linux-kernel - Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

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Date:	Fri, 18 Dec 2009 03:10:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	utrace-devel@...hat.com
Subject: Re: x86: do_debug && PTRACE_SINGLESTEP broken by
	08d68323d1f0c34452e614263b212ca556dae47f

On 12/17, Roland McGrath wrote:
>
> Comparing to the old (2.6.32) logic, I think it might be this (untested).
> I also note this is the sole use of get_si_code, seems like it should
> just be rolled in here.

Well, it is too late for me to even try to read this patch ;)

but...

> @@ -569,14 +568,15 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
>  	 * We already checked v86 mode above, so we can check for kernel mode
>  	 * by just checking the CPL of CS.
>  	 */
> +	dr6 = tsk->thread.debugreg6;

why? we have "tsk->thread.debugreg6 = dr6" above

>  	if ((dr6 & DR_STEP) && !user_mode(regs)) {
>  		tsk->thread.debugreg6 &= ~DR_STEP;
>  		set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
>  		regs->flags &= ~X86_EFLAGS_TF;

this looks strange... we set TIF_SINGLESTEP but clear X86_EFLAGS_TF

> +	} else if (dr6 & (DR_STEP | DR_TRAP_BITS)) {
> +		send_sigtrap(tsk, regs, error_code, get_si_code(dr6));
>  	}
> -	si_code = get_si_code(tsk->thread.debugreg6);
> -	if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS))
> -		send_sigtrap(tsk, regs, error_code, si_code);
> +

can't understand how this change can fix the problem. We should always
send SIGTRAP if the task returns to user-mode with X86_EFLAGS_TF?

OK. I blindly applied this patch, step-simple still fails.

Oleg.

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