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

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Message-ID: <20210113202315.GI4641@sirena.org.uk>
Date:   Wed, 13 Jan 2021 20:23:15 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 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?

> > +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.

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