Re: [PATCH v4 bpf-next 05/14] bpf: Remove btf_id helpers resolving

From: Yonghong Song <yhs@fb.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, <netdev@vger.kernel.org>,
	<bpf@vger.kernel.org>, Song Liu <songliubraving@fb.com>,
	Martin KaFai Lau <kafai@fb.com>, David Miller <davem@redhat.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Wenbo Zhang <ethercflow@gmail.com>,
	KP Singh <kpsingh@chromium.org>, Andrii Nakryiko <andriin@fb.com>,
	Brendan Gregg <bgregg@netflix.com>,
	Florent Revest <revest@chromium.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v4 bpf-next 05/14] bpf: Remove btf_id helpers resolving
Date: Sun, 28 Jun 2020 13:59:54 -0700	[thread overview]
Message-ID: <7c0a5ea0-9425-071d-0f41-b7e5c5ef04f0@fb.com> (raw)
In-Reply-To: <20200628201608.GG2988321@krava>

On 6/28/20 1:16 PM, Jiri Olsa wrote:
> On Fri, Jun 26, 2020 at 02:36:37PM -0700, Yonghong Song wrote:
> 
> SNIP
> 
>>> -	}
>>> -
>>> -	t = btf_type_by_id(btf_vmlinux, t->type);
>>> -	if (!btf_type_is_ptr(t))
>>> -		return -EFAULT;
>>> -	t = btf_type_by_id(btf_vmlinux, t->type);
>>> -	if (!btf_type_is_func_proto(t))
>>> -		return -EFAULT;
>>> -
>>> -	args = (const struct btf_param *)(t + 1);
>>> -	if (arg >= btf_type_vlen(t)) {
>>> -		bpf_log(log, "bpf helper %s doesn't have %d-th argument\n",
>>> -			fnname, arg);
>>> +	if (WARN_ON_ONCE(!fn->btf_id))
>>
>> The original code does not have this warning. It directly did
>> "ret = READ_ONCE(*btf_id);" after testing reg arg type ARG_PTR_TO_BTF_ID.
> 
> not sure why I put it in there, it's probably enough guarded
> by arg_type having ARG_PTR_TO_BTF_ID, will remove
> 
>>
>>>    		return -EINVAL;
>>> -	}
>>> -	t = btf_type_by_id(btf_vmlinux, args[arg].type);
>>> -	if (!btf_type_is_ptr(t) || !t->type) {
>>> -		/* anything but the pointer to struct is a helper config bug */
>>> -		bpf_log(log, "ARG_PTR_TO_BTF is misconfigured\n");
>>> -		return -EFAULT;
>>> -	}
>>> -	btf_id = t->type;
>>> -	t = btf_type_by_id(btf_vmlinux, t->type);
>>> -	/* skip modifiers */
>>> -	while (btf_type_is_modifier(t)) {
>>> -		btf_id = t->type;
>>> -		t = btf_type_by_id(btf_vmlinux, t->type);
>>> -	}
>>> -	if (!btf_type_is_struct(t)) {
>>> -		bpf_log(log, "ARG_PTR_TO_BTF is not a struct\n");
>>> -		return -EFAULT;
>>> -	}
>>> -	bpf_log(log, "helper %s arg%d has btf_id %d struct %s\n", fnname + 4,
>>> -		arg, btf_id, __btf_name_by_offset(btf_vmlinux, t->name_off));
>>> -	return btf_id;
>>> -}
>>> +	id = fn->btf_id[arg];
>>
>> The corresponding BTF_ID definition here is:
>>    BTF_ID_LIST(bpf_skb_output_btf_ids)
>>    BTF_ID(struct, sk_buff)
>>
>> The bpf helper writer needs to ensure proper declarations
>> of BTF_IDs like the above matching helpers definition.
>> Support we have arg1 and arg3 as BTF_ID. then the list
>> definition may be
>>
>>    BTF_ID_LIST(bpf_skb_output_btf_ids)
>>    BTF_ID(struct, sk_buff)
>>    BTF_ID(struct, __unused)
>>    BTF_ID(struct, task_struct)
>>
>> This probably okay, I guess.
> 
> right, AFAIK we don't have such case yet, but would be good
> to be ready and have something like
> 
>    BTF_ID(struct, __unused)
> 
> maybe adding new type for that will be better:
> 
>    BTF_ID(none, unused)

Maybe we can have a separate macro BTF_ID_UNUSED macro
which simply adds 4 bytes hole in the .btf_ids* section.

> 
> jirka
> 