On Fri, Apr 09, 2021 at 04:37:41PM -0500, Josh Poimboeuf wrote:

On Fri, Apr 09, 2021 at 01:09:09PM +0100, Mark Rutland wrote:

quoted

Further, I believe all the special cases are assembly functions, and
most of those are already in special sections to begin with. I reckon
it'd be simpler and more robust to reject unwinding based on the
section. If we need to unwind across specific functions in those
sections, we could opt-in with some metadata. So e.g. we could reject
all functions in ".entry.text", special casing the EL0 entry functions
if necessary.

Couldn't this also end up being somewhat fragile?  Saying "certain
sections are deemed unreliable" isn't necessarily obvious to somebody
who doesn't already know about it, and it could be overlooked or
forgotten over time.  And there's no way to enforce it stays that way.

Anything in this area is going to have some opportunity for fragility
and missed assumptions somewhere.  I do find the idea of using the
SYM_CODE annotations that we already have and use for other purposes to
flag code that we don't expect to be suitable for reliable unwinding
appealing from that point of view.  It's pretty clear at the points
where they're used that they're needed, even with a pretty surface level
review, and the bit actually pushing things into a section is going to
be in a single place where the macro is defined.  That seems relatively
robust as these things go, it seems no worse than our reliance on
SYM_FUNC to create BTI annotations.  Missing those causes oopses when we
try to branch to the function.