Re: Kernel warnings in fs/inode.c:302 drop_nlink+0x28/0x40

From: Miklos Szeredi <miklos@szeredi.hu>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Phasip <phasip@gmail.com>, overlayfs <linux-unionfs@vger.kernel.org>
Subject: Re: Kernel warnings in fs/inode.c:302 drop_nlink+0x28/0x40
Date: Mon, 23 Mar 2020 15:53:24 +0100	[thread overview]
Message-ID: <CAJfpeguKujUqW-z75F+6mCh0uwHF6rz2cK4OWUCFe83QNmaSrQ@mail.gmail.com> (raw)
In-Reply-To: <CAJfpegtCn-HLhuDB98G4dO8L-t2PMcqcwDw+0TiknU5LGvBacQ@mail.gmail.com>

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On Mon, Mar 23, 2020 at 3:21 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Mon, Mar 23, 2020 at 2:24 PM Amir Goldstein <amir73il@gmail.com> wrote:

> > IDGI. coming from vfs_unlink() and vfs_rename() it doesn't look like
> > it is possible for victim inode not to have a hashed alias, so the
> > alias test seems futile.
>
> Yeah, needs a comment: both ovl_remove_upper() and
> ovl_remove_and_whiteout() unhash the dentry before returning, so
> d_find_alias() will find another hashed dentry or none.

Except that doesn't seem to be true for the overwriting rename case...

Attached patch should work for both.

Thanks,
Miklos

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---
 fs/overlayfs/dir.c |   25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -819,6 +819,27 @@ static bool ovl_pure_upper(struct dentry
 	       !ovl_test_flag(OVL_WHITEOUTS, d_inode(dentry));
 }
 
+static void ovl_drop_nlink(struct dentry *dentry)
+{
+	struct inode *inode = d_inode(dentry);
+	struct dentry *alias;
+
+	spin_lock(&inode->i_lock);
+	hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
+ 		if (alias != dentry && !d_unhashed(alias))
+			break;
+	}
+	spin_unlock(&inode->i_lock);
+
+	/*
+	 * Changes to underlying layers may cause i_nlink to lose sync with
+	 * reality.  In this case prevent the link count from going to zero
+	 * prematurely.
+	 */
+	if (inode->i_nlink > !!alias)
+		drop_nlink(inode);
+}
+
 static int ovl_do_remove(struct dentry *dentry, bool is_dir)
 {
 	int err;
@@ -856,7 +877,7 @@ static int ovl_do_remove(struct dentry *
 		if (is_dir)
 			clear_nlink(dentry->d_inode);
 		else
-			drop_nlink(dentry->d_inode);
+			ovl_drop_nlink(dentry);
 	}
 	ovl_nlink_end(dentry);
 
@@ -1201,7 +1222,7 @@ static int ovl_rename(struct inode *oldd
 		if (new_is_dir)
 			clear_nlink(d_inode(new));
 		else
-			drop_nlink(d_inode(new));
+			ovl_drop_nlink(new);
 	}
 
 	ovl_dir_modified(old->d_parent, ovl_type_origin(old) ||
