[PATCH v2 04/16] overlayfs: Document critical override_creds() operations

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Add a comment to these operations that cannot use the _light version
of override_creds()/revert_creds(), because during the critical
section the struct cred .usage counter might be modified.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 fs/overlayfs/copy_up.c | 6 +++++-
 fs/overlayfs/dir.c     | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index a5ef2005a2cc..7dec275e08cd 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -718,8 +718,12 @@ static int ovl_prep_cu_creds(struct dentry *dentry, struct ovl_cu_creds *cc)
 	if (err < 0)
 		return err;
 
-	if (cc->new)
+	if (cc->new) {
+		/* Do not use the _light version, the credentials
+		 * ->usage might be modified.
+		 */
 		cc->old = override_creds(cc->new);
+	}
 
 	return 0;
 }
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index ab65e98a1def..851945904385 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -584,6 +584,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 	const struct cred *old_cred;
 	struct dentry *parent = dentry->d_parent;
 
+	/* Do not use the _light version, cred->usage might be modified */
 	old_cred = ovl_override_creds(dentry->d_sb);
 
 	/*
@@ -1159,6 +1160,7 @@ static int ovl_rename(struct mnt_idmap *idmap, struct inode *olddir,
 			goto out;
 	}
 
+	/* Do not use the _light version, cred->usage might be modified */
 	old_cred = ovl_override_creds(old->d_sb);
 
 	if (!list_empty(&list)) {
@@ -1314,6 +1316,7 @@ static int ovl_create_tmpfile(struct file *file, struct dentry *dentry,
 	int flags = file->f_flags | OVL_OPEN_FLAGS;
 	int err;
 
+	/* Do not use the _light version, cred->usage might be modified */
 	old_cred = ovl_override_creds(dentry->d_sb);
 	err = ovl_setup_cred_for_create(dentry, inode, mode, old_cred);
 	if (err)
-- 
2.46.0