On Fri, Dec 11, 2020 at 1:13 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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!
