On Fri, Nov 22, 2019 at 3:25 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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);
Not sure whether it's gonna be any cleaner.
Intuitively doesn't feel so.
