netdev - Re: [PATCH bpf-next v2 7/8] bpf, x86: emit patchable direct jump as tail call

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Message-ID: <20191123061805.grankibnqpae4tnd@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 22 Nov 2019 22:18:07 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 7/8] bpf, x86: emit patchable direct jump as
 tail call

On Fri, Nov 22, 2019 at 09:00:35PM -0800, Andrii Nakryiko wrote:
> On Fri, Nov 22, 2019 at 6:28 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Fri, Nov 22, 2019 at 3:25 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> > > >> +       case BPF_MOD_CALL_TO_NOP:
> > > >> +       case BPF_MOD_JUMP_TO_NOP:
> > > >> +               if (old_addr && !new_addr) {
> > > >> +                       memcpy(new_insn, nop_insn, X86_PATCH_SIZE);
> > > >> +
> > > >> +                       prog = old_insn;
> > > >> +                       ret = emit_patch_fn(&prog, old_addr, ip);
> > > >> +                       if (ret)
> > > >> +                               return ret;
> > > >> +                       break;
> > > >> +               }
> > > >> +               return -ENXIO;
> > > >> +       default:
> > > >
> > > > There is this redundancy between BPF_MOD_xxx enums and
> > > > old_addr+new_addr (both encode what kind of transition it is), which
> > > > leads to this cumbersome logic. Would it be simpler to have
> > > > old_addr/new_addr determine whether it's X-to-NOP, NOP-to-Y, or X-to-Y
> > > > transition, while separate bool or simple BPF_MOD_CALL/BPF_MOD_JUMP
> > > > enum determining whether it's a call or a jump that we want to update.
> > > > Seems like that should be a simpler interface overall and cleaner
> > > > implementation?
> > >
> > > Right we can probably simplify it further, I kept preserving the original
> > > switch from Alexei's code where my assumption was that having the transition
> > > explicitly spelled out was preferred in here and then based on that doing
> > > the sanity checks to make sure we don't get bad input from any call-site
> > > since we're modifying kernel text, e.g. in the bpf_trampoline_update() as
> > > one example the BPF_MOD_* is a fixed constant input there.
> >
> > I guess we can try adding one more argument
> > bpf_arch_text_poke(ip, BPF_MOD_NOP, old_addr, BPF_MOD_INTO_CALL, new_addr);
> 
> I was thinking along the lines of:
> 
> bpf_arch_text_poke(ip, BPF_MOD_CALL (or BPF_MOD_JMP), old_addr, new_addr);
> 
> old_addr/new_addr being possibly NULL determine NOP/not-a-NOP.

I see. Something like:
if (BPF_MOD_CALL) {
   if (old_addr)
       memcmp(ip, old_call_insn);
   else
       memcmp(ip, nop_insn);
} else if (BPF_MOD_JMP) {
   if (old_addr)
       memcmp(ip, old_jmp_insn);
   else
       memcmp(ip, nop_insn);
}
I guess that can work.