Messages in this thread | | | Date | Thu, 7 Sep 2006 02:25:56 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] proc: readdir race fix (take 3) |
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On 09/06, Jean Delvare wrote: > > On Wednesday 6 September 2006 11:01, Jean Delvare wrote: > > Eric, Kame, thanks a lot for working on this. I'll be giving some good > > testing to this patch today, and will return back to you when I'm done. > > The original issue is indeed fixed, but there's a problem with the patch. > When stressing /proc (to verify the bug was fixed), my test machine ended > up crashing. Here are the 2 traces I found in the logs: > > Sep 6 12:06:00 arrakis kernel: BUG: warning at > kernel/fork.c:113/__put_task_struct() > Sep 6 12:06:00 arrakis kernel: [<c0115f93>] __put_task_struct+0xf3/0x100 > Sep 6 12:06:00 arrakis kernel: [<c019666a>] proc_pid_readdir+0x13a/0x150 > Sep 6 12:06:00 arrakis kernel: [<c01745f0>] vfs_readdir+0x80/0xa0 > Sep 6 12:06:00 arrakis kernel: [<c0174750>] filldir+0x0/0xd0 > Sep 6 12:06:00 arrakis kernel: [<c017488c>] sys_getdents+0x6c/0xb0 > Sep 6 12:06:00 arrakis kernel: [<c0174750>] filldir+0x0/0xd0 > Sep 6 12:06:00 arrakis kernel: [<c0102fb7>] syscall_call+0x7/0xb
I think there is a bug in next_tgid(),
> -static struct task_struct *next_tgid(struct task_struct *start) > -{ > - struct task_struct *pos; > + task = NULL; > rcu_read_lock(); > - pos = start; > - if (pid_alive(start)) > - pos = next_task(start); > - if (pid_alive(pos) && (pos != &init_task)) { > - get_task_struct(pos); > - goto done; > +retry: > + pid = find_ge_pid(tgid); > + if (pid) { > + tgid = pid->nr + 1; > + task = pid_task(pid, PIDTYPE_PID); > + /* What we to know is if the pid we have find is the > + * pid of a thread_group_leader. Testing for task > + * being a thread_group_leader is the obvious thing > + * todo but there is a window when it fails, due to > + * the pid transfer logic in de_thread. > + * > + * So we perform the straight forward test of seeing > + * if the pid we have found is the pid of a thread > + * group leader, and don't worry if the task we have > + * found doesn't happen to be a thread group leader. > + * As we don't care in the case of readdir. > + */ > + if (!task || !has_group_leader_pid(task)) > + goto retry; > + get_task_struct(task); > } > - pos = NULL; > -done: > rcu_read_unlock(); > - put_task_struct(start); > - return pos; > + return task; > }
If the task found is not a group leader, we go to retry, but the task != NULL.
Now, if find_ge_pid(tgid) returns NULL, we return that wrong task, and it was not get_task_struct()'ed.
Oleg.
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