netdev - Re: [PATCH rfc bpf-next 1/8] bpf, x86: generalize and extend bpf_arch_text

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Date:   Fri, 15 Nov 2019 15:22:59 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 rfc bpf-next 1/8] bpf, x86: generalize and extend
 bpf_arch_text_poke for direct jumps

On Thu, Nov 14, 2019 at 5:04 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Add BPF_MOD_{NOP_TO_JUMP,JUMP_TO_JUMP,JUMP_TO_NOP} patching for x86
> JIT in order to be able to patch direct jumps or nop them out. We need
> this facility in order to patch tail call jumps and in later work also
> BPF static keys.
>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---

just naming nits, looks good otherwise


>  arch/x86/net/bpf_jit_comp.c | 64 ++++++++++++++++++++++++++-----------
>  include/linux/bpf.h         |  6 ++++
>  2 files changed, 52 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2e586f579945..66921f2aeece 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -203,8 +203,9 @@ struct jit_context {
>  /* Maximum number of bytes emitted while JITing one eBPF insn */
>  #define BPF_MAX_INSN_SIZE      128
>  #define BPF_INSN_SAFETY                64
> -/* number of bytes emit_call() needs to generate call instruction */
> -#define X86_CALL_SIZE          5
> +
> +/* Number of bytes emit_patchable() needs to generate instructions */
> +#define X86_PATCHABLE_SIZE     5
>
>  #define PROLOGUE_SIZE          25
>
> @@ -215,7 +216,7 @@ struct jit_context {
>  static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf)
>  {
>         u8 *prog = *pprog;
> -       int cnt = X86_CALL_SIZE;
> +       int cnt = X86_PATCHABLE_SIZE;
>
>         /* BPF trampoline can be made to work without these nops,
>          * but let's waste 5 bytes for now and optimize later
> @@ -480,64 +481,91 @@ static void emit_stx(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, int off)
>         *pprog = prog;
>  }
>
> -static int emit_call(u8 **pprog, void *func, void *ip)
> +static int emit_patchable(u8 **pprog, void *func, void *ip, u8 b1)

I'd strongly prefer opcode instead of b1 :) also would emit_patch() be
a terrible name?

>  {
>         u8 *prog = *pprog;
>         int cnt = 0;
>         s64 offset;
>

[...]

>         case BPF_MOD_CALL_TO_NOP:
> -               if (memcmp(ip, old_insn, X86_CALL_SIZE))
> +       case BPF_MOD_JUMP_TO_NOP:
> +               if (memcmp(ip, old_insn, X86_PATCHABLE_SIZE))
>                         goto out;
> -               text_poke_bp(ip, ideal_nops[NOP_ATOMIC5], X86_CALL_SIZE, NULL);
> +               text_poke_bp(ip, ideal_nops[NOP_ATOMIC5], X86_PATCHABLE_SIZE,


maybe keep it shorter with X86_PATCH_SIZE?

> +                            NULL);
>                 break;
>         }
>         ret = 0;

[...]