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

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Date:	Sun, 16 Mar 2008 15:31:38 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...sign.ru>
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

> 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.

> 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();

We are on the way soon to having no arch callers of ptrace_get_task_struct
left, so only the two kernel/ptrace.c uses will survive.  (x86 and ia64
switchovers to arch_ptrace/compat_arch_ptrace are already in the pipeline,
and maybe s390 too.)  So let's worry about the cleanup removing this
function once those wither away.

Thanks,
Roland
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