Re: [PATCH 04/29] x86/livepatch: Validate __fentry__ location

[Peter Zijlstra]

On Wed, Feb 23, 2022 at 03:49:41PM +0100, Peter Zijlstra wrote:

> > Since we would want rec->ip if the pointer is before the ftrace
> > instruction. But we would need to audit all use cases and make sure this is
> > not called from any hot paths (in a callback).
> > 
> > This will affect kprobes and BPF as they both use ftrace_location() as well.
> 
> Yes, I already fixed kprobes, still trying to (re)discover how to run
> the bpf-selftests, that stuff is too painful :-(

Ok, I think I managed... I'm obviously hitting the WARN_ON_ONCE() in
is_ftrace_location(). Funnily, no dead kernel, so that's something I
suppose.

Now, I'm trying to make sense of that code, but all that !ftrace_managed
code scares me to death.

At the very least __bpf_arch_text_poke() needs a bunch of help. Let me
go prod it with something sharp to see what falls out ...