[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>

1 files changed, 16 insertions, 13 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c7f855441573..f0db7f616ac3 100644
--- a/fs/proc/base.c
+++ b/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;