Re: [PATCH bpf-next v6 5/9] bpf: look into the types of the fields of a struct type recursively.

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-05-20 at 13:40 -0700, Kui-Feng Lee wrote:
> The verifier has field information for specific special types, such as
> kptr, rbtree root, and list head. These types are handled
> differently. However, we did not previously examine the types of fields of
> a struct type variable. Field information records were not generated for
> the kptrs, rbtree roots, and linked_list heads that are not located at the
> outermost struct type of a variable.
> 
> For example,
> 
>   struct A {
>     struct task_struct __kptr * task;
>   };
> 
>   struct B {
>     struct A mem_a;
>   }
> 
>   struct B var_b;
> 
> It did not examine "struct A" so as not to generate field information for
> the kptr in "struct A" for "var_b".
> 
> This patch enables BPF programs to define fields of these special types in
> a struct type other than the direct type of a variable or in a struct type
> that is the type of a field in the value type of a map.
> 
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

> +	/* Look into variables of struct types */
> +	if ((field_type == BPF_KPTR_REF || !field_type) &&
> +	    __btf_type_is_struct(var_type)) {

This code would have looked nicer (handled inside the same switch as
all other branches) if we had BPF_NESTED_FIELD in `btf_field_type` or
something like that. But that's probably be an overkill.

> +		sz = var_type->size;
> +		if (expected_size && expected_size != sz * nelems)
> +			return 0;
> +		ret = btf_find_nested_struct(btf, var_type, off, nelems, field_mask,
> +					     &info[0], info_cnt);
> +		return ret;
> +	}

[...]