Re: [PATCH v3 07/19] unwind: Add user space unwinding API

[Josh Poimboeuf]

On Wed, Dec 11, 2024 at 03:53:26PM +0100, Jens Remus wrote:
> On 09.12.2024 21:54, Josh Poimboeuf wrote:
> > if (cfa <= state->sp)
> > 	goto the_end;
> 
> Assuming the x86 definition of the CFA (CFA == SP at call site) this
> translates into:
> 
> if (sp <= state->sp)
> 	goto the_end;
> 
> That won't work for architectures that pass the return address in a
> register instead of on the stack, such as s390. At least in the
> topmost frame the unwound SP may be unchanged. For instance when in
> the function prologue or when in a leaf function.
> 
> One of my patches for s390 support introduces a state->first flag,
> indicating whether it is the topmost user space frame. Using that
> your check could be extended to:
> 
> if ((state->first && sp < state->sp) || (!state->first && sp <= state->sp))
> 	goto the_end;
> 
> Which could be simplified to:
> 
> if (sp <= state->sp - state->first)
> 	goto the_end;

Since my patches are x86-only, how about I leave the "sp <= state->sp"
check and then you add something like that in your patches on top?

> Btw. neither would work for architectures with an upwards-growing
> stack, such as hppa. Not sure if that needs to be considered.

I don't think that's needed until if/when sframe becomes supported for
such an arch.

-- 
Josh