linux-kernel - Re: [PATCH] man ptrace: add extended description of various ptrace quirks

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Message-ID: <20110721165137.GA21345@redhat.com>
Date:	Thu, 21 Jul 2011 18:51:37 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	mtk.manpages@...il.com, Jan Kratochvil <jan.kratochvil@...hat.com>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] man ptrace: add extended description of various ptrace
	quirks

On 07/21, Denys Vlasenko wrote:
>
> Deleted several outright false statements:
> - pid 1 can be traced
> - tracer is not shown as parent in ps output
> - PTRACE_ATTACH is not "the same behavior as if tracee had done
>   a PTRACE_TRACEME": PTRACE_ATTACH delivers a SIGSTOP.
> - SIGSTOP _can_ be injected.

Yes, this is correct, thanks.

> +Tracer can not assume that tracee ALWAYS ends its life by reporting
> +WIFEXITED(status) or WIFSIGNALED(status).
> +.LP
> +.\" or can it? Do we include such a promise into ptrace API?

IIRC, we already discussed this... The traced group leader can
disappear during mt-exec, otherwise the tracee can never go away
silently.

> +Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0). This
> +operation is deprecated, use kill(SIGKILL) or tgkill(SIGKILL) instead.
> +The problem with this operation is that it requires tracee to be in
> +signal-delivery-stop, otherwise it may not work (may complete
> +successfully but won't kill the tracee),

In short, ptrace(PTRACE_KILL) is more or less ptrace(PTRACE_CONT, SIGKILL),
but it always returns 0. IOW, it never worked as decribed in the man
page. And I guess today nobody can explain why PTRACE_KILL exists.

Oleg.

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