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Date: Sat, 23 Nov 2019 10:24:16 +0100 From: Daniel Borkmann <daniel@...earbox.net> To: Alexei Starovoitov <alexei.starovoitov@...il.com>, Andrii Nakryiko <andrii.nakryiko@...il.com> Cc: 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 11/23/19 7:18 AM, Alexei Starovoitov wrote: > 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@...il.com> wrote: >>> 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); >> >> 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. Ok, will see to come up with a clean simplification in the evening. Thanks, Daniel
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