linux-kernel - Re: [PATCH] uprobes: don't enable/disable signle step if the user did it

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Date:	Thu, 26 Jul 2012 19:31:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] uprobes: don't enable/disable signle step if the user
	did it

Well. I agree, this needs changes. To begin with, uprobe should avoid
user_enable_single_step() which does access_process_vm(). And I suspect
uprobes have the problems with TIF_FORCED_TF logic.

But I am not sure about this patch...

On 07/26, Sebastian Andrzej Siewior wrote:
>
> @@ -1528,7 +1528,10 @@ static void handle_swbp(struct pt_regs *regs)
>
>  	utask->state = UTASK_SSTEP;
>  	if (!pre_ssout(uprobe, regs, bp_vaddr)) {
> -		user_enable_single_step(current);
> +		if (test_tsk_thread_flag(current, TIF_SINGLESTEP))
> +			uprobe->flags |= UPROBE_USER_SSTEP;
> +		else
> +			user_enable_single_step(current);

This is x86 specific, TIF_SINGLESTEP is not defined on every arch.

> @@ -1569,7 +1572,10 @@ static void handle_singlestep(struct uprobe_task *utask, struct pt_regs *regs)
>  	put_uprobe(uprobe);
>  	utask->active_uprobe = NULL;
>  	utask->state = UTASK_RUNNING;
> -	user_disable_single_step(current);
> +	if (uprobe->flags & UPROBE_USER_SSTEP)
> +		uprobe->flags &= ~UPROBE_USER_SSTEP;
> +	else
> +		user_disable_single_step(current);

This is not enough (and I am not sure this is portable).

If SINGLESTEP was set, we should send SIGTRAP here. With this patch
we return with X86_EFLAGS_TF set, gdb will be notified only after the
next insn. And if we notify gdb, there is no need to keep X86_EFLAGS_TF.

I'm afraid this needs more thinking and new arch-dependant helpers.

Oleg.

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