RE: objtool/ORC generation for noreturn functions

[David Laight <David.Laight@ACULAB.COM>]

> On Wed, Jan 13, 2021 at 11:44:22AM +0100, vanessa.hack@fau.de wrote:
> >    Hi,
> >    I am currently writing my final thesis at university on the topic of stack
> >    unwinding. My goal is to implement and evaluate stack unwinders for
> >    research operating system ports to x86 32 and 64 bit architectures and
> >    SPARC V8.
> >    For the x86 ports I chose ORC as unwinding format due to its simplicity
> >    and reliability. So far, it works quite well (although I've ran into some
> >    minor issues with objtool as the research OS is written in C++).
> >    But now I have some problems with functions that are explicitly marked as
> >    noreturn with the [[noreturn]] attribute, all following unwinding steps
> >    are unreliable. I have read in the objtool documentation that such
> >    functions have to be added to the objtool global_noreturn array.
> >    Unfortunately, I do not understand the purpose of that array and the
> >    intended ORC behaviour for noreturn functions. Are the unwinding steps
> >    that follow a noreturn intended to be unreliable?

There was an 'interesting' unwinder I saw a few years ago.
(Which couldn't handle 'noreturn' functions.)

The idea is to follow forwards through the code while keeping
track of %sp and %fp until a return instruction is found.
You need to be able to detect loops, and then continue from
the other target of an earlier conditional branch.
Provided function calls don't change %sp they can be ignored.
If the %fp isn't used as a frame pointer it won't get reloaded
into %sp so it doesn't matter

This works (most of the time) with no debug info and no symbol
table.

Adding a code marker after a call to a 'notreturn' function (eg 'jmp .')
might be the simplest way of stopping them being an issue.
The extra instruction is unlikely to be an issue.

	David

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