Function start_transaction() can return ERR_PTR(1) when flush is
BTRFS_RESERVE_FLUSH_LIMIT, so the call graph is
start_transaction (return ERR_PTR(1))
-> btrfs_block_rsv_add (return 1)
-> reserve_metadata_bytes (return 1)
-> flush_space (return 1)
-> do_chunk_alloc (return 1)
With BTRFS_RESERVE_FLUSH_LIMIT, if flush_space is already on the
flush_state of ALLOC_CHUNK and it successfully allocates a new
chunk, then instead of trying to reserve space again,
reserve_metadata_bytes returns 1 immediately.
Eventually the callers who call start_transaction() usually just
do the IS_ERR() check which ERR_PTR(1) can pass, then it'll get
a panic when dereferencing a pointer which is ERR_PTR(1).
This makes flush_space() translate 'ret = 1' to 'ret = 0'.
Signed-off-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
---
We found this 'NULL pointer dereference' on an old 3.8 kernel but
it's not going to happen on the upstream since there is no caller
of btrfs_start_transaction_lflush().
fs/btrfs/extent-tree.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 7a35c9d..a00fb67 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4457,6 +4457,15 @@ void check_system_chunk(struct btrfs_trans_handle *trans,
}
}
+/*
+ * If force is CHUNK_ALLOC_FORCE:
+ * - return 1 if it successfully allocates a chunk,
+ * - return errors including -ENOSPC otherwise.
+ * If force is NOT CHUNK_ALLOC_FORCE:
+ * - return 0 if it doesn't need to allocate a new chunk,
+ * - return 1 if it successfully allocates a chunk,
+ * - return errors including -ENOSPC otherwise.
+ */
static int do_chunk_alloc(struct btrfs_trans_handle *trans,
struct btrfs_root *extent_root, u64 flags, int force)
{
@@ -4857,7 +4866,7 @@ static int flush_space(struct btrfs_root *root,
btrfs_get_alloc_profile(root, 0),
CHUNK_ALLOC_NO_FORCE);
btrfs_end_transaction(trans, root);
- if (ret == -ENOSPC)
+ if (ret == -ENOSPC || ret == 1)
ret = 0;
break;
case COMMIT_TRANS:
--
2.5.5
--
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