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@xxxxxxxxx> wrote:
> >
> > 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);
>
> 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.
