On Fri, Jul 01, 2022 at 08:40:02PM +0200, Helge Deller wrote:

In the kernel image vmlinux.lds.S linker scripts the .altinstructions
and __bug_table sections are 32- or 64-bit aligned because they hold 32-
and/or 64-bit values.

But for modules the module.lds.S linker script doesn't define a default
alignment yet, so the linker chooses the default byte alignment, which
then leads to unnecessary unaligned memory accesses at runtime.

Usually such unaligned accesses are unnoticed, because either the
hardware (as on x86 CPUs) or in-kernel exception handlers (e.g. on hppa
or sparc) emulate and fix them up at runtime.

On hppa the 32-bit unalignment exception handler was temporarily broken
due another bad commit, and as such wrong values were returned on
unaligned accesses to the altinstructions table.

OK so some bad commit broke something which caused bad alignment access
on altinstructions... But why on modules?!

I am not aware of modules using alternatives, given that alternatives
are hacks to help with bootup. For modules we can use other things
like jump labels, static keys.

So I don't understand still how this happened yet.

This then led to
undefined behaviour because wrong kernel addresses were patched and we
suddenly faced lots of unrelated bugs, as can be seen in this mail
thread:
https://lore.kernel.org/all/07d91863-dacc-a503-aa2b-05c3b92a1e39@bell.net/T/#mab602dfa32be5e229d5e192ab012af196d04d75d

This patch adds the missing natural alignment for kernel modules to
avoid unnecessary (hard- or software-based) fixups.

Is it correct to infer that issue you found through a bad commit was
then through code inspection after the bad commit made the kernel do
something stupid with unaligned access to some module altinstructions
section ? Ie, that should not have happened.

I'd like to determine if this is a stable fix, a regression, etc. And
this is not yet clear.

  Luis