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Date: Thu, 22 Jun 2017 23:14:11 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>, Ananth N
 Mavinakayanahalli <ananth@linux.vnet.ibm.com>, Masami Hiramatsu
 <mhiramat@kernel.org>, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3 4/6] powerpc/64s: Un-blacklist system_call() from
 kprobes
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On Thu, 22 Jun 2017 21:14:21 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Nicholas Piggin <npiggin@gmail.com> writes:
> 
> > On Thu, 22 Jun 2017 00:08:40 +0530
> > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> >  
> >> It is actually safe to probe system_call() in entry_64.S, but only till
> >> we unset MSR_RI. To allow this, add a new symbol system_call_exit()
> >> after the mtmsrd and blacklist that. Though the mtmsrd instruction
> >> itself is now whitelisted, we won't be allowed to probe on it as we
> >> don't allow probing on rfi and mtmsr instructions (checked for in
> >> arch_prepare_kprobe()).  
> >
> > Can you add a little comment to say probes aren't allowed, and it's
> > located after the mtmsr in order to avoid contaminating traces?
> >
> > Also I wonder if a slightly different name would be more instructive?
> > I don't normally care, but the system_call_common code isn't trivial
> > to follow. system_call_exit might give the impression that it is the
> > entire exit path (which would pair with system_call for entry).  
> 
> It is the entire path in the happy case isn't it? I'm not sure I know
> what you mean.

Oh, yes you're right, I thought it was moved down further.

Thanks,
Nick