Re: [PATCH bpf-next v3 08/11] bpf,x86: Implement JIT support for stack arguments

[Yonghong Song <yonghong.song@linux.dev>]

On 4/5/26 11:20 AM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 32864dbc2c4e..206f342a0ca0 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
> [ ... ]
>
>> @@ -2449,6 +2529,41 @@ st:			if (is_imm8(insn->off))
>>   			}
>>   			if (!imm32)
>>   				return -EINVAL;
>> +
>> +			if (src_reg == BPF_PSEUDO_CALL && outgoing_stack_arg_depth > 0) {
>> +				s32 outgoing_base = -(prog_stack_depth + outgoing_stack_arg_depth);
>> +				int n_args = outgoing_stack_arg_depth / 8;
>> +
>> +				ip += push_stack_args(&prog, outgoing_base, n_args, 1);
>> +			}
>> +
>> +			if (src_reg != BPF_PSEUDO_CALL && insn->off > 0) {
>> +				/* Kfunc calls: arg 6 -> R9, args 7+ -> push. */
>> +				s32 outgoing_base = -(prog_stack_depth + outgoing_stack_arg_depth);
>> +				int kfunc_stack_args = insn->off;
>> +
>> +				stack_args = kfunc_stack_args > 1 ? kfunc_stack_args - 1 : 0;
>> +
>> +				/* Push args 7+ in reverse order */
>> +				if (stack_args > 0)
>> +					ip += push_stack_args(&prog, outgoing_base, kfunc_stack_args, 2);
>> +
>> +				/* mov r9, [rbp + outgoing_base] (arg 6) */
>> +				if (is_imm8(outgoing_base)) {
>> +					EMIT4(0x4C, 0x8B, 0x4D, outgoing_base);
>> +					ip += 4;
>> +				} else {
>> +					EMIT3_off32(0x4C, 0x8B, 0x8D, outgoing_base);
>> +					ip += 7;
>> +				}
>> +			}
>>   			if (priv_frame_ptr) {
>>   				push_r9(&prog);
>>   				ip += 2;
> When priv_frame_ptr is active (private stack in use), R9 holds the
> private frame pointer throughout the program. The existing push_r9 /
> pop_r9 around calls preserves R9 across the function call.
>
> With this new code, two issues arise when stack args and
> priv_frame_ptr are both active.
>
> For kfunc calls with stack args: the MOV R9, [rbp + outgoing_base]
> overwrites the priv_frame_ptr in R9 with arg6 before push_r9 saves
> it. After the call, pop_r9 restores arg6 into R9 instead of the
> priv_frame_ptr, so subsequent BPF_REG_FP accesses (which map to R9
> when priv_frame_ptr is set) would reference the wrong memory.
>
> For both kfunc and BPF-to-BPF calls: push_r9 inserts an extra 8
> bytes onto the stack between the pushed args and the CALL
> instruction. The callee then sees the wrong values for its stack
> arguments. For example, with a BPF-to-BPF call the callee's
> incoming arg formula expects [rbp + 16] to be the first stack arg,
> but instead it finds the caller's saved R9 value:
>
>      push_stack_args:  [RSP] = arg1, [RSP+8] = arg2
>      push_r9:          [RSP] = R9,   [RSP+8] = arg1, [RSP+16] = arg2
>      CALL:             [RSP] = ret,  [RSP+8] = R9 ...
>      callee prologue:  [rbp+16] = R9  (expected arg1)
>
> Both priv_stack_mode and stack_arg_depth are set independently per
> subprog in jit_subprogs(), so can this combination actually be
> reached?

You are right. Similar to my previous comments, private stack will
be disabled if outgoing stack arguments are not empty.

>
>> @@ -2458,6 +2573,14 @@ 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_arg_depth > 0) {
>> +				/* add rsp, outgoing_stack_arg_depth */
>> +				EMIT4(0x48, 0x83, 0xC4, outgoing_stack_arg_depth);
>> +			}
>>   			break;
>>   		}
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/24006922024