Re: [PATCH] uprobes: don't enable/disable signle step if the userdid it

[Sebastian Andrzej Siewior]

On 07/26/2012 07:31 PM, Oleg Nesterov wrote:
> 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.

Why? Shouldn't wee keep the trap flag if the instruction on which we
placed the uprobe activates it?

> 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.

It is not defined on every arch but I wouldn't say it is 86 specific.
From the architectures which have user_enable_single_step() defined I
see

 avr32	TIF_SINGLE_STEP
 m68k	TIF_DELAYED_TRACE
 s390	TIF_SINGLE_STEP

which means those three could rename their flag so things are
consistent. The remaining architectures are

 alpha
 cris
 h8300
 score

and they don't set a flag and it seems they change the register
directly.

> > @@ -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.

Sending SIGTRAP is, yes.

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

Sebastian
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