ovl: take lower dir inode mutex outside upper sb_writers lock

The functions ovl_lower_positive() and ovl_check_empty_dir() both take
inode mutex on the real lower dir under ovl_want_write() which takes
the upper_mnt sb_writers lock.

While this is not a clear locking order or layering violation, it creates
an undesired lock dependency between two unrelated layers for no good
reason.

This lock dependency materializes to a false(?) positive lockdep warning
when calling rmdir() on a nested overlayfs, where both nested and
underlying overlayfs both use the same fs type as upper layer.

rmdir() on the nested overlayfs creates the lock chain:
  sb_writers of upper_mnt (e.g. tmpfs) in ovl_do_remove()
  ovl_i_mutex_dir_key[] of lower overlay dir in ovl_lower_positive()

rmdir() on the underlying overlayfs creates the lock chain in
reverse order:
  ovl_i_mutex_dir_key[] of lower overlay dir in vfs_rmdir()
  sb_writers of nested upper_mnt (e.g. tmpfs) in ovl_do_remove()

To rid of the unneeded locking dependency, move both ovl_lower_positive()
and ovl_check_empty_dir() to before ovl_want_write() in rmdir() and
rename() implementation.

This change spreads the pieces of ovl_check_empty_and_clear() directly
inside the rmdir()/rename() implementations so the helper is no longer
needed and removed.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index beb945e1963c..926248e1de04 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -774,6 +774,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 	struct ovl_entry *oe = dentry->d_fsdata;
 	struct ovl_entry *poe = dentry->d_parent->d_fsdata;
 	const struct qstr *name = &dentry->d_name;
+	const struct cred *old_cred;
 	unsigned int i;
 	bool positive = false;
 	bool done = false;
@@ -789,6 +790,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 	if (!ovl_dentry_upper(dentry))
 		return true;
 
+	old_cred = ovl_override_creds(dentry->d_sb);
 	/* Positive upper -> have to look up lower to see whether it exists */
 	for (i = 0; !done && !positive && i < poe->numlower; i++) {
 		struct dentry *this;
@@ -818,6 +820,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 			dput(this);
 		}
 	}
+	revert_creds(old_cred);
 
 	return positive;
 }