[PATCH] simplify/fix first_tid()

first_tid:

	/* If nr exceeds the number of threads there is nothing todo */
	if (nr) {
		if (nr >= get_nr_threads(leader))
			goto done;
	}

This is not reliable: sub-threads can exit after this check, so the
'for' loop below can overlap and proc_task_readdir() can return an
already filldir'ed dirents.

	for (; pos && pid_alive(pos); pos = next_thread(pos)) {
		if (--nr > 0)
			continue;

Off-by-one error, will return 'leader' when nr == 1.

This patch tries to fix these problems and simplify the code.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6092a6e..5ee46d3 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2227,38 +2227,34 @@ int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
 static struct task_struct *first_tid(struct task_struct *leader,
                                        int tid, int nr)
 {
-       struct task_struct *pos = NULL;
+       struct task_struct *pos;
 
        rcu_read_lock();
        /* Attempt to start with the pid of a thread */
        if (tid && (nr > 0)) {
                pos = find_task_by_pid(tid);
-               if (pos && (pos->group_leader != leader))
-                       pos = NULL;
-               if (pos)
-                       nr = 0;
+               if (pos && (pos->group_leader == leader))
+                       goto found;
        }
 
        /* If nr exceeds the number of threads there is nothing todo */
-       if (nr) {
-               if (nr >= get_nr_threads(leader))
-                       goto done;
-       }
+       pos = NULL;
+       if (nr && nr >= get_nr_threads(leader))
+               goto out;
 
-       /* If we haven't found our starting place yet start with the
-        * leader and walk nr threads forward.
+       /* If we haven't found our starting place yet start
+        * with the leader and walk nr threads forward.
         */
-       if (!pos && (nr >= 0))
-               pos = leader;
-
-       for (; pos && pid_alive(pos); pos = next_thread(pos)) {
-               if (--nr > 0)
-                       continue;
-               get_task_struct(pos);
-               goto done;
+       for (pos = leader; nr > 0; --nr) {
+               pos = next_thread(pos);
+               if (pos == leader) {
+                       pos = NULL;
+                       goto out;
+               }
        }
-       pos = NULL;
-done:
+found:
+       get_task_struct(pos);
+out:
        rcu_read_unlock();
        return pos;
 }