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

[Kui-Feng Lee <sinquersw@gmail.com>]

On 5/21/24 11:12, Eduard Zingerman wrote:
> 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.

After thinking twice, I will change btf_get_field_type() to return 
BPF_KPTR_REF only if the type is not struct.

> 
>> +		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;
>> +	}
> 
> [...]