Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in retpoline

[Zhenzhong Duan]

On 2018/10/31 22:00, Peter Zijlstra wrote:
> On Wed, Oct 31, 2018 at 02:53:20PM +0100, Peter Zijlstra wrote:
> > On Wed, Oct 31, 2018 at 02:01:20PM +0800, Zhenzhong Duan wrote:
> > > On 2018/10/30 16:36, Peter Zijlstra wrote:
> > > > On Mon, Oct 29, 2018 at 11:55:06PM -0700, Zhenzhong Duan wrote:
> > > > > Since CONFIG_RETPOLINE hard depends on compiler support now, so
> > > > > replacing indirect-jump check with the range check is safe in that case.
> > > >
> > > > Can we put kprobes on module init text before we run alternatives on it?
> > >
> > > Forgive me I doesn't understand your question. Do you mean this patch impact
> > > kprobes on module init text?
> >
> > In that case we would still see the indirect paravirt calls for example,
> > and we'd still need that cascade you took out.

Understood.
In another case when loading a non-retpoline module, we suffer the same.
> > Now, I'm not at all sure we're able to use kprobes at those times, so it
> > might be a non-issue.

Not sure, but if it's possible then alternative patching may cover the 
kprobes, it looks like a bug.
> Hmm, what about the case where we have RETPOLINE runtime disabled? Then
> the CALL_NOSPEC alternative patches in an indirect call again, and the
> retpolines are gone.

Is RETPOLINE runtime toggle supported in upstream? I don't see such code.
> Does that not need the __insn_is_indirect_jump() thing?

Yes it's needed if RETPOLINE runtime disabled.

Based on all above reasons, I'd like to drop this patch.

Thanks
Zhenzhong