locks: require that flock->l_pid be set to 0 for file-private locks

Neil Brown suggested potentially overloading the l_pid value as a "lock
context" field for file-private locks. While I don't think we will
probably want to do that here, it's probably a good idea to ensure that
in the future we could extend this API without breaking existing
callers.

Typically the l_pid value is ignored for incoming struct flock
arguments, serving mainly as a place to return the pid of the owner if
there is a conflicting lock. For file-private locks, require that it
currently be set to 0 and return EINVAL if it isn't. If we eventually
want to make a non-zero l_pid mean something, then this will help ensure
that we don't break legacy programs that are using file-private locks.

Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/locks.c

@@ -1931,6 +1931,10 @@ int fcntl_getlk(struct file *filp, unsigned int cmd, struct flock __user *l)
 		goto out;
 
 	if (cmd == F_GETLKP) {
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_GETLK;
 		file_lock.fl_flags |= FL_FILE_PVT;
 		file_lock.fl_owner = (fl_owner_t)filp;
@@ -2062,11 +2066,19 @@ again:
 	 */
 	switch (cmd) {
 	case F_SETLKP:
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_SETLK;
 		file_lock->fl_flags |= FL_FILE_PVT;
 		file_lock->fl_owner = (fl_owner_t)filp;
 		break;
 	case F_SETLKPW:
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_SETLKW;
 		file_lock->fl_flags |= FL_FILE_PVT;
 		file_lock->fl_owner = (fl_owner_t)filp;
@@ -2121,6 +2133,10 @@ int fcntl_getlk64(struct file *filp, unsigned int cmd, struct flock64 __user *l)
 		goto out;
 
 	if (cmd == F_GETLKP) {
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_GETLK64;
 		file_lock.fl_flags |= FL_FILE_PVT;
 		file_lock.fl_owner = (fl_owner_t)filp;
@@ -2185,11 +2201,19 @@ again:
 	 */
 	switch (cmd) {
 	case F_SETLKP:
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_SETLK64;
 		file_lock->fl_flags |= FL_FILE_PVT;
 		file_lock->fl_owner = (fl_owner_t)filp;
 		break;
 	case F_SETLKPW:
+		error = -EINVAL;
+		if (flock.l_pid != 0)
+			goto out;
+
 		cmd = F_SETLKW64;
 		file_lock->fl_flags |= FL_FILE_PVT;
 		file_lock->fl_owner = (fl_owner_t)filp;