Re: [PATCH] powerpc/ftrace: restore r2 to caller's stack on livepatch sibling call

[Joe Lawrence <joe.lawrence@redhat.com>]

On 8/15/24 12:07, Ryan Sullivan wrote:
> Hi Michael,
> 
> The r2 value is stored to the livepatch stack prior to entering into 
> the livepatched code, so accessing it will gurantee the previous value
> is restored.
> 
> Also, yes, this bug is caused by tooling that "scoops out" pre-compiled
> code and places it into the livepatch handler (e.g. kpatch). However, 
> since the large majority of customers interact with the livepatch 
> subsystem through tooling, and this fix would not pose any serious risk
> to either usability or security (other than those already present in 
> livepatching), plus it would solve a large problem for these tools with
> a simple fix, I feel as though this would be a useful update to 
> livepatch.
> 

Right, for kpatch-build binary-diff livepatch creation, the tooling
scans modified code for a sibling call pattern and errors out with a
report to the user:

https://github.com/dynup/kpatch/blob/master/kpatch-build/create-diff-object.c#L3886

and we advise users to manually turn sibling calls off as needed:

https://github.com/dynup/kpatch/blob/master/doc/patch-author-guide.md#sibling-calls

If I understand Ryan's patch, it would remove this restriction from
kpatch creation -- assuming that it is safe and sane for the kernel's
ftrace handler to do so.

-- 
Joe