Re: [PATCH bpf-next v4 2/6] bpf: introduce BPF dispatcher

[Peter Zijlstra]

On Mon, Aug 15, 2022 at 07:31:23AM -0700, Alexei Starovoitov wrote:
> On Mon, Aug 15, 2022 at 7:13 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Wed, 11 Dec 2019 13:30:13 +0100
> > Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
> >
> > > From: Björn Töpel <bjorn.topel@xxxxxxxxx>
> > >
> > > The BPF dispatcher is a multi-way branch code generator, mainly
> > > targeted for XDP programs. When an XDP program is executed via the
> > > bpf_prog_run_xdp(), it is invoked via an indirect call. The indirect
> > > call has a substantial performance impact, when retpolines are
> > > enabled. The dispatcher transform indirect calls to direct calls, and
> > > therefore avoids the retpoline. The dispatcher is generated using the
> > > BPF JIT, and relies on text poking provided by bpf_arch_text_poke().
> > >
> > > The dispatcher hijacks a trampoline function it via the __fentry__ nop
> >
> > Why was the ftrace maintainers not Cc'd on this patch?  I would have NACKED
> > it. Hell, it wasn't even sent to LKML! This was BPF being sneaky in
> > updating major infrastructure of the Linux kernel without letting the
> > stakeholders of this change know about it.
> >
> > For some reason, the BPF folks think they own the entire kernel!
> >
> > When I heard that ftrace was broken by BPF I thought it was something
> > unique they were doing, but unfortunately, I didn't investigate what they
> > were doing at the time.
> 
> ftrace is still broken and refusing to accept the fact doesn't make it
> non-broken.

Alexei, stop this. The 'call __fentry__' sites are owned by ftrace.
Always have been. If BPF somehow thinks it can use them without telling
ftrace then it's BPF that's broken.

> > Then they started sending me patches to hide fentry locations from ftrace.
> > And even telling me that fentry != ftrace
> 
> It sounds that you've invented nop5 and kernel's ability
> to replace nop5 with a jump or call.

Ftrace has introduced the mcount/fentry patching into the kernel and has
always owned it for those sites. There is a lot of other text writing
not owned by ftrace. But the fentry sites are ftrace's.

Ftrace was also the one that got us the text_poke_bp() infrastructure
and got it reviewed by the CPU vendors.

Since then we've grown static_branch and static_call, they have their
own patch sites and do no interfere with ftrace.

> ftrace should really stop trying to own all of the kernel text rewrites.
> It's in the way. Like this case.

It doesn't. It hasn't. But it *does* own the fentry sites.

> It was implemented long before static_calls made it to the kernel
> and it's different.

It wasn't long before. Yes it landed a few months prior to the
static_call work, but the whole static_call thing was in progress for a
long long time.

Anyway, yes it is different. But it's still very much broken. You simply
cannot step on __fentry__ sites like that.