[PATCH bpf-next 2/3] bpf, arm64: Add JIT support for stack arguments

[Puranjay Mohan]

On Tue, Apr 21, 2026 at 3:58 AM Alexei Starovoitov
<alexei.starovoitov at gmail.com> wrote:
>
> On Mon, Apr 20, 2026 at 8:36 AM Puranjay Mohan <puranjay at kernel.org> wrote:
> >
>
> nice and clean. I like how it maps to arm64 calling convention.
>
> > +       if (prog->aux->stack_arg_depth > prog->aux->incoming_stack_arg_depth) {
> > +               u16 outgoing = prog->aux->stack_arg_depth - prog->aux->incoming_stack_arg_depth;
> > +               int nr_on_stack = outgoing / sizeof(u64) - NR_STACK_ARG_REGS;
> > +
> > +               if (nr_on_stack > 0)
> > +                       ctx.stack_arg_size = round_up(nr_on_stack * sizeof(u64), 16);
> > +       }
>
> I'm struggling to understand this part.
> Why do this when this func calls more than what callee passed in?
> Looks fishy. I'd like to see selftests with more than 6,7,8 args.
> Because only then this logic will kick in?

Your confusion stems from the naming of "incoming_stack_arg_depth" and
"stack_arg_depth" (this should be called total_stack_arg_depth in my
opinion)

So, if you see fixups.c

                func[i]->aux->incoming_stack_arg_depth =
env->subprog_info[i].incoming_stack_arg_depth;
                func[i]->aux->stack_arg_depth =
env->subprog_info[i].incoming_stack_arg_depth +

env->subprog_info[i].outgoing_stack_arg_depth;

prog->aux->stack_arg_depth doesn't store outgoing stack depth, rather
it has the sum of both incoming and outgoing, that means if a func
doesn't call any function with more than 5 arguments but receives more
than five arguments, incoming_stack_arg_depth will be equal to
stack_arg_depth.

if (prog->aux->stack_arg_depth > prog->aux->incoming_stack_arg_depth)

This check is for - "Does this function call any function with more
than 5 arguments", if yes, is it more than 8? if yes allocate stack
space, otherwise stack space is not needed because argument 6,7,8 can
live in arm64 registers.

I hope this clears the confusion.

Thanks,
Puranjay