Re: [PATCH bpf-next v13 14/14] bpf: pass btf object id in bpf_map_info.

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

On 12/14/23 23:46, Martin KaFai Lau wrote:
> On 12/8/23 4:27 PM, thinker.li@gmail.com wrote:
>> From: Kui-Feng Lee <thinker.li@gmail.com>
>>
>> Include btf object id (btf_obj_id) in bpf_map_info so that tools (ex:
>> bpftools struct_ops dump) know the correct btf from the kernel to look up
>> type information of struct_ops types.
>>
>> Since struct_ops types can be defined and registered in a module. The
>> type information of a struct_ops type are defined in the btf of the
>> module defining it.  The userspace tools need to know which btf is for
>> the module defining a struct_ops type.
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   include/linux/bpf.h            | 1 +
>>   include/uapi/linux/bpf.h       | 2 +-
>>   kernel/bpf/bpf_struct_ops.c    | 7 +++++++
>>   kernel/bpf/syscall.c           | 2 ++
>>   tools/include/uapi/linux/bpf.h | 2 +-
>>   5 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index c881befa35f5..26103d8a4374 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -3350,5 +3350,6 @@ struct bpf_struct_ops_##_name 
>> {                    \
>>   int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
>>                    struct btf *btf,
>>                    struct bpf_verifier_log *log);
>> +void bpf_map_struct_ops_info_fill(struct bpf_map_info *info, struct 
>> bpf_map *map);
> 
> This needs to be in the CONFIG_BPF_JIT guard also.

Got it!
> 
>>   #endif /* _LINUX_BPF_H */
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 5c3838a97554..716c6b28764d 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -6534,7 +6534,7 @@ struct bpf_map_info {
>>       __u32 btf_id;
>>       __u32 btf_key_type_id;
>>       __u32 btf_value_type_id;
>> -    __u32 :32;    /* alignment pad */
>> +    __u32 btf_obj_id;
> 
> may be "btf_vmlinux_id" to make it clear it is a kernel btf and should 
> be used with map_info->btf_vmlinux_value_type_id.

Sure!

> 
>>       __u64 map_extra;
>>   } __attribute__((aligned(8)));
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index fd26716fa0f9..51c0de75aa85 100644
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -979,3 +979,10 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
>>       kfree(link);
>>       return err;
>>   }
>> +
>> +void bpf_map_struct_ops_info_fill(struct bpf_map_info *info, struct 
>> bpf_map *map)
>> +{
>> +    struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map 
>> *)map;
>> +
>> +    info->btf_obj_id = btf_obj_id(st_map->btf);
>> +}
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 4aced7e58904..3cab56cd02ff 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -4715,6 +4715,8 @@ static int bpf_map_get_info_by_fd(struct file 
>> *file,
>>           info.btf_value_type_id = map->btf_value_type_id;
>>       }
>>       info.btf_vmlinux_value_type_id = map->btf_vmlinux_value_type_id;
>> +    if (map->map_type == BPF_MAP_TYPE_STRUCT_OPS)
>> +        bpf_map_struct_ops_info_fill(&info, map);
> 
> This patch should be moved earlier in the set instead of after the 
> selftest patch 13. May be after patch 5 where st_map->btf is added.

No problem

> 
> and where is the test for this?

I use bpf_map_info as a part of calling bpf_map_create() while
testmod.ko is unloaded. It check if this change work as well.

> 
>>       if (bpf_map_is_offloaded(map)) {
>>           err = bpf_map_offload_info_fill(&info, map);
>> diff --git a/tools/include/uapi/linux/bpf.h 
>> b/tools/include/uapi/linux/bpf.h
>> index 5c3838a97554..716c6b28764d 100644
>> --- a/tools/include/uapi/linux/bpf.h
>> +++ b/tools/include/uapi/linux/bpf.h
>> @@ -6534,7 +6534,7 @@ struct bpf_map_info {
>>       __u32 btf_id;
>>       __u32 btf_key_type_id;
>>       __u32 btf_value_type_id;
>> -    __u32 :32;    /* alignment pad */
>> +    __u32 btf_obj_id;
>>       __u64 map_extra;
>>   } __attribute__((aligned(8)));
>