Re: [PATCH] x86: introduce push/pop macros which generate CFI_REL_OFFSET and CFI_RESTORE

[Borislav Petkov]

On Mon, Jan 12, 2015 at 11:46:53AM -0800, Andy Lutomirski wrote:
> Dumb example:
> 
>     pushq_cfi $__KERNEL_DS /* ss */
> 
> This doesn't save anything that the unwinder would care about.

And? The unwinder or whatever looks at that info simply ignores stuff it
is not interested in, no?

> Better example:
> 
>     pushq_cfi \child_rip /* rip */
>     CFI_REL_OFFSET    rip,0
> 
> Doing this with a macro would need a fancier macro.

I'd ask first whether we really need this at all.

> Then there's crap like:
> 
>     pushq_cfi %rdi
>     SCHEDULE_USER
>     popq_cfi %rdi

I guess we can add a gas regname argument optional and if it is set, use
it and if not, use the reg itself... Or something like that in the best
effort type of approach.

> I would need to look a lot more carefully to figure out whether this
> would need CFI_REL_OFFSET.
> 
> If we actually had a DWARF unwinder in the kernel, maybe we could have
> real test cases :-/

I don't think that's ever going to happen.

I'd say we do the CFI annotation on a best effort basis but not
sacrifice readability in the process. If it can't be annotated, well,
tough luck.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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