Re: [External] Re: [PATCH bpf-next] libbpf: Let any two INT/UNION compatible if their names and sizes match

[Feng Zhou <zhoufeng.zf@bytedance.com>]

在 2021/12/1 下午12:17, Andrii Nakryiko 写道:
> On Tue, Nov 30, 2021 at 7:55 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>>
>> commit:67c0496e87d193b8356d2af49ab95e8a1b954b3c(kernfs: convert
>> kernfs_node->id from union kernfs_node_id to u64).
>>
>> The bpf program compiles on the kernel version after this commit and
>> then tries to run on the kernel before this commit, libbpf will report
>> an error. The reverse is also same.
>>
>> libbpf: prog 'tcp_retransmit_synack_tp': relo #4: kind <byte_off> (0),
>> spec is [342] struct kernfs_node.id (0:9 @ offset 104)
>> libbpf: prog 'tcp_retransmit_synack_tp': relo #4: non-matching candidate
>> libbpf: prog 'tcp_retransmit_synack_tp': relo #4: non-matching candidate
>> libbpf: prog 'tcp_retransmit_synack_tp': relo #4: no matching targets
>> found
>>
>> The type before this commit:
>>          union kernfs_node_id    id;
>>          union kernfs_node_id {
>>                  struct {
>>                          u32             ino;
>>                          u32             generation;
>>                  };
>>                  u64                     id;
>>          };
>>
>> The type after this commit:
>>          u64 id;
>>
>> We can find that the variable name and size have not changed except for
>> the type change.
>> So I added some judgment to let any two INT/UNION are compatible, if
>> their names and sizes match.
>>
>> Reported-by: Chengming Zhou <zhouchengming@bytedance.com>
>> Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
>> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
>> ---
> This should be handled by application, not by hacking libbpf's CO-RE
> relocation logic. See [0] for how this should be done with existing
> BPF CO-RE mechanisms.
>
>    [0] https://nakryiko.com/posts/bpf-core-reference-guide/#handling-incompatible-field-and-type-changes

This is very useful to me, thank you very much.

>>   tools/lib/bpf/relo_core.c | 21 +++++++++++++++++----
>>   1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/lib/bpf/relo_core.c b/tools/lib/bpf/relo_core.c
>> index b5b8956a1be8..ff7f4e97bafb 100644
>> --- a/tools/lib/bpf/relo_core.c
>> +++ b/tools/lib/bpf/relo_core.c
>> @@ -294,6 +294,7 @@ static int bpf_core_parse_spec(const struct btf *btf,
>>    *   - any two FLOATs are always compatible;
>>    *   - for ARRAY, dimensionality is ignored, element types are checked for
>>    *     compatibility recursively;
>> + *   - any two INT/UNION are compatible, if their names and sizes match;
>>    *   - everything else shouldn't be ever a target of relocation.
>>    * These rules are not set in stone and probably will be adjusted as we get
>>    * more experience with using BPF CO-RE relocations.
>> @@ -313,8 +314,14 @@ static int bpf_core_fields_are_compat(const struct btf *local_btf,
>>
>>          if (btf_is_composite(local_type) && btf_is_composite(targ_type))
>>                  return 1;
>> -       if (btf_kind(local_type) != btf_kind(targ_type))
>> -               return 0;
>> +       if (btf_kind(local_type) != btf_kind(targ_type)) {
>> +               if (local_type->size == targ_type->size &&
>> +                   (btf_is_union(local_type) || btf_is_union(targ_type)) &&
>> +                   (btf_is_int(local_type) || btf_is_int(targ_type)))
>> +                       return 1;
>> +               else
>> +                       return 0;
>> +       }
>>
>>          switch (btf_kind(local_type)) {
>>          case BTF_KIND_PTR:
>> @@ -384,11 +391,17 @@ static int bpf_core_match_member(const struct btf *local_btf,
>>          targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
>>          if (!targ_type)
>>                  return -EINVAL;
>> -       if (!btf_is_composite(targ_type))
>> -               return 0;
>>
>>          local_id = local_acc->type_id;
>>          local_type = btf__type_by_id(local_btf, local_id);
>> +       if (!btf_is_composite(targ_type)) {
>> +               if (local_type->size == targ_type->size &&
>> +                   btf_is_union(local_type) && btf_is_int(targ_type))
>> +                       return 1;
>> +               else
>> +                       return 0;
>> +       }
>> +
>>          local_member = btf_members(local_type) + local_acc->idx;
>>          local_name = btf__name_by_offset(local_btf, local_member->name_off);
>>
>> --
>> 2.11.0
>>