Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

Josh Poimboeuf
2021-11-12T01:50:12Z

On Wed, Nov 10, 2021 at 12:20:47PM +0000, David Laight wrote:
> > > Wouldn't moving part of a function to .text.cold (or .text.unlikely)
> > > generate the same problems with the stack backtrace code as the
> > > .text.fixup section you are removing had??
> >
> > GCC can already split a function into func and func.cold today (or
> > worse: func, func.isra.N, func.cold, func.isra.N.cold etc..).
> >
> > I'm assuming reliable unwind and livepatch know how to deal with this.
>
> They'll have 'proper' function labels at the top - so backtrace
> stands a chance.
> Indeed you (probably) want it to output "func.irsa.n.cold" rather
> than just "func" to help show which copy it is in. >
> I guess that livepatch will need separate patches for each
> version of the function - which might be 'interesting' if
> all the copies actually need patching at the same time.
> You'd certainly want a warning if there seemed to be multiple
> copies of the function.

Hm, I think there is actually a livepatch problem here.

If the .cold (aka "child") function actually had a fentry hook then we'd
be fine. Then we could just patch both "parent" and "child" functions
at the same time. We already have the ability to patch multiple
functions having dependent interface changes.

But there's no fentry hook in the child, so we can only patch the
parent.

If the child schedules out, and then the parent gets patched, things can
go off-script if the child later jumps back to the unpatched version of
the parent, and then for example the old parent tries to call another
patched function with a since-changed ABI.

Granted, it's like three nested edge cases, so it may not be all that
likely to happen.

Some ideas to fix:

a) Add a field to 'klp_func' which allows the patch module to specify a
function's .cold counterpart?

b) Detect such cold counterparts in klp_enable_patch()? Presumably it
would require searching kallsyms for "<func>.cold", which is somewhat
problematic as there might be duplicates.

c) Update the reliable stacktrace code to mark the stack unreliable if
it has a function with ".cold" in the name?

d) Don't patch functions with .cold counterparts? (Probably not a viable
long-term solution, there are a ton of .cold functions because calls
to printk are marked cold)

e) Disable .cold optimization?

f) Add fentry hooks to .cold functions?

I'm thinking a) seems do-able, and less disruptive / more precise than
most others, but it requires more due diligence on behalf of the patch
creation. It sounds be pretty easy for kpatch-build to handle at least.

--
Josh