Re: [PATCH bpf-next] bpf: Add rcu ptr in btf_id_sock_common_types

[Philo Lu <lulie@linux.alibaba.com>]

On 2024/10/11 06:07, Martin KaFai Lau wrote:
> On 10/8/24 7:23 PM, Philo Lu wrote:
>>
>>
>> On 2024/10/9 03:05, Martin KaFai Lau wrote:
>>> On 10/8/24 1:09 AM, Philo Lu wrote:
>>>> Sometimes sk is dereferenced as an rcu ptr, such as skb->sk in tp_btf,
>>>> which is a valid type of sock common. Then helpers like bpf_skc_to_*()
>>>> can be used with skb->sk.
>>>>
>>>> For example, the following prog will be rejected without this patch:
>>>> ```
>>>> SEC("tp_btf/tcp_bad_csum")
>>>> int BPF_PROG(tcp_bad_csum, struct sk_buff* skb)
>>>> {
>>>>     struct sock *sk = skb->sk;
>>>>     struct tcp_sock *tp;
>>>>
>>>>     if (!sk)
>>>>         return 0;
>>>>     tp = bpf_skc_to_tcp_sock(sk);
>>>
>>> If the use case is for reading the fields in tp, please use the 
>>> bpf_core_cast from the libbpf's bpf_core_read.h. bpf_core_cast is 
>>> using the bpf_rdonly_cast kfunc underneath.
>>>
>>
>> Thank you! This works for me so this patch is unnecessary then.
>>
>> Just curious is there any technical issue to include rcu_ptr into 
>> btf_id_sock_common_types? AFAICT rcu_ptr should also be a valid ptr 
>> type, and then btf_id_sock_common_types will behave like 
>> (PTR_TO_BTF_ID + &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON]) in 
>> bpf_func_proto.
> 
> bpf_skc_to_*() returns a PTR_TO_BTF_ID which can be passed into other 
> helpers that takes ARG_PTR_TO_BTF_ID_SOCK_COMMON. There are helpers that 
> change the sk. e.g. bpf_setsockopt() changes the sk and needs sk to be 
> locked. Other non tracing hooks do have a hold on the skb also. I did 
> take a quick look at the bpf_setsockopt situation and looks ok. I am 
> positive there are other helpers that need to audit first.
> 
> Tracing use case should only read the sk. bpf_core_cast() is the correct 
> one to use. The bpf_sk_storage_{get,delete}() should be the only allowed 
> helper that can change the sk.

Thank you for explanation, Martin. This helps me a lot.
-- 
Philo