Re: [PATCH bpf-next v7 6/8] libbpf: Update a bpf_link with another struct_ops.

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

On 3/17/23 12:42, Martin KaFai Lau wrote:
> On 3/15/23 7:36 PM, Kui-Feng Lee wrote:
>> Introduce bpf_link__update_map(), which allows to atomically update
>> underlying struct_ops implementation for given struct_ops BPF link
>>
>> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
>> ---
>>   tools/lib/bpf/libbpf.c   | 30 ++++++++++++++++++++++++++++++
>>   tools/lib/bpf/libbpf.h   |  1 +
>>   tools/lib/bpf/libbpf.map |  1 +
>>   3 files changed, 32 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 6dbae7ffab48..63ec1f8fe8a0 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -11659,6 +11659,36 @@ struct bpf_link 
>> *bpf_map__attach_struct_ops(const struct bpf_map *map)
>>       return &link->link;
>>   }
>> +/*
>> + * Swap the back struct_ops of a link with a new struct_ops map.
>> + */
>> +int bpf_link__update_map(struct bpf_link *link, const struct bpf_map 
>> *map)
>> +{
>> +    struct bpf_link_struct_ops *st_ops_link;
>> +    __u32 zero = 0;
>> +    int err, fd;
>> +
>> +    if (!bpf_map__is_struct_ops(map) || map->fd < 0)
>> +        return -EINVAL;
>> +
>> +    st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
>> +    /* Ensure the type of a link is correct */
>> +    if (st_ops_link->map_fd < 0)
>> +        return -EINVAL;
>> +
>> +    err = bpf_map_update_elem(map->fd, &zero, 
>> map->st_ops->kern_vdata, 0);
>> +    if (err && err != -EBUSY)
>> +        return err;
>> +
>> +    fd = bpf_link_update(link->fd, map->fd, NULL);
> 
> bpf_link_update() returns ok/error. Using "fd = ..." is confusing. Use 
> "err = ..." instead and remove the need of "int fd;".

got it.

> 
>> +    if (fd < 0)
>> +        return fd;
>> +
>> +    st_ops_link->map_fd = map->fd;
>> +
>> +    return 0;
>> +}
>> +
>>   typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct 
>> perf_event_header *hdr,
>>                                 void *private_data);
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index db4992a036f8..1615e55e2e79 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -719,6 +719,7 @@ bpf_program__attach_freplace(const struct 
>> bpf_program *prog,
>>   struct bpf_map;
>>   LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct 
>> bpf_map *map);
>> +LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const 
>> struct bpf_map *map);
>>   struct bpf_iter_attach_opts {
>>       size_t sz; /* size of this struct for forward/backward 
>> compatibility */
>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>> index 50dde1f6521e..cc05be376257 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -387,6 +387,7 @@ LIBBPF_1.2.0 {
>>       global:
>>           bpf_btf_get_info_by_fd;
>>           bpf_link_get_info_by_fd;
>> +        bpf_link__update_map;
>>           bpf_map_get_info_by_fd;
>>           bpf_prog_get_info_by_fd;
>>   } LIBBPF_1.1.0;
>