On Mon, Jan 18, 2021 at 03:02:31PM +0100, Petr Mladek wrote:

Hi Mark,

first, thanks a lot for writing this.

On Fri 2021-01-15 17:16:17, Mark Brown wrote:

quoted

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.

The above looks enough for the commit message. Well, Josh, typically
asks for a directive style, example:

Instead of "I've tried to give rationale...", please use something like
"The documentation gives rationale...".

True, we do try to use imperative form like "Try to give rationale...".

Though documentation is less technical than code, so maybe technical
language is less important.

quoted

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.

This looks like a background that is typically part of the cover
leter. It mentions some Mark's doubts.

Could anyone please answer whether the above assumptions are correct
or not? We should remove them from the commit message. If any
assumption is wrong, we should fix the documentation.

Agreed, this section can probably be dropped.

-- 
Josh