linux-kernel - Re: [PATCH] Revert "kprobes: Warn if optprobe handler tries to change execution path"

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Message-Id: <20171023135819.g5yo3x65vkekshvk@naverao1-tp.localdomain>
Date:   Mon, 23 Oct 2017 19:28:19 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "kprobes: Warn if optprobe handler tries to
 change execution path"

On 2017/10/19 09:38PM, Masami Hiramatsu wrote:
> On Tue, 17 Oct 2017 13:48:34 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
> > This reverts commit e863d539614641 ("kprobes: Warn if optprobe handler
> > tries to change execution path").
> > 
> > On powerpc, we place a probe at kretprobe_trampoline to catch function
> > returns and with CONFIG_OPTPROBES, this probe gets optimized. This
> > works for us due to the way we handle the optprobe as described in
> > commit 762df10bad6954 ("powerpc/kprobes: Optimize kprobe
> > in kretprobe_trampoline()"). With the above commit, we end up with a
> > warning. As such, revert this change.
> 
> OK, so for ppc, optprobe can handle nip change.
> 
> Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> 
> I'll update the document too.

Thanks. We don't quite handle the nip change (yet), but just work around 
it for kretprobe trampoline by using the link register, and issuing a 
'blr' after the trampoline code.

Regards,
Naveen