Subject: Re: What is ptrace flag PT_TRACESYSGOOD for?
From: "Charles P. Wright" <cwright@cs.sunysb.edu>
To: Chuck Ebbert <76306.1226@compuserve.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Roland McGrath <roland@redhat.com>, Linus Torvalds <torvalds@osdl.org>
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References: <200603140531_MC3-1-BAA0-B3C3@compuserve.com>
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Date: Tue, 14 Mar 2006 09:01:54 -0500
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On Tue, 2006-03-14 at 05:26 -0500, Chuck Ebbert wrote:
> I am trying to document PTRACE_SETOPTIONS and I can't figure out what
> the option PTRACE_O_TRACESYSGOOD is used for.  Google is no help;
> I can't find an explanation for _why_ it's there.  All I can see is that
> it causes ptrace() to deliver syscall stops with SIGTRAP | 0x80
> instead of just SIGTRAP and it can be used with PTRACE_SYSEMU.
Chuck,

The PTRACE_O_TRACESYSGOOD is useful, because it allows you to
differentiate between a standard SIGTRAP and a system call entry or
exit.  For example, if you have a ptrace monitor and receive a SIGTRAP,
without O_SYSGOOD, it isn't clear if the kernel returned from wait (1)
because someone did kill -TRAP pid, or (2) the process was entering a
system call.

Charles

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