Re: [PATCH v14 16/19] unwind_user/sframe: Add support for SFrame V3 flexible FDEs

On Wed, 13 May 2026 15:50:29 +0200
Jens Remus <[email protected]> wrote:

> >>
> >> @Indu:  Although the SFrame spec does only state "unused Unused bit." I
> >> think it would be good for the logic to reject any value other than zero
> >> as that could be used in future extensions of the SFrame format.  Do you
> >> agree?
> >>  
> > 
> > For unused bits, it will be ideal to not associate any "observable"
> > behaviour on the consumer side, before the bits find their use.  So I
> > suggest the recommended way is to ignore the unused bits completely,
> > i.e., mask them out and not report any error (if they are set).  
> 
> I disagree.  Masking unused/reserved bits would cause the unwinder to
> ignore those even if they get assigned a meaning in the future that it
> needs to respect.  It could thus cause the unwinder to erroneously
> proceed with wrong results instead of stopping.

Correct. "reserved bits" should always be zero unless there is another way
to know when they will become used. For instance, if using these bits
requires a version change, then it would be OK to ignore them. But if they
can ever be used without some other way to inform the kernel that they are
to be ignored, then they must be zero.

-- Steve