Re: [PATCH bpf-next 07/10] bpf,x86: Implement JIT support for stack arguments

Yonghong Song
2026-04-02T23:27:03Z

On 4/2/26 3:26 PM, Amery Hung wrote:
> On Wed, Apr 1, 2026 at 6:28 PM Yonghong Song <yonghong.song@linux.dev> wrote:
> [...]
>
>> case BPF_JMP | BPF_CALL: {
>> + int off, base_off, n_stack_args, kfunc_stack_args = 0, stack_args = 0;
>> + u16 outgoing_stack_args = stack_arg_depth - incoming_stack_arg_depth;
>> u8 *ip = image + addrs[i - 1];
>>
>> func = (u8 *) __bpf_call_base + imm32;
>> @@ -2449,6 +2549,29 @@ st: if (is_imm8(insn->off))
>> }
>> if (!imm32)
>> return -EINVAL;
>> +
>> + if (src_reg == BPF_PSEUDO_CALL && outgoing_stack_args > 0) {
>> + n_stack_args = outgoing_stack_args / 8;
>> + base_off = -(prog_stack_depth + incoming_stack_arg_depth);
>> + ip += push_stack_args(&prog, base_off, n_stack_args, 1);
>> + }
>> +
>> + if (src_reg != BPF_PSEUDO_CALL && insn->off > 0) {
>> + kfunc_stack_args = insn->off;
>> + stack_args = kfunc_stack_args > 1 ? kfunc_stack_args - 1 : 0;
>> + base_off = -(prog_stack_depth + incoming_stack_arg_depth);
>> + ip += push_stack_args(&prog, base_off, kfunc_stack_args, 2);
>> +
>> + /* mov r9, [rbp + base_off - 8] */
>> + off = base_off - 8;
>> + if (is_imm8(off)) {
>> + EMIT4(0x4C, 0x8B, 0x4D, off);
>> + ip += 4;
>> + } else {
>> + EMIT3_off32(0x4C, 0x8B, 0x8D, off);
>> + ip += 7;
>> + }
>> + }
> Do we need to make sure RSP is 16-byte aligned before passing arg
> through stack per x86-64 ABI?

Good question. Without this patch set, looks like bpf jit here does not enforce 16 byte
aligned. IIUC, 16 byte aligned requirement will be necessary for some special
SSE/AVX/128-bit XMM etc. The bpf jit does not really emit them, so I guess it is okay?

>
>> if (priv_frame_ptr) {
>> push_r9(&prog);
>> ip += 2;
>> @@ -2458,6 +2581,14 @@ st: if (is_imm8(insn->off))
>> return -EINVAL;
>> if (priv_frame_ptr)
>> pop_r9(&prog);
>> + if (stack_args > 0) {
>> + /* add rsp, stack_args * 8 */
>> + EMIT4(0x48, 0x83, 0xC4, stack_args * 8);
>> + }
>> + if (src_reg == BPF_PSEUDO_CALL && outgoing_stack_args > 0) {
>> + /* add rsp, outgoing_stack_args */
>> + EMIT4(0x48, 0x83, 0xC4, outgoing_stack_args);
>> + }
>> break;
>> }
>>
>> --
>> 2.52.0
>>
>>