linux-kernel - Re: ptrace.2: PTRACE_KILL needs a stopped process too

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Date:	Sun, 22 Apr 2012 22:04:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	pacman@...h.dhis.org, linux-man <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Pedro Alves <palves@...hat.com>
Subject: Re: ptrace.2: PTRACE_KILL needs a stopped process too

On 04/23, Michael Kerrisk (man-pages) wrote:
>
> [widening CC]

add more CC's

> The man page says "For requests other than PTRACE_KILL,

Argh, PTRACE_KILL again.

You know, I simply do not know what it was supposed to do. I can only
see what the code actually does.

> the child process
> must be stopped."

Yes and no.

Yes, ptrace(PTRACE_KILL) "succeeds" even if the tracee is not stopped.

No, it has no effect if the tracee is not stopped.

All I can say is: PTRACE_KILL should never exist. If you want to kill
the tracee, you can do kill(SIGKILL).

Roughly, ptrace(PTRACE_KILL) is equal to ptrace(PTRACE_CONT, SIGKILL)
except it always returns 0.

> If the man page is describing actual intended kernel behavior, then it's a
> fairly long-standing kernel bug.

Perhaps. May be it should simply do kill(SIGKILL), but then it is not
clear why do we have PTRACE_KILL. And once again, I was never able to
understand the supposed behaviour.

Personally, I think we should fix the documentation. And imho the only
possible fix is to add this note: do not ever use PTRACE_KILL.

Oleg.

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