linux-kernel - Re: [PATCH] Documentation: livepatch: document reliable stacktrace

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Date:   Thu, 14 Jan 2021 11:54:18 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>, linux-doc@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH] Documentation: livepatch: document reliable stacktrace

On Wed, Jan 13, 2021 at 01:33:13PM -0600, Josh Poimboeuf wrote:
> On Wed, Jan 13, 2021 at 04:57:43PM +0000, Mark Brown wrote:
> > From: Mark Rutland <mark.rutland@....com>
> > +There are several ways an architecture may identify kernel code which is deemed
> > +unreliable to unwind from, e.g.
> > +
> > +* Using metadata created by objtool, with such code annotated with
> > +  SYM_CODE_{START,END} or STACKFRAME_NON_STANDARD().
> 
> I'm not sure why SYM_CODE_{START,END} is mentioned here, but it doesn't
> necessarily mean the code is unreliable, and objtool doesn't treat it as
> such.  Its mention can probably be removed unless there was some other
> point I'm missing.
> 
> Also, s/STACKFRAME/STACK_FRAME/

When I wrote this, I was under the impression that (for x86) code marked
as SYM_CODE_{START,END} wouldn't be considered as a function by objtool.
Specifically SYM_FUNC_END() marks the function with SYM_T_FUNC whereas
SYM_CODE_END() marks it with SYM_T_NONE, and IIRC I thought that objtool
only generated ORC for SYM_T_FUNC functions, and hence anything else
would be considered not unwindable due to the absence of ORC.

Just to check, is that understanding for x86 correct, or did I get that
wrong?

If that's right, it might be worth splitting this into two points, e.g.

| * Using metadata created by objtool, with such code annotated with
|   STACKFRAME_NON_STANDARD().
|
|
| * Using ELF symbol attributes, with such code annotated with
|   SYM_CODE_{START,END}, and not having a function type.

If that's wrong, I suspect there are latent issues here?

Thanks,
Mark.