here.
-- 
2.39.3