Re: [PATCH bpf-next 0/2] Fix bpf_perf_event_data ABI breakage

[Heiko Carstens <hca@linux.ibm.com>]

On Sun, Feb 06, 2022 at 10:23:19PM -0800, Andrii Nakryiko wrote:
> I'm not sure the origins of the need for user_pt_regs (as opposed to
> using pt_regs directly, like x86-64 does), but with CO-RE and
> vmlinux.h it would be more reliable and straightforward to just stick
> to kernel-internal struct pt_regs everywhere. And for non-CO-RE macros
> maybe just using an offset within struct pt_regs (i.e.,
> offsetofend(gprs)) would do it?

user_pt_regs was introduced on s390 because struct pt_regs is _not_
stable. Only the first n entries (aka user_pt_regs) are supposed to be
stable.

We could of course reduce struct pt_regs to the bare minimum, which seems
to be the current user_pt_regs plus orig_gpr2; which semantically would
match more or less what x86 has.

Then move pt_regs to uapi, so it is clear that it cannot be changed
anymore, and have additional data in a separate structure on the stack,
which has pt_regs at the beginning, and access this additional data with
container_of & friends.

I guess that could work, even though this requires to keep user_pt_regs
"for historical reasons".