Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

[Florian Weimer]

* Chris Kennelly:

> When glibc provides registration, is the anticipated use case that a
> library would unregister and reregister each thread to "upgrade" it to
> the most modern version of interface it knows about provided by the
> kernel?

Absolutely not, that is likely to break other consumers because an
expected rseq area becomes dormant instead.

> There, I could assume an all-or-nothing registration of the new
> feature--limited only by kernel availability for thread
> homogeneity--but inconsistencies across early adopter libraries would
> mean each thread would have to examine its own TLS to determine if a
> feature were available.

Exactly.  Certain uses of seccomp can also have this effect,
presenting a non-homogeneous view.