netdev - Re: [PATCH] bpf,x64: pad NOPs to make images converge more easily

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Date:   Fri, 11 Dec 2020 18:24:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Gary Lin <glin@...e.com>, Networking <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        andreas.taschner@...e.com
Subject: Re: [PATCH] bpf,x64: pad NOPs to make images converge more easily

On Fri, Dec 11, 2020 at 1:13 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >> +                       }
> >>   emit_jmp:
> >>                          if (is_imm8(jmp_offset)) {
> >> +                               if (jmp_padding)
> >> +                                       cnt += emit_nops(&prog, INSN_SZ_DIFF - 2);

Could you describe all possible numbers of bytes in padding?
Is it 0, 2, 4 ?
Would be good to add warn_on_once to make sure the number
of nops is expected.

> >>   struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >>   {
> >>          struct bpf_binary_header *header = NULL;
> >> @@ -1981,6 +1997,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >>          struct jit_context ctx = {};
> >>          bool tmp_blinded = false;
> >>          bool extra_pass = false;
> >> +       bool padding = prog->padded;
> >
> > can this ever be true on assignment? I.e., can the program be jitted twice?
>
> Yes, progs can be passed into the JIT twice, see also jit_subprogs(). In one of
> the earlier patches it would still potentially change the image size a second
> time which would break subprogs aka bpf2bpf calls.

Right. I think memorized padded flag shouldn't be in sticky bits
of the prog structure.
It's only needed between the last pass and extra pass for bpf2bpf calls.
I think it would be cleaner to keep it in struct x64_jit_data *jit_data.

As others have said the selftests are must have.
Especially for bpf2bpf calls where one subprog is padded.

Thanks!