linux-kernel - Re: [RFC PATCH] set TASK_TRACED before arch

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Date:	Tue,  3 Jun 2008 14:01:25 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	Luming Yu <luming.yu@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org
Subject: Re: [RFC PATCH] set TASK_TRACED before arch_ptrace code to fix a
 race

> It's definitely a bug in strace. For some reason (I don't care about)
> the execve() syscall produces an extra notification. However, this
> notification message is suppressed when SIGTRAP is blocked. This
> explains why the test case fails only when SIGTRAP is blocked.

What do you mean by "extra"?  There is a SIGTRAP sent after execve
completes when ptraced, even when PTRACE_SYSCALL is not being used.
So for an execve that succeeds under PTRACE_SYSCALL, there is a
ptrace_notify at syscall entry, then a SIGTRAP queued (i.e., not seen
by ptrace if blocked), then a ptrace_notify at syscall exit.  If
that's what's happening (including the blocked SIGTRAP not being seen
by the ptracer, i.e. strace), then there is no mystery (and no bug).

Thanks,
Roland
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