btrfs: exit gracefully if reloc roots don't match

[BUG]
Syzbot reported a crash that an ASSERT() got triggered inside
prepare_to_merge().

[CAUSE]
The root cause of the triggered ASSERT() is we can have a race between
quota tree creation and relocation.

This leads us to create a duplicated quota tree in the
btrfs_read_fs_root() path, and since it's treated as fs tree, it would
have ROOT_SHAREABLE flag, causing us to create a reloc tree for it.

The bug itself is fixed by a dedicated patch for it, but this already
taught us the ASSERT() is not something straightforward for
developers.

[ENHANCEMENT]
Instead of using an ASSERT(), let's handle it gracefully and output
extra info about the mismatch reloc roots to help debug.

Also with the above ASSERT() removed, we can trigger ASSERT(0)s inside
merge_reloc_roots() later.
Also replace those ASSERT(0)s with WARN_ON()s.

CC: stable@vger.kernel.org # 5.15+
Reported-by: syzbot+ae97a827ae1c3336bbb4@syzkaller.appspotmail.com
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 37 insertions, 8 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 25a3361caedc49..46c3c1d5726686 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1916,7 +1916,39 @@ again:
 				err = PTR_ERR(root);
 			break;
 		}
-		ASSERT(root->reloc_root == reloc_root);
+
+		if (unlikely(root->reloc_root != reloc_root)) {
+			if (root->reloc_root) {
+				btrfs_err(fs_info,
+"reloc tree mismatch, root %lld has reloc root key (%lld %u %llu) gen %llu, expect reloc root key (%lld %u %llu) gen %llu",
+					  root->root_key.objectid,
+					  root->reloc_root->root_key.objectid,
+					  root->reloc_root->root_key.type,
+					  root->reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &root->reloc_root->root_item),
+					  reloc_root->root_key.objectid,
+					  reloc_root->root_key.type,
+					  reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &reloc_root->root_item));
+			} else {
+				btrfs_err(fs_info,
+"reloc tree mismatch, root %lld has no reloc root, expect reloc root key (%lld %u %llu) gen %llu",
+					  root->root_key.objectid,
+					  reloc_root->root_key.objectid,
+					  reloc_root->root_key.type,
+					  reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &reloc_root->root_item));
+			}
+			list_add(&reloc_root->root_list, &reloc_roots);
+			btrfs_put_root(root);
+			btrfs_abort_transaction(trans, -EUCLEAN);
+			if (!err)
+				err = -EUCLEAN;
+			break;
+		}
 
 		/*
 		 * set reference count to 1, so btrfs_recover_relocation
@@ -1989,7 +2021,7 @@ again:
 		root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
 					 false);
 		if (btrfs_root_refs(&reloc_root->root_item) > 0) {
-			if (IS_ERR(root)) {
+			if (WARN_ON(IS_ERR(root))) {
 				/*
 				 * For recovery we read the fs roots on mount,
 				 * and if we didn't find the root then we marked
@@ -1998,17 +2030,14 @@ again:
 				 * memory.  However there's no reason we can't
 				 * handle the error properly here just in case.
 				 */
-				ASSERT(0);
 				ret = PTR_ERR(root);
 				goto out;
 			}
-			if (root->reloc_root != reloc_root) {
+			if (WARN_ON(root->reloc_root != reloc_root)) {
 				/*
-				 * This is actually impossible without something
-				 * going really wrong (like weird race condition
-				 * or cosmic rays).
+				 * This can happen if on-disk metadata has some
+				 * corruption, e.g. bad reloc tree key offset.
 				 */
-				ASSERT(0);
 				ret = -EINVAL;
 				goto out;
 			}