Oleg Nesterov <[email protected]> writes:
> Eric W. Biederman wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -699,7 +699,17 @@ static int de_thread(struct task_struct
>> remove_parent(current);
>> remove_parent(leader);
>>
>> - switch_exec_pids(leader, current);
>> +
>> + /* Become a process group leader with the old leader's pid.
>> + * Note: The old leader also uses thispid until release_task
>> + * is called. Odd but simple and correct.
>> + */
>> + detach_pid(current, PIDTYPE_PID);
>> + current->pid = leader->pid;
>> + attach_pid(current, PIDTYPE_PID, current->pid);
>
> What happens after de_thread() unlocks tasklist_lock and before
> it is taken again in release_task() ?
>
> In that window find_task_by_pid() will return dead leader, not
> the new leader of thread group. This means we can miss tkill()
> or ptrace(), for example.
A reasonable concern. For some reason I had it in my
head that find_pid didn't need that tasklist_lock
but it does. kthread and vmscan need to be fixed.
All I have done is enlarged the window where this
race is possible. So for tkill I am not concerned,
as it wants a particular thread. Nor am I concerned
about anything else that wants a particular thread.
The fact that the group_leader does not point
at the actual thread group leader might be a problem,
as I have opened a window where that is now the case.
For signals that is not a problem as signals are still shared.
This applies to most other resources as well.
Looking through all of the callers of find_task_by_pid
I couldn't see anything that looks like it cares.
So until we spot that case I'm ready to put this down
of one of those cases in de_thread that looks wrong
but happens to work. Now if there is a way to make
it work more cleanly that may be worth looking at.
Eric
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