[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>

1 files changed, 17 insertions, 21 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6092a6e2c5a9..5ee46d3a5cac 100644
--- 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;
 }