Re: [PATCH bpf-next 03/10] bpf: Support stack arguments for bpf functions

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

On 4/2/26 4:38 PM, Alexei Starovoitov wrote:
> On Wed, Apr 1, 2026 at 6:27 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>> Currently BPF functions (subprogs) are limited to 5 register arguments.
>> With [1], the compiler can emit code that passes additional arguments
>> via a dedicated stack area through bpf register
>> BPF_REG_STACK_ARG_BASE (r12), introduced in the previous patch.
>>
>> The following is an example to show how stack arguments are saved
>> and transferred between caller and callee:
>>
>>    int foo(int a1, int a2, int a3, int a4, int a5, int a6, int a7) {
>>      ...
>>      bar(a1, a2, a3, a4, a5, a6, a7, a8);
>>      ...
>>    }
>>
>> The following is a illustration of stack allocation:
>>
>>     Caller (foo)                           Callee (bar)
>>     ============                           ============
>>     r12-relative stack arg area:           r12-relative stack arg area:
>>
>>     r12-8:  [incoming arg 6]          +--> r12-8:  [incoming arg 6] (from caller's outgoing r12-24)
>>     r12-16: [incoming arg 7]          |+-> r12-16: [incoming arg 7] (from caller's outgoing r12-32)
>>                                       ||+> r12-24: [incoming arg 8] (from caller's outgoing r12-40)
>>     ---- incoming/outgoing boundary   |||  ---- incoming/outgoing boundary
>>     r12-24: [outgoing arg 6 to callee]+||   ...
>>     r12-32: [outgoing arg 7 to callee]-+|
>>     r12-40: [outgoing arg 8 to callee]--+
> I haven't looked at the patch itself only at this diagram.
> How does it suppose to map to x86 calling convention?
> The shift is unusual.
> x86 is using fp-N for outgoing and fp+N for incoming.
> Why can't we use the same?
>
This is not for jit. The above transfer is for verification purpose.
For example, for callee, a load 'rX = *(u64 *)(r12 - 8)' can easily
get the value rX in callee since the value is copied from caller to callee.