linux-kernel - Re: [RFC 1/1] orc: mark it as reliable

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Date:   Wed, 4 Oct 2017 09:04:22 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/1] orc: mark it as reliable

On Wed, Oct 04, 2017 at 11:23:15AM +0200, Jiri Slaby wrote:
> We need a reliable stack unwinder for kernel live patching, but we do
> not want to enable frame pointers for performance reasons. So let ORC be
> a reliable stack unwinder on x86 as it performs nicely wrt reliability
> of traces.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> ---
> 
> I am sending this as an RFC. Do you still consider ORC to be not-enough
> reliable?

Off the top of my head, at least the following is missing:

- save_stack_trace_reliable() assumes that kernel mode pt_regs on the
  stack make the stack trace unreliable.  This is an FP-specific
  assumption which no longer applies for ORC.

- The ORC unwinder needs to set unwind_state.error if it doesn't reach
  all the way to the end (user pt_regs).

-- 
Josh