[PATCH] proc: optimize proc_check_dentry_visible

The code doesn't need to sleep to when making this check so I can just do the
comparison and not worry about the reference counts.

TODO: While looking at this I realized that my original cleanup did not push
the permission check far enough down into the stack.  The call of
proc_check_dentry_visible needs to move out of the generic proc
readlink/follow link code and into the individual get_link instances.
Otherwise the shared resources checks are not quite correct (shared
files_struct does not require a shared fs_struct), and there are races with
unshare.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/base.c

@@ -1074,24 +1074,27 @@ static int proc_check_dentry_visible(struct inode *inode,
 	 * namespace, or are simply process local (like pipes).
 	 */
 	struct task_struct *task;
-	struct files_struct *task_files, *files;
 	int error = -EACCES;
 
 	/* See if the the two tasks share a commone set of
 	 * file descriptors.  If so everything is visible.
 	 */
-	task = get_proc_task(inode);
-	if (!task)
-		goto out;
-	files = get_files_struct(current);
-	task_files = get_files_struct(task);
-	if (files && task_files && (files == task_files))
-		error = 0;
-	if (task_files)
-		put_files_struct(task_files);
-	if (files)
-		put_files_struct(files);
-	put_task_struct(task);
+	rcu_read_lock();
+	task = tref_task(proc_tref(inode));
+	if (task) {
+		struct files_struct *task_files, *files;
+		/* This test answeres the question:
+		 * Is there a point in time since we looked up the
+		 * file descriptor where the two tasks share the
+		 * same files struct?
+		 */
+		rmb();
+		files = current->files;
+		task_files = task->files;
+		if (files && (files == task_files))
+			error = 0;
+	}
+	rcu_read_unlock();
 	if (!error)
 		goto out;