btrfs: clear 'ret' in btrfs_check_shared() loop

PATCH RESEND In review Mark Fasheh 2 participants 2 messages May 19, 2015 Btrfs file system

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Mark Fasheh

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2015-05-19 19:49 UTC #0 lore raw

PATCH RESEND 1/1 btrfs: clear 'ret' in btrfs_check_shared() loop

btrfs_check_shared() is leaking a return value of '1' from find_parent_nodes(). As a result, callers (in this case, extent_fiemap()) are told extents are shared when they are not. This in turn broke fiemap on btrfs for kernels v3.18 and up.

The fix is simple - we just have to clear 'ret' after we are done processing the results of find_parent_nodes().

It wasn't clear to me at first what was happening with return values in btrfs_check_shared() and find_parent_nodes() - thanks to Josef for the help on irc. I added documentation to both functions to make things more clear for the next hacker who might come across them.

If we could queue this up for -stable too that would be great.

Signed-off-by Mark Fasheh <[email protected]>

---
 fs/btrfs/backref.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

fs/btrfs/backref.c +17 -0

index f55721f..8d47380 100644
@@ -880,6 +880,8 @@ static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
  * indirect refs to their parent bytenr.
  * When roots are found, they're added to the roots list
  *
+ * NOTE: This can return values > 0
+ *
  * FIXME some caching might speed things up
  */
 static int find_parent_nodes(struct btrfs_trans_handle *trans,

@@ -1198,6 +1200,19 @@ int btrfs_find_all_roots(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+/**
+ * btrfs_check_shared - tell us whether an extent is shared
+ *
+ * @trans: optional trans handle
+ *
+ * btrfs_check_shared uses the backref walking code but will short
+ * circuit as soon as it finds a root or inode that doesn't match the
+ * one passed in. This provides a significant performance benefit for
+ * callers (such as fiemap) which want to know whether the extent is
+ * shared but do not need a ref count.
+ *
+ * Return: 0 if extent is not shared, 1 if it is shared, < 0 on error.
+ */
 int btrfs_check_shared(struct btrfs_trans_handle *trans,
 		       struct btrfs_fs_info *fs_info, u64 root_objectid,
 		       u64 inum, u64 bytenr)

@@ -1226,11 +1241,13 @@ int btrfs_check_shared(struct btrfs_trans_handle *trans,
 		ret = find_parent_nodes(trans, fs_info, bytenr, elem.seq, tmp,
 					roots, NULL, root_objectid, inum);
 		if (ret == BACKREF_FOUND_SHARED) {
+			/* this is the only condition under which we return 1 */
 			ret = 1;
 			break;
 		}
 		if (ret < 0 && ret != -ENOENT)
 			break;
+		ret = 0;
 		node = ulist_next(tmp, &uiter);
 		if (!node)
 			break;

Signature · 1 line

2.1.2

Josef Bacik

Reviewed-by on v1 1/1

2015-05-19 20:35 UTC #1 lore raw

Quoted 8 lines · Mark Fasheh

btrfs_check_shared() is leaking a return value of '1' from find_parent_nodes(). As a result, callers (in this case, extent_fiemap()) are told extents are shared when they are not. This in turn broke fiemap on btrfs for kernels v3.18 and up.
The fix is simple - we just have to clear 'ret' after we are done processing the results of find_parent_nodes().

Quoted 8 lines

It wasn't clear to me at first what was happening with return values in btrfs_check_shared() and find_parent_nodes() - thanks to Josef for the help on irc. I added documentation to both functions to make things more clear for the next hacker who might come across them.
If we could queue this up for -stable too that would be great.
Signed-off-by Mark Fasheh <[email protected]>

Reviewed-by Josef Bacik <[email protected]>

Thanks,

Josef