linux-kernel - Re: [PATCH v2 4/4] uprobes/powerpc: Make use of generic routines to enable single step

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Date:	Tue, 18 Dec 2012 11:06:03 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Suzuki K. Poulose" <suzuki@...ibm.com>, bigeasy@...utronix.de,
	srikar@...ux.vnet.ibm.com, peterz@...radead.org,
	benh@...nel.crashing.org, mingo@...e.hu, anton@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] uprobes/powerpc: Make use of generic routines to
	enable single step

On Fri, Dec 14, 2012 at 09:02:41PM +0100, Oleg Nesterov wrote:
> On 12/03, Suzuki K. Poulose wrote:
> >
> > Replace the ptrace helpers with the powerpc generic routines to
> > enable/disable single step. We save/restore the MSR (and DCBR for BookE)
> > across for the operation. We don't have to disable the single step,
> > as restoring the MSR/DBCR would restore the previous state.
> 
> Obviously I can't review this series (although it looks fine to me).
> 
> Just one note,
> 
> > @@ -121,7 +132,7 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> >
> >  	WARN_ON_ONCE(current->thread.trap_nr != UPROBE_TRAP_NR);
> >
> > -	uprobe_restore_context_sstep(&utask->autask);
> > +	uprobe_restore_context_sstep(&utask->autask, regs);
> 
> I am not sure ppc needs this, but note that x86 does a bit more.
> 
> Not only we need to restore the "single-step" state, we need to
> send SIGTRAP if it was not set by us. The same for _skip_sstep.

Do you mean restoring the TF equivalent on powerpc to what it was before?

If so, powerpc has always been unique in this aspect -- the single-step
exception handler *always* resets the sstep bit in MSR. Any user needing
to continue single-stepping has to explicitly set it again.

Ananth

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