locks: only validate the lock vs. f_mode in F_SETLK codepaths

v2: replace missing break in switch statement (as pointed out by Dave
Jones)

commit bce7560d4946 (locks: consolidate checks for compatible
filp->f_mode values in setlk handlers) introduced a regression in the
F_GETLK handler.

flock64_to_posix_lock is a shared codepath between F_GETLK and F_SETLK,
but the f_mode checks should only be applicable to the F_SETLK codepaths
according to POSIX.

Instead of just reverting the patch, add a new function to do this
checking and have the F_SETLK handlers call it.

Cc: Dave Jones <[e-mail]>
Reported-and-Tested-by: Reuben Farrelly <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>

fs/locks.c

···
 	fl->fl_ops = NULL;
 	fl->fl_lmops = NULL;
 
-	/* Ensure that fl->fl_filp has compatible f_mode */
-	switch (l->l_type) {
-	case F_RDLCK:
-		if (!(filp->f_mode & FMODE_READ))
-			return -EBADF;
-		break;
-	case F_WRLCK:
-		if (!(filp->f_mode & FMODE_WRITE))
-			return -EBADF;
-		break;
-	}
-
 	return assign_type(fl, l->l_type);
 }
 
···
 	return error;
 }
 
+/* Ensure that fl->fl_filp has compatible f_mode for F_SETLK calls */
+static int
+check_fmode_for_setlk(struct file_lock *fl)
+{
+	switch (fl->fl_type) {
+	case F_RDLCK:
+		if (!(fl->fl_file->f_mode & FMODE_READ))
+			return -EBADF;
+		break;
+	case F_WRLCK:
+		if (!(fl->fl_file->f_mode & FMODE_WRITE))
+			return -EBADF;
+	}
+	return 0;
+}
+
 /* Apply the lock described by l to an open file descriptor.
  * This implements both the F_SETLK and F_SETLKW commands of fcntl().
  */
···
 
 again:
 	error = flock_to_posix_lock(filp, file_lock, &flock);
+	if (error)
+		goto out;
+
+	error = check_fmode_for_setlk(file_lock);
 	if (error)
 		goto out;
 
···
 
 again:
 	error = flock64_to_posix_lock(filp, file_lock, &flock);
+	if (error)
+		goto out;
+
+	error = check_fmode_for_setlk(file_lock);
 	if (error)
 		goto out;