>>> As in, do you substitute forward declarations for types that are
>>> never directly used? If not, that's going to be very suboptimal for
>>> something like task_struct and any other type that's part of a big
>>> cluster of types.
>> We decided to include the whole types and all direct and indirect
>> types referenced from a structure field for type-based relocations.
>> Our reasoning is that we don't know if the matching algorithm of
>> libbpf could be changed to require more information in the future and
>> type-based relocations are few compared to field based relocations.
> It will depend on application and which type is used in relocation.
> task_struct reaches tons of types and will add a very noticeable size
> to minimized BTF, for no good reason, IMO. If we discover that we do
> need those types, we'll update bpftool to generate more.
Just to see if I understood this part correctly. IIRC, we started type
based relocations support in btfgen because of this particular case:
union kernfs_node_id {
struct {
u32 ino;
u32 generation;
};
u64 id;
};
struct kernfs_node___older_v55 {
const char *name;
union kernfs_node_id id;
};
struct kernfs_node___rh8 {
const char *name;
union {
u64 id;
struct {
union kernfs_node_id id;
} rh_kabi_hidden_172;
union { };
};
};
So we have 3 situations:
(struct kernfs_node *)->id as u64
[29] STRUCT 'kernfs_node' size=128 vlen=1
'id' type_id=42 bits_offset=832
[42] TYPEDEF 'u64' type_id=10
(struct kernfs_node___older_v55 *)->id as u64 (union kernfs_node_id)->id
[79] STRUCT 'kernfs_node' size=128 vlen=1
'id' type_id=69 bits_offset=832
[69] UNION 'kernfs_node_id' size=8 vlen=2
'(anon)' type_id=132 bits_offset=0
'id' type_id=40 bits_offset=0
[40] TYPEDEF 'u64' type_id=12
(struct kernfs_node___rh8 *)->id = (anon union)->id
[56] STRUCT 'kernfs_node' size=128 vlen=1
'(anon)' type_id=24 bits_offset=832
[24] UNION '(anon)' size=8 vlen=1
'id' type_id=40 bits_offset=0
[40] TYPEDEF 'u64' type_id=11
We're finding needed BTF types, that should be added to generated BTF,
based on fields/members of CORE relo info. How we would know we had to
add the anon union of the last case if it does not exist in the local
BTF ? What is your suggestion ?
Thanks!
-rafaeldtinoco
