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Date: Fri, 22 Nov 2019 18:28:42 -0800 From: Alexei Starovoitov <alexei.starovoitov@...il.com> To: Daniel Borkmann <daniel@...earbox.net> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, 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 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); Not sure whether it's gonna be any cleaner. Intuitively doesn't feel so.
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