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

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Message-ID: <CAADnVQKijkstBqV_JhQSaBXZ+65coY+8=UGw8tPi8-boXV=AOQ@mail.gmail.com>
Date:   Mon, 15 Aug 2022 08:19:16 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Edward Cree <ecree@...arflare.com>,
        Toke Høiland-Jørgensen <thoiland@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH bpf-next v4 2/6] bpf: introduce BPF dispatcher

On Mon, Aug 15, 2022 at 8:16 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon, 15 Aug 2022 07:31:23 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > >
> > > 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.
>
> I extended Jiri's patch to make it work again.
>
> >
> > > 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.
>
> Actually I did invent it.
>
>    https://lore.kernel.org/lkml/20080210072109.GR4100@elte.hu/
>
>
> I'm the one that introduced the code to convert mcount into the 5 byte nop,
> and did the research and development to make it work at run time. I had one
> hiccup along the way that caused the e1000e network card breakage.
>
> The "daemon" approach was horrible, and then I created the recordmcount.pl
> perl script to accomplish the same thing at compile time.
>
> > ftrace should really stop trying to own all of the kernel text rewrites.
> > It's in the way. Like this case.
>
> It's not trying to own all modifications (static_calls is not ftrace). But
> the code at the start of functions with fentry does belong to it.
>
> >
> > >    https://lore.kernel.org/all/CAADnVQJTT7h3MniVqdBEU=eLwvJhEKNLSjbUAK4sOrhN=zggCQ@mail.gmail.com/
> > >
> > > Even though fentry was created for ftrace
> > >
> > >    https://lore.kernel.org/lkml/1258720459.22249.1018.camel@gandalf.stny.rr.com/
> > >
> > > and all the work with fentry was for the ftrace infrastructure. Ftrace
> > > takes a lot of care for security and use cases for other users (like
> > > live kernel patching). But BPF has the NIH syndrome, and likes to own
> > > everything and recreate the wheel so that they have full control.
> > >
> > > > of the trampoline. One dispatcher instance currently supports up to 64
> > > > dispatch points. A user creates a dispatcher with its corresponding
> > > > trampoline with the DEFINE_BPF_DISPATCHER macro.
> > >
> > > Anyway, this patch just looks like a re-implementation of static_calls:
> >
> > It was implemented long before static_calls made it to the kernel
> > and it's different. Please do your home work.
>
> Long before? This code made it into the kernel in Dec 2019. Yes static calls
> finally made it into the kernel in 2020, but it was first introduced in
> October 2018:
>
>   https://lore.kernel.org/all/20181006015110.653946300@goodmis.org/
>
> If you had Cc'd us on this patch, we could have collaborated and come up
> with something that would have worked for you.
>
> It took time to get in because we don't just push our features in, we make
> sure that they are generic and work for others, and is secure and robust.
>
> I sent a proof of concept, Josh took over, Linus had issues, and finally
> Peter pushed it through the gate. It's a long process, but we don't break
> others code while doing it!

Replied in the other thread. Let's stick to one thread please.