readlinkat(), fchownat() and fstatat() with empty relative pathnames

For readlinkat() we simply allow empty pathname; it will fail unless
we have dfd equal to O_PATH-opened symlink, so we are outside of
POSIX scope here.  For fchownat() and fstatat() we allow AT_EMPTY_PATH;
let the caller explicitly ask for such behaviour.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -573,13 +573,15 @@ SYSCALL_DEFINE5(fchownat, int, dfd, const char __user *, filename, uid_t, user,
 {
 	struct path path;
 	int error = -EINVAL;
-	int follow;
+	int lookup_flags;
 
-	if ((flag & ~AT_SYMLINK_NOFOLLOW) != 0)
+	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
 		goto out;
 
-	follow = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
-	error = user_path_at(dfd, filename, follow, &path);
+	lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
+	if (flag & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (error)
 		goto out;
 	error = mnt_want_write(path.mnt);

fs/stat.c

@@ -75,13 +75,16 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
 	int error = -EINVAL;
 	int lookup_flags = 0;
 
-	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT)) != 0)
+	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
+		      AT_EMPTY_PATH)) != 0)
 		goto out;
 
 	if (!(flag & AT_SYMLINK_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
 	if (flag & AT_NO_AUTOMOUNT)
 		lookup_flags |= LOOKUP_NO_AUTOMOUNT;
+	if (flag & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
 
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (error)
@@ -297,7 +300,7 @@ SYSCALL_DEFINE4(readlinkat, int, dfd, const char __user *, pathname,
 	if (bufsiz <= 0)
 		return -EINVAL;
 
-	error = user_path_at(dfd, pathname, 0, &path);
+	error = user_path_at(dfd, pathname, LOOKUP_EMPTY, &path);
 	if (!error) {
 		struct inode *inode = path.dentry->d_inode;