btrfs-progs: check/lowmem: detect invalid file extents for symbolic links

[BUG]
There is a recent bug that btrfs/012 fails and kernel rejects to read a
symbolic link which is backed by a regular extent.

Furthremore in that case, "btrfs check --mode=lowmem" doesn't detect such
problem at all.

[CAUSE]
For symbolic links, we only allow inline extents, and this means we should
only have a symbolic link target which is smaller than 4K.

But lowmem mode btrfs check doesn't handle symbolic link inodes any
differently, thus it doesn't check if the file extents are inlined or not,
nor reporting this problem as an error.

[FIX]
When processing data extents, if we find the owning inode is a symbolic
link, and the file extent is regular/preallocated, report an error for
the bad file extent item.

Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

check/mode-lowmem.c

@@ -3351,6 +3351,31 @@ static int repair_extent_data_item(struct btrfs_root *root,
 	return err;
 }
 
+static int read_inode_item(struct btrfs_root *root,
+			   u64 ino, struct btrfs_inode_item *ret_ii)
+{
+	struct btrfs_path path = { 0 };
+	struct btrfs_key key = {
+		.objectid = ino,
+		.type = BTRFS_INODE_ITEM_KEY,
+		.offset = 0
+	};
+	int ret;
+
+	ret = btrfs_search_slot(NULL, root, &key, &path, 0, 0);
+	if (ret > 0)
+		ret = -ENOENT;
+	if (ret < 0)
+		goto out;
+
+	read_extent_buffer(path.nodes[0], ret_ii,
+			   btrfs_item_ptr_offset(path.nodes[0], path.slots[0]),
+			   sizeof(*ret_ii));
+out:
+	btrfs_release_path(&path);
+	return ret;
+}
+
 /*
  * Check EXTENT_DATA item, mainly for its dbackref in extent tree
  *
@@ -3371,6 +3396,7 @@ static int check_extent_data_item(struct btrfs_root *root,
 	struct btrfs_extent_item *ei;
 	struct btrfs_extent_inline_ref *iref;
 	struct btrfs_extent_data_ref *dref;
+	struct btrfs_inode_item inode_item;
 	u64 owner;
 	u64 disk_bytenr;
 	u64 disk_num_bytes;
@@ -3400,6 +3426,24 @@ static int check_extent_data_item(struct btrfs_root *root,
 	extent_num_bytes = btrfs_file_extent_num_bytes(eb, fi);
 	offset = btrfs_file_extent_offset(eb, fi);
 
+	/*
+	 * There is a regular/preallocated data extent. Make sure the owning
+	 * inode is not a symbolic link.
+	 * As symbolic links can only have inline data extents.
+	 */
+	ret = read_inode_item(root, fi_key.objectid, &inode_item);
+	if (ret < 0) {
+		errno = -ret;
+		error("failed to grab the inode item for inode %llu: %m",
+		      fi_key.objectid);
+		err |= INODE_ITEM_MISSING;
+	}
+	if (S_ISLNK(inode_item.mode)) {
+		error("symbolic link at root %lld ino %llu has regular/preallocated extents",
+		      root->root_key.objectid, fi_key.objectid);
+		err |= FILE_EXTENT_ERROR;
+	}
+
 	/* Check unaligned disk_bytenr, disk_num_bytes and num_bytes */
 	if (!IS_ALIGNED(disk_bytenr, gfs_info->sectorsize)) {
 		error(