Linux的bpf过滤器的nop空指令
https://elixir.bootlin.com/linux/latest/source/Documentation/networking/filter.txt
ja 6 Jump to label
BPF_JUMP(BPF_JMP + BPF_JA, 0, 0, 0),
像上面那样插入一个jmp 跳转到到下一行代码 ，最后应该会被jit生成器给删掉，也就是空指令的效果吧，不过和nop指令还是有区别的。
https://elixir.bootlin.com/linux/latest/source/arch/x86/net/bpf_jit_comp.c
case BPF_JMP | BPF_JA:
if (insn->off == -1)
/* -1 jmp instructions will always jump
* backwards two bytes. Explicitly handling
* this case avoids wasting too many passes
* when there are long sequences of replaced
* dead code.
*/
jmp_offset = -2;
else
jmp_offset = addrs[i + insn->off] - addrs[i];
if (!jmp_offset)
/* Optimize out nop jumps */
break;
emit_jmp:
if (is_imm8(jmp_offset)) {
EMIT2(0xEB, jmp_offset);
} else if (is_simm32(jmp_offset)) {
EMIT1_off32(0xE9, jmp_offset);
} else {
pr_err("jmp gen bug %llx\n", jmp_offset);
return -EFAULT;
}
break;