linux-kernel - Re: [PATCH 5/5] ptrace: it is fun to strace /sbin/init

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Date:	Mon, 17 Mar 2008 02:17:51 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>,
	Laurent Riffard <laurent.riffard@...e.fr>,
	Pavel Emelyanov <xemul@...nvz.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] ptrace: it is fun to strace /sbin/init

On 03/16, Roland McGrath wrote:
>
> > Ptracing of /sbin/init is not allowed. Of course, this is very dangerous, but
> > may be useful. Introduce the kernel boot parameter to allow this.
> 
> Personally I wouldn't mind losing all the ptrace/signals special cases for
> init.  (Just don't have a buggy init and expect not to crash, don't be root
> and kill init, etc.)  So this is fine by me.  The conservative route of
> changing it only with a boot option is the wise thing to do.

Great.

> > Unless I missed something, ptrace_get_task_struct() is pointless. It does not
> > need to check "pid == 1", ptrace_attach() does this. It doesn't need tasklist.
> 
> Agreed.  It's a hold-over from when there was more hair in there.
> 
> > It should be replaced with the generic find_get_task_by_vpid() which does not
> > exist yet.
> 
> I didn't see enough other uses to really warrant it.  Most
> find_task_by_vpid calls don't actually do get_task_struct.
> Those that do want to do some other check inside rcu_read_lock
> before deciding to bother with get_task_struct anyway.
> So there is nothing wrong with ptrace just open-coding:
> 
> 	rcu_read_lock();
> 	child = find_task_by_vpid(pid);
> 	if (child)
> 		get_task_struct(child);
> 	rcu_read_unlock();

proc_task_lookup(), fill_pid(), attach_task_by_pid(), can use the new helper.

But yes sure, we can open-code this.

Oleg.

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