Re: [PATCH bpf-next v3 1/7] bpf: Allow struct argument in trampoline based programs

[Yonghong Song <yhs@fb.com>]

On 8/30/22 5:11 AM, Jiri Olsa wrote:
> On Sat, Aug 27, 2022 at 07:54:43PM -0700, Yonghong Song wrote:
> 
> SNIP
> 
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 903719b89238..4a081bfb4c8a 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -5328,6 +5328,31 @@ static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
>>   	return btf_type_is_int(t);
>>   }
>>   
>> +static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
>> +			   int off)
>> +{
>> +	const struct btf_param *args;
>> +	const struct btf_type *t;
>> +	u32 offset = 0, nr_args;
>> +	int i;
>> +
>> +	nr_args = btf_type_vlen(func_proto);
>> +	args = (const struct btf_param *)(func_proto + 1);
>> +	for (i = 0; i < nr_args; i++) {
>> +		t = btf_type_skip_modifiers(btf, args[i].type, NULL);
>> +		offset += btf_type_is_ptr(t) ? 8 : roundup(t->size, 8);
>> +		if (off < offset)
>> +			return i;
>> +	}
>> +
>> +	t = btf_type_skip_modifiers(btf, func_proto->type, NULL);
>> +	offset += btf_type_is_ptr(t) ? 8 : roundup(t->size, 8);
>> +	if (off < offset)
>> +		return nr_args;
>> +
>> +	return nr_args + 1;
>> +}
>> +
>>   bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>   		    const struct bpf_prog *prog,
>>   		    struct bpf_insn_access_aux *info)
>> @@ -5347,7 +5372,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>   			tname, off);
>>   		return false;
>>   	}
>> -	arg = off / 8;
>> +	arg = t == NULL ? (off / 8) :  get_ctx_arg_idx(btf, t, off);
> 
> is the t == NULL check needed? we relied on t being defined later in the code

t is defined earlier in
    const struct btf_type *t = prog->aux->attach_func_proto;
so we should be fine.

I guess I will fold
	if (!t)
		return off / 8;
inside get_ctx_arg_idx() to make interface cleaner.
Will do this in the next revision.

> 
> jirka
> 
>>   	args = (const struct btf_param *)(t + 1);
>>   	/* if (t == NULL) Fall back to default BPF prog with
>>   	 * MAX_BPF_FUNC_REG_ARGS u64 arguments.
>> @@ -5417,7 +5442,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>   	/* skip modifiers */
>>   	while (btf_type_is_modifier(t))
>>   		t = btf_type_by_id(btf, t->type);
>> -	if (btf_type_is_small_int(t) || btf_is_any_enum(t))
>> +	if (btf_type_is_small_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
>>   		/* accessing a scalar */
>>   		return true;
>>   	if (!btf_type_is_ptr(t)) {
>> @@ -5881,7 +5906,7 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
>>   	if (btf_type_is_ptr(t))
>>   		/* kernel size of pointer. Not BPF's size of pointer*/
>>   		return sizeof(void *);
>> -	if (btf_type_is_int(t) || btf_is_any_enum(t))
>> +	if (btf_type_is_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
>>   		return t->size;
>>   	return -EINVAL;
> 
> SNIP