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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH] move cred_guard_mutex from task_struct to signal_struct
Cc: kosaki.motohiro@jp.fujitsu.com, Roland McGrath <roland@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        pageexec@freemail.hu,
        "Brad Spengler <spender@grsecurity.net>, Eugene Teo" 
	<eugene@redhat.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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> 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.