On Mon, Mar 23, 2020 at 9:50 AM Phasip <phasip@xxxxxxxxx> wrote:
>
> Hello!
>
> I have stumbled upon two ways of producing kernel warnings when using the overlayfs, both seem to be results of the same issue.
>
> The issue seems to be related to handling of hard links that are created directly in the upperdir.
> Below is my system details and then two samples with a list of commands to reproduce and the corresponding kernel warning
Hi,
Thanks for the report.
The problem is that i_nlink is not kept in sync with changes to
underlying layers. That would not in itself be an issue, since
modification of the underlying layers may result in
undefined/unexpected behavior. The problem is that this manifests
itself as a kernel warning.
Since unlink/rename is synchronized on the victim inode (the one that
is getting removed) it is possible to detect this condition and
prevent drop_nlink() from being called.
Attached patch fixes both of your testcases.
We'll need an xfstests case for this as well.
Thanks,
Miklos
---
fs/overlayfs/dir.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -819,6 +819,20 @@ static bool ovl_pure_upper(struct dentry
!ovl_test_flag(OVL_WHITEOUTS, d_inode(dentry));
}
+static void ovl_drop_nlink(struct inode *inode)
+{
+ struct dentry *alias = d_find_alias(inode);
+
+ dput(alias);
+ /*
+ * 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 +870,7 @@ 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->d_inode);
}
ovl_nlink_end(dentry);
@@ -1201,7 +1215,7 @@ 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(d_inode(new));
}
ovl_dir_modified(old->d_parent, ovl_type_origin(old) ||
