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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Mark Brown <broonie@kernel.org>
Cc: linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
	Jiri Kosina <jikos@kernel.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>, Miroslav Benes <mbenes@suse.cz>,
	Petr Mladek <pmladek@suse.com>,
	linux-doc@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [PATCH] Documentation: livepatch: document reliable stacktrace
Date: Wed, 13 Jan 2021 16:25:41 -0600	[thread overview]
Message-ID: <20210113222541.ysvtievx4o5r42ym@treble> (raw)
In-Reply-To: <20210113202315.GI4641@sirena.org.uk>

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


  reply	other threads:[~2021-01-14  2:09 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-13 16:57 [PATCH] Documentation: livepatch: document reliable stacktrace Mark Brown
2021-01-13 19:33 ` Josh Poimboeuf
2021-01-13 20:23   ` Mark Brown
2021-01-13 22:25     ` Josh Poimboeuf [this message]
2021-01-14 18:10       ` Mark Rutland
2021-01-15  0:03         ` Josh Poimboeuf
2021-01-14 11:54   ` Mark Rutland
2021-01-14 14:36     ` Josh Poimboeuf
2021-01-14 17:49       ` Mark Rutland
2021-01-14 20:03         ` Josh Poimboeuf
  -- strict thread matches above, loose matches on Subject: below --
2020-10-23 15:35 Mark Rutland
2020-10-27 11:16 ` Petr Mladek
2020-10-29 10:04 ` Miroslav Benes

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