Re: [PATCH bpf-next 01/18] bpf: Support stack arguments for bpf functions

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Fri Apr 24, 2026 at 10:14 AM PDT, Yonghong Song wrote:
> @@ -1669,6 +1669,8 @@ struct bpf_prog_aux {
>  	u32 max_pkt_offset;
>  	u32 max_tp_access;
>  	u32 stack_depth;
> +	u16 incoming_stack_arg_depth;
> +	u16 stack_arg_depth; /* both incoming and max outgoing of stack arguments */
>  	u32 id;
>  	u32 func_cnt; /* used by non-func prog as the number of func progs */
>  	u32 real_func_cnt; /* includes hidden progs, only used for JIT and freeing progs */

...

> @@ -739,10 +759,13 @@ struct bpf_subprog_info {
>  	bool keep_fastcall_stack: 1;
>  	bool changes_pkt_data: 1;
>  	bool might_sleep: 1;
> -	u8 arg_cnt:3;
> +	u8 arg_cnt:4;
>  
>  	enum priv_stack_mode priv_stack_mode;
> -	struct bpf_subprog_arg_info args[MAX_BPF_FUNC_REG_ARGS];
> +	struct bpf_subprog_arg_info args[MAX_BPF_FUNC_ARGS];
> +	u16 incoming_stack_arg_depth;
> +	u16 stack_arg_depth; /* incoming + max outgoing */
> +	u16 max_out_stack_arg_depth;
>  };

I asked before but if there was an answer it got lost in all emails.
So will ask again: why duplicate incoming_stack_arg_depth
and stack_arg_depth in two places? One should be enough.

And another question:
max_out_stack_arg_depth is computed only to error like this in bpf_fixup_call_args().

+               u16 outgoing = subprog->stack_arg_depth - subprog->incoming_stack_arg_depth;
+
+               if (subprog->max_out_stack_arg_depth > outgoing) {
+                       verbose(env,
+                               "func#%d writes stack arg slot at depth %u, but calls only require %u bytes\n",
+                               i, subprog->max_out_stack_arg_depth, outgoing);
+                       return -EINVAL;

why bother? What will go wrong if it's not there?