linux-kernel - Re: [PATCH v2] On ppc64le we HAVE_RELIABLE

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Message-ID: <20180312153536.7avozx64ku4lvd3e@treble>
Date:   Mon, 12 Mar 2018 10:35:36 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Torsten Duwe <duwe@....de>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Jiri Kosina <jkosina@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v2] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Fri, Mar 09, 2018 at 05:47:18PM +0100, Torsten Duwe wrote:
> On Thu, 8 Mar 2018 10:26:16 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > This doesn't seem to address some of my previous concerns:
> 
> You're right. That discussion quickly headed towards objtool
> and I forgot about this one paragraph with the remarks.
> 
> > - Bailing on interrupt/exception frames
> 
> That is a good question. My current code keeps unwinding as long
> as the trace looks sane. If the exception frame has a valid code
> pointer in the LR slot it will continue. Couldn't there be cases
> where this is desirable?

I thought we established in the previous discussion that this could
cause some functions to get skipped in the stack trace:

  https://lkml.kernel.org/r/20171219214652.u7qeb7fxov62ttke@treble

> Should this be configurable? Not that
> I have an idea how this situation could occur for a thread
> that is current or sleeping...

Page faults and preemption.

> Michael, Balbir: is that possible? Any Idea how to reliably detect
> an exception frame? My approach would be to look at the next return
> address and compare it to the usual suspects (i.e. collect all 
> "b ret" addresses in the EXCEPTION_COMMON macro, for BookS).

It looks like show_stack() already knows how to do this:

		/*
		 * See if this is an exception frame.
		 * We look for the "regshere" marker in the current frame.
		 */
		if (validate_sp(sp, tsk, STACK_INT_FRAME_SIZE)
		    && stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {

So you could do something similar.

> > - Function graph tracing return address conversion
> > 
> > - kretprobes return address conversion
> 
> You mean like in arch/x86/kernel/unwind_frame.c the call to
> ftrace_graph_ret_addr ?
> 
> Forgive me my directness but I don't see why these should be handled in
> arch-dependent code, other than maybe a hook, if inevitable, that calls
> back into the graph tracer / kretprobes in order to get the proper
> address,

I don't really follow, where exactly would you propose calling
ftrace_graph_ret_addr() from?

> or simply call the trace unreliable in case it finds such a
> return address.

If you're going to make livepatch incompatible with function graph
tracing, there needs to be a good justification for it (and we'd need to
make sure existing users are fine with it).

-- 
Josh