Date: Fri, 22 May 2015 12:00:08 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: David Long <dave.long@linaro.org>
Cc: "Jon Medhurst (Tixy)" <tixy@linaro.org>,
        Steve Capper <steve.capper@linaro.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        sandeepa.s.prabhu@gmail.com,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Russell King <linux@arm.linux.org.uk>,
        William Cohen <wcohen@redhat.com>, davem@davemloft.net,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 3/6] arm64: Kprobes with single stepping support
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On Thu, May 21, 2015 at 12:44:45AM -0400, David Long wrote:
> On 05/20/15 12:39, Catalin Marinas wrote:
> >On Mon, Apr 20, 2015 at 04:19:44PM -0400, David Long wrote:
> >>Add support for basic kernel probes(kprobes) and jump probes
> >>(jprobes) for ARM64.
> >>
> >>Kprobes utilizes software breakpoint and single step debug
> >>exceptions supported on ARM v8.
> >>
> >>A software breakpoint is placed at the probe address to trap the
> >>kernel execution into the kprobe handler.
> >>
> >>ARM v8 supports enabling single stepping before the break exception
> >>return (ERET), with next PC in exception return address (ELR_EL1). The
> >>kprobe handler prepares an executable memory slot for out-of-line
> >>execution with a copy of the original instruction being probed, and
> >>enables single stepping. The PC is set to the out-of-line slot address
> >>before the ERET. With this scheme, the instruction is executed with the
> >>exact same register context except for the PC (and DAIF) registers.
> >
> >I wonder whether it would be simpler to use another software breakpoint
> >after the out of line instruction copy. You won't run the instructions
> >that change the PC anyway.
> 
> We put quite a bit of work into making single-step work.  I don't see any
> obvious advantage to trying to switch to a software breakpoint. Both are
> debug exceptions but SS does leave open the possibility of maybe eventually
> running some instructions that do change the PC.

I'm not trying to get this re-written, just as a potential workaround
for the unexpected single-step error reported but I need to read some
more before I understand it properly (and I can see patches already for
fixing this).

> >Since an unconditional branch instruction within the kernel address
> >space can reach any point in the kernel (and modules), could we go a
> >step further and avoid the software breakpoint altogether, just generate
> >a branch instruction to the original location (after the software
> >breakpoint)?
> 
> Wouldn't a branch instruction have to make use of a register in order to
> span the whole address space?  How could you do that and have all the
> registers unmolested when you land back after the original probe point?  The
> thing that really kills this though is the fact we need to be able to run
> the pre and post functions before and *after* the XOL stepping.

A "b #imm" would be able to cover a wide range (+/-128MB), but, as you
said, it doesn't help with the post function call.

Any plans to post an updated version with the "unexpected single-step
error" fixed?

-- 
Catalin
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