[PATCH v2] xfs: fix an undefined behaviour in _da3_path_shift

[Qian Cai <cai@lca.pw>]

xfs_da3_path_shift() could see state->path.blk[-1] because
state->path.active == 1 is a valid state when it tries to add an entry
to a single dir leaf block and then to shift forward to see if
there's a sibling block that would be a better place to put the new
entry.

 UBSAN: Undefined behaviour in fs/xfs/libxfs/xfs_da_btree.c:1989:14
 index -1 is out of range for type 'xfs_da_state_blk_t [5]'
 Call trace:
  dump_backtrace+0x0/0x2c8
  show_stack+0x20/0x2c
  dump_stack+0xe8/0x150
  __ubsan_handle_out_of_bounds+0xe4/0xfc
  xfs_da3_path_shift+0x860/0x86c [xfs]
  xfs_da3_node_lookup_int+0x7c8/0x934 [xfs]
  xfs_dir2_node_addname+0x2c8/0xcd0 [xfs]
  xfs_dir_createname+0x348/0x38c [xfs]
  xfs_create+0x6b0/0x8b4 [xfs]
  xfs_generic_create+0x12c/0x1f8 [xfs]
  xfs_vn_mknod+0x3c/0x4c [xfs]
  xfs_vn_create+0x34/0x44 [xfs]
  do_last+0xd4c/0x10c8
  path_openat+0xbc/0x2f4
  do_filp_open+0x74/0xf4
  do_sys_openat2+0x98/0x180
  __arm64_sys_openat+0xf8/0x170
  do_el0_svc+0x170/0x240
  el0_sync_handler+0x150/0x250
  el0_sync+0x164/0x180

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: update the commit log thanks to Darrick.
    simplify the code.

 fs/xfs/libxfs/xfs_da_btree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_da_btree.c b/fs/xfs/libxfs/xfs_da_btree.c
index 875e04f82541..e864c3d47f60 100644
--- a/fs/xfs/libxfs/xfs_da_btree.c
+++ b/fs/xfs/libxfs/xfs_da_btree.c
@@ -1986,7 +1986,8 @@ static inline int xfs_dabuf_nfsb(struct xfs_mount *mp, int whichfork)
 	ASSERT(path != NULL);
 	ASSERT((path->active > 0) && (path->active < XFS_DA_NODE_MAXDEPTH));
 	level = (path->active-1) - 1;	/* skip bottom layer in path */
-	for (blk = &path->blk[level]; level >= 0; blk--, level--) {
+	for (; level >= 0; level--) {
+		blk = &path->blk[level];
 		xfs_da3_node_hdr_from_disk(dp->i_mount, &nodehdr,
 					   blk->bp->b_addr);
 
-- 
1.8.3.1