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

Alexei Starovoitov
2026-04-02T23:38:44Z

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?