file: simplify logic in __close_range()

It never looked too pleasant and it doesn't really buy us anything
anymore now that CLOSE_RANGE_CLOEXEC exists and we need to retake the
current maximum under the lock for it anyway. This also makes the logic
easier to follow.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Giuseppe Scrivano <gscrivan@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/file.c

@@ -701,7 +701,6 @@ static inline void __range_close(struct files_struct *cur_fds, unsigned int fd,
  */
 int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
 {
-	unsigned int cur_max;
 	struct task_struct *me = current;
 	struct files_struct *cur_fds = me->files, *fds = NULL;
 
@@ -711,26 +710,26 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
 	if (fd > max_fd)
 		return -EINVAL;
 
-	rcu_read_lock();
-	cur_max = files_fdtable(cur_fds)->max_fds;
-	rcu_read_unlock();
-
-	/* cap to last valid index into fdtable */
-	cur_max--;
-
 	if (flags & CLOSE_RANGE_UNSHARE) {
 		int ret;
 		unsigned int max_unshare_fds = NR_OPEN_MAX;
 
 		/*
-		 * If the requested range is greater than the current maximum,
-		 * we're closing everything so only copy all file descriptors
-		 * beneath the lowest file descriptor.
-		 * If the caller requested all fds to be made cloexec copy all
-		 * of the file descriptors since they still want to use them.
+		 * If the caller requested all fds to be made cloexec we always
+		 * copy all of the file descriptors since they still want to
+		 * use them.
+		 */
+		if (!(flags & CLOSE_RANGE_CLOEXEC)) {
+			/*
+			 * If the requested range is greater than the current
+			 * maximum, we're closing everything so only copy all
+			 * file descriptors beneath the lowest file descriptor.
 			 */
-		if (!(flags & CLOSE_RANGE_CLOEXEC) && (max_fd >= cur_max))
+			rcu_read_lock();
+			if (max_fd >= last_fd(files_fdtable(cur_fds)))
 				max_unshare_fds = fd;
+			rcu_read_unlock();
+		}
 
 		ret = unshare_fd(CLONE_FILES, max_unshare_fds, &fds);
 		if (ret)
@@ -744,8 +743,6 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
 			swap(cur_fds, fds);
 	}
 
-	max_fd = min(max_fd, cur_max);
-
 	if (flags & CLOSE_RANGE_CLOEXEC)
 		__range_cloexec(cur_fds, fd, max_fd);
 	else