[PATCH 27/35] union-mount: Implement union-aware access()/faccessat()

[Valerie Aurora <vaurora@redhat.com>]

For union mounts, a file located on the lower layer will incorrectly
return EROFS on an access check.  To fix this, use the new
path_permission() call, which ignores a read-only lower layer file
system if the target will be copied up to the topmost file system.
---
 fs/open.c |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index e17f544..686fcd2 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -454,7 +454,10 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 	const struct cred *old_cred;
 	struct cred *override_cred;
 	struct path path;
+	struct nameidata nd;
+	struct vfsmount *mnt;
 	struct inode *inode;
+	char *tmp;
 	int res;
 
 	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
@@ -478,10 +481,17 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 
 	old_cred = override_creds(override_cred);
 
-	res = user_path_at(dfd, filename, LOOKUP_FOLLOW, &path);
+	res = user_path_nd(dfd, filename, LOOKUP_FOLLOW,
+				   &nd, &path, &tmp);
 	if (res)
 		goto out;
 
+	/* For union mounts, use the topmost mnt's permissions */
+	if (IS_UNIONED_DIR(&nd.path))
+		mnt = nd.path.mnt;
+	else
+		mnt = path.mnt;
+
 	inode = path.dentry->d_inode;
 
 	if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
@@ -490,11 +500,11 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 		 * with the "noexec" flag.
 		 */
 		res = -EACCES;
-		if (path.mnt->mnt_flags & MNT_NOEXEC)
+		if (mnt->mnt_flags & MNT_NOEXEC)
 			goto out_path_release;
 	}
 
-	res = inode_permission(inode, mode | MAY_ACCESS);
+	res = path_permission(&path, &nd.path, mode | MAY_ACCESS);
 	/* SuS v2 requires we report a read only fs too */
 	if (res || !(mode & S_IWOTH) || special_file(inode->i_mode))
 		goto out_path_release;
@@ -508,11 +518,13 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 	 * inherently racy and know that the fs may change
 	 * state before we even see this result.
 	 */
-	if (__mnt_is_readonly(path.mnt))
+	if (__mnt_is_readonly(mnt))
 		res = -EROFS;
 
 out_path_release:
 	path_put(&path);
+	path_put(&nd.path);
+	putname(tmp);
 out:
 	revert_creds(old_cred);
 	put_cred(override_cred);
-- 
1.6.3.3