linux-kernel - Re: [PATCH] prevent sending wrong signals to a traced process whose tracer gets killed

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Date:	Wed, 05 Dec 2007 17:57:30 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	linux-kernel@...r.kernel.org
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] prevent sending wrong signals to a traced process whose
 tracer gets killed

Petr Tesarik wrote:
> Hi,
> 
> I experienced troubles when tracing a process with strace. Sometimes,
> when I killed the strace process (SIGKILL), the traced process was also
> killed. I found out that it was getting SIGTRAP and, indeed, when the
> traced process set up a signal handler for SIGTRAP, it no longer died.
> 
> I noticed that normally, when the traced process is continued (via
> PTRACE_CONT or similar), the signal to be sent to it is stored in
> current->exit_code, which is then examined by the arch-specific code and
> usually leads to something like:
> 
>                 send_sig(current->exit_code, current, 1);
> 
> The exit_code is set in ptrace_stop(), but the tracing process may go
> away while the traced process waits for it, and in that case exit_code
> is left as-is. I think we must set it to zero in ptrace_untrace().

My patch was very wrong, but at least I produced a test case. It fails
on all systems I could test. The only correct solution (TM) could be
achieved if we could tell the tracee in ptrace_stop() that the tracer
actually died and it should use nostop_code instead of exit_code.
Possibly a new flag?

Regards,
Petr Tesarik


View attachment "tracerkill.c" of type "text/x-csrc" (3891 bytes)