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 20:10:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ananth N Mavinakayanahalli <ananth@...ibm.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 12/18, Ananth N Mavinakayanahalli wrote:
>
> On Fri, Dec 14, 2012 at 09:02:41PM +0100, Oleg Nesterov wrote:
> > >
> > > -	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.

I meant another thing.

Suppose that, say, gdb tries to single-step over the probed insn.
In this case we need to send SIGTRAP after xol/emulate. Please look at
send_sig(SIGTRAP) in arch/x86/kernel/uprobes.c:arch_uprobe_post_xol()
and arch_uprobe_skip_sstep().

Oleg.

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