Re: [PATCH bpf-next v5 4/8] libbpf: Create a bpf_link in bpf_map__attach_struct_ops().

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/8/23 4:22 PM, Kui-Feng Lee wrote:
> 
> 
> On 3/8/23 13:42, Martin KaFai Lau wrote:
>> On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
>>> @@ -11566,22 +11591,34 @@ struct bpf_link *bpf_program__attach(const struct 
>>> bpf_program *prog)
>>>       return link;
>>>   }
>>> +struct bpf_link_struct_ops {
>>> +    struct bpf_link link;
>>> +    int map_fd;
>>> +};
>>> +
>>>   static int bpf_link__detach_struct_ops(struct bpf_link *link)
>>>   {
>>> +    struct bpf_link_struct_ops *st_link;
>>>       __u32 zero = 0;
>>> -    if (bpf_map_delete_elem(link->fd, &zero))
>>> -        return -errno;
>>> +    st_link = container_of(link, struct bpf_link_struct_ops, link);
>>> -    return 0;
>>> +    if (st_link->map_fd < 0) {
>>
>> map_fd < 0 should always be true?
> 
> If the user pass a wrong link, it can fail.

I may have missed something. How can user directly pass a link to this static 
function?

> I check it here explicitly even the kernel returns
> an error for deleting an element of a struct_ops w/ link.
Yep, the kernel should have stopped the delete if the user somehow corrupted the 
map_fd to -1.

> 
>>
>>> +        /* Fake bpf_link */
>>> +        if (bpf_map_delete_elem(link->fd, &zero))
>>> +            return -errno;
>>> +        return 0;
>>> +    }
>>> +
>>> +    /* Doesn't support detaching. */
>>> +    return -EOPNOTSUPP;