[Devel] [PATCH RH7] ext4: work around deleting a file with i_nlink == 0 safely

[Vasily Averin]

ML commit c7df4a1ecb8579838ec8c56b2bb6a6716e974f37
Author: Theodore Ts'o <tytso at mit.edu>
Date:   Mon Nov 11 22:18:13 2019 -0500

    ext4: work around deleting a file with i_nlink == 0 safely

    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@mit.edu
    Signed-off-by: Theodore Ts'o <tytso at mit.edu>
    Cc: stable at kernel.org
    Reviewed-by: Andreas Dilger <adilger at dilger.ca>

[vvs@] taken from RHEl7.9 beta kernel
2020-03-11 Jan Stancek <jstancek at redhat.com> [3.10.0-1128.el7]
    - [fs] ext4: work around deleting a file with i_nlink == 0 safely (Carlos Maiolino) [1801046]

Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 fs/ext4/namei.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 24d8617685b1..c8c56ef9281d 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2846,19 +2846,18 @@ static int ext4_unlink(struct inode *dir, struct dentry *dentry)
 	if (inode == EXT4_SB(dir->i_sb)->s_balloon_ino)
 		goto end_unlink;
 
-	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);
-- 
2.17.1