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

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Date:	Thu, 22 May 2008 10:47:25 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Luming Yu <luming.yu@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org,
	Roland McGrath <roland@...hat.com>
Subject: Re: [RFC PATCH] set TASK_TRACED before arch_ptrace code to fix a
	race

On Thu, 2008-05-22 at 10:47 +0800, Luming Yu wrote:
> Hello list,
> 
> The following patch is to fixed a race in ptrace_stop handling which
> causes "strace" hang if the target process blocks SIGTRAP with the
> test case filed at
> https://bugzilla.redhat.com/show_bug.cgi?id=446200#c16.
> Please note this is just IA64 problem because just IA64 has
> arch_ptrace_stop_needed defined, and has arch_ptrace_stop defined that
> would set notify_resume flags for syncing rbs...but it also opens the
> door to invoke ia64_do_signal->get_signal_to_deliver before setting
> current PTRACED flag. Please help review.
> 
> **The patch is enclosed in text attachment*
> **I'm using web client to send the patch* *

I'm inlining the patch for sake of convenience:

> 
> Signed-off-by: Yu Luming <luming.yu@...el.com>
> --------------------------------------
>  signal.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- 0/kernel/signal.c	2008-05-14 02:24:51.000000000 +0800
> +++ 1/kernel/signal.c	2008-05-22 13:54:42.000000000 +0800
> @@ -1488,6 +1488,9 @@
>  {
>  	int killed = 0;
>  
> +	/* Let the debugger run.  */
> +	__set_current_state(TASK_TRACED);
> +

That's probably not what we want. What happens if the task then sleeps
during the user-space access? Unless I forgot something obvious, it will
never get scheduled again...

Petr Tesarik

>  	if (arch_ptrace_stop_needed(exit_code, info)) {
>  		/*
>  		 * The arch code has something special to do before a
> @@ -1516,8 +1519,6 @@
>  	current->last_siginfo = info;
>  	current->exit_code = exit_code;
>  
> -	/* Let the debugger run.  */
> -	__set_current_state(TASK_TRACED);
>  	spin_unlock_irq(&current->sighand->siglock);
>  	read_lock(&tasklist_lock);
>  	if (!unlikely(killed) && may_ptrace_stop()) {

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