[PATCH 3.16 107/148] ext4: work around deleting a file with i_nlink == 0 safely

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@xxxxxxx>

commit c7df4a1ecb8579838ec8c56b2bb6a6716e974f37 upstream.

If the file system is corrupted such that a file's i_links_count is
too small, then it's possible that when unlinking that file, i_nlink
will already be zero.  Previously we were working around this kind of
corruption by forcing i_nlink to one; but we were doing this before
trying to delete the directory entry --- and if the file system is
corrupted enough that ext4_delete_entry() fails, then we exit with
i_nlink elevated, and this causes the orphan inode list handling to be
FUBAR'ed, such that when we unmount the file system, the orphan inode
list can get corrupted.

A better way to fix this is to simply skip trying to call drop_nlink()
if i_nlink is already zero, thus moving the check to the place where
it makes the most sense.

https://bugzilla.kernel.org/show_bug.cgi?id=205433

Link: https://lore.kernel.org/r/20191112032903.8828-1-tytso@xxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
[bwh: Backported to 3.16: Log message and function are different]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2830,19 +2830,18 @@ static int ext4_unlink(struct inode *dir
 	if (IS_DIRSYNC(dir))
 		ext4_handle_sync(handle);
 
-	if (!inode->i_nlink) {
-		ext4_warning(inode->i_sb,
-			     "Deleting nonexistent file (%lu), %d",
-			     inode->i_ino, inode->i_nlink);
-		set_nlink(inode, 1);
-	}
 	retval = ext4_delete_entry(handle, dir, de, bh);
 	if (retval)
 		goto end_unlink;
 	dir->i_ctime = dir->i_mtime = ext4_current_time(dir);
 	ext4_update_dx_flag(dir);
 	ext4_mark_inode_dirty(handle, dir);
-	drop_nlink(inode);
+	if (inode->i_nlink == 0)
+		ext4_warning(inode->i_sb,
+			     "Deleting nonexistent file (%lu), %d",
+			     inode->i_ino, inode->i_nlink);
+	else
+		drop_nlink(inode);
 	if (!inode->i_nlink)
 		ext4_orphan_add(handle, inode);
 	inode->i_ctime = ext4_current_time(inode);