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

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Date:   Wed, 13 Jan 2021 16:25:41 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        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 08:23:15PM +0000, Mark Brown wrote:
> On Wed, Jan 13, 2021 at 01:33:13PM -0600, Josh Poimboeuf wrote:
> 
> > I think it's worth mentioning a little more about objtool.  There are a
> > few passing mentions of objtool's generation of metadata (i.e. ORC), but
> > objtool has another relevant purpose: stack validation.  That's
> > particularly important when it comes to frame pointers.
> 
> > For some architectures like x86_64 and arm64 (but not powerpc/s390),
> > it's far too easy for a human to write asm and/or inline asm which
> > violates frame pointer protocol, silently causing the violater's callee
> > to get skipped in the unwind.  Such architectures need objtool
> > implemented for CONFIG_STACK_VALIDATION.
> 
> This basically boils down to just adding a statement saying "you may
> need to depend on objtool" I think?

Right, but maybe it would be a short paragraph or two.

> > > +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.
> 
> I was reading that as being a thing that the architecture could possibly
> do, especially as a first step - it does seem like a reasonable thing to
> consider using anyway.  I guess you could also use it the other way
> around and do additional checks for things that are supposed to be
> regular functions that you relax for SYM_CODE() sections.

Makes sense, but we have to be careful not to imply that objtool already
does something like that :-)

-- 
Josh