ppc64le STRICT_MODULE_RWX and livepatch apply_relocate_add() crashes

[Russell Currey]

On Sun, 2021-10-31 at 22:43 -0400, Joe Lawrence wrote:
> Starting with 5.14 kernels, I can reliably reproduce a crash [1] on
> ppc64le when loading livepatches containing late klp-relocations [2].
> These are relocations, specific to livepatching, that are resolved not
> when a livepatch module is loaded, but only when a livepatch-target
> module is loaded.

Hey Joe, thanks for the report.

> I haven't started looking at a fix yet, but in the case of the x86 code
> update, its apply_relocate_add() implementation was modified to use a
> common text_poke() function to allowed us to drop
> module_{en,dis}ble_ro() games by the livepatching code.

It should be a similar fix for Power, our patch_instruction() uses a
text poke area but apply_relocate_add() doesn't use it and does its own
raw patching instead.

> I can take a closer look this week, but thought I'd send out a report
> in case this may be a known todo for STRICT_MODULE_RWX on Power.

I'm looking into this now, will update when there's progress.  I
personally wasn't aware but Jordan flagged this as an issue back in
August [0].  Are the selftests in the klp-convert tree sufficient for
testing?  I'm not especially familiar with livepatching & haven't used
the userspace tools.

- Russell

[0] https://github.com/linuxppc/issues/issues/375

> 
> -- Joe