[PATCH] ovl: fix WARN_ON nlink drop to zero

[Amir Goldstein <amir73il@xxxxxxxxx>]

Changes to underlying layers should not cause WARN_ON(), but this repro
does:

 mkdir w l u mnt
 sudo mount -t overlay -o workdir=w,lowerdir=l,upperdir=u overlay mnt
 touch mnt/h
 ln u/h u/k
 rm -rf mnt/k
 rm -rf mnt/h
 dmesg

 ------------[ cut here ]------------
 WARNING: CPU: 1 PID: 116244 at fs/inode.c:302 drop_nlink+0x28/0x40

After upper hardlinks were added while overlay is mounted, unlinking all
overlay hardlinks drops overlay nlink to zero before all upper inodes
are unlinked.

Detect too low i_nlink before unlink/rename and set the overlay nlink
to the upper inode nlink (minus one for an index entry).

Reported-by: Phasip <phasip@xxxxxxxxx>
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/overlayfs/util.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

Miklos,

This fix passed the reported reproducers (with index=off),
overlay/034 with (index=on) and overlay/034 with s/LOWER/UPPER:

 -lowerdir=$OVL_BASE_SCRATCH_MNT/$OVL_LOWER
 +lowerdir=$OVL_BASE_SCRATCH_MNT/$OVL_UPPER
  workdir=$OVL_BASE_SCRATCH_MNT/$OVL_WORK

As well as the rest of overlay/quick group.

I will post the overlay/034 fork as a separate test later.

Thanks,
Amir.

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 36b60788ee47..e894a14857c7 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -734,7 +734,9 @@ static void ovl_cleanup_index(struct dentry *dentry)
 int ovl_nlink_start(struct dentry *dentry)
 {
 	struct inode *inode = d_inode(dentry);
+	struct inode *iupper = ovl_inode_upper(inode);
 	const struct cred *old_cred;
+	int index_nlink;
 	int err;
 
 	if (WARN_ON(!inode))
@@ -764,7 +766,26 @@ int ovl_nlink_start(struct dentry *dentry)
 	if (err)
 		return err;
 
-	if (d_is_dir(dentry) || !ovl_test_flag(OVL_INDEX, inode))
+	if (d_is_dir(dentry))
+		goto out;
+
+	/* index adds +1 to upper nlink */
+	index_nlink = !!ovl_test_flag(OVL_INDEX, inode);
+	if (iupper && (iupper->i_nlink - index_nlink) > inode->i_nlink) {
+		pr_warn_ratelimited("inode nlink too low (%pd2, ino=%lu, nlink=%u, upper_nlink=%u)\n",
+				    dentry, inode->i_ino, inode->i_nlink,
+				    iupper->i_nlink - index_nlink);
+		/*
+		 * Upper hardlinks were added while overlay is mounted.
+		 * Unlinking all overlay hardlinks would drop overlay nlink to
+		 * zero before all upper inodes are unlinked.  As a safety
+		 * measure, when that situation is detected, set the overlay
+		 * nlink to the upper inode nlink minus one for the index entry.
+		 */
+		set_nlink(d_inode(dentry), iupper->i_nlink - index_nlink);
+	}
+
+	if (!index_nlink)
 		goto out;
 
 	old_cred = ovl_override_creds(dentry->d_sb);
-- 
2.17.1