Re: [PATCH v3] Documentation: livepatch: document reliable stacktrace

On Fri, Jan 15, 2021 at 02:24:46PM +0000, Mark Brown wrote:

From: Mark Rutland <mark.rutland@arm.com>

Add documentation for reliable stacktrace. This is intended to describe
the semantics and to be an aid for implementing architecture support for
HAVE_RELIABLE_STACKTRACE.

Unwinding is a subtle area, and architectures vary greatly in both
implementation and the set of concerns that affect them, so I've tried
to avoid making this too specific to any given architecture. I've used
examples from both x86_64 and arm64 to explain corner cases in more
detail, but I've tried to keep the descriptions sufficient for those who
are unfamiliar with the particular architecture.

I've tried to give rationale for all the recommendations/requirements,
since that makes it easier to spot nearby issues, or when a check
happens to catch a few things at once. I believe what I have written is
sound, but as some of this was reverse-engineered I may have missed
things worth noting.

I've made a few assumptions about preferred behaviour, notably:

* If you can reliably unwind through exceptions, you should (as x86_64
  does).

* It's fine to omit ftrace_return_to_handler and other return
  trampolines so long as these are not subject to patching and the
  original return address is reported. Most architectures do this for
  ftrace_return_handler, but not other return trampolines.

* For cases where link register unreliability could result in duplicate
  entries in the trace or an inverted trace, I've assumed this should be
  treated as unreliable. This specific case shouldn't matter to
  livepatching, but I assume that that we want a reliable trace to have
  the correct order.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <redacted>
Cc: Mark Brown <broonie@kernel.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>
Cc: linux-doc@vgert.kernel.org
Cc: live-patching@vger.kernel.org
[Updates following review -- broonie]
Signed-off-by: Mark Brown <broonie@kernel.org>
---

v3:
 - Incorporated objtool section from Mark.
 - Deleted confusing notes about using annotations.

Acked-by: Josh Poimboeuf <redacted>

-- 
Josh