Re: [PATCH] move cred_guard_mutex from task_struct to signal_struct

[KOSAKI Motohiro <kosaki.motohiro () jp fujitsu com>]

> On 09/10, KOSAKI Motohiro wrote:
> > 1) moving cread_guard_mutex itself
> >    - no increase execve overhead
> >     -> very good
> >    - it also prevent parallel ptrace
>
> No, it doesn't. Only PTRACE_ATTACH needs this mutex, and as Roland
> pointed out it also needs write_lock(tasklist) which is worse. So
> this change doesn't make any practical harm for ptrace.

I see, thanks.

> > 2) move in_exec_mm to signal_struct too
> >    -> very hard. oom-killer can use very few lock because it's called
> >       from various place. now both ->mm and ->in_exec_mm are protected
> >       task_lock() and it help to avoid messy.
>
> Yes. But, if ->in_exec_mm is only used by oom_badness(), then I think
> you can use task_lock(tsk->group_leader). oom_badness() needs tasklist
> anyway, this means it can't race with de_thread() changing the leader.
> But up to you.

Good idea. will fix.

> Another very minor nit (but again, up to you). Perhaps exec_mmap()
> could clear ->in_exec_mm (in task_struct or signal_struct, this doesnt
> matter), it takes task_lock(current) anyway (and at this point current
> is always the group leader).

Thanks. will fix.


> > Let's move ->cred_guard_mutex from task_struct to signal_struct. It
> > naturally prevent multiple-threads-inside-exec.
>
> Reviewed-by: Oleg Nesterov <oleg () redhat com>
>
>
> This is very minor, but perhaps you can also fix a couple of comments
> which mention task->cred_guard_mutex,
>
>       fs/exec.c:1109          the caller must hold current->cred_guard_mutex
>       kernel/cred.c:328       The caller must hold current->cred_guard_mutex
>       include/linux/tracehook.h:153   @task->cred_guard_mutex

Will fix, of cource.