Re: [PATCH bpf-next] bpftool: cast pointers for shadow types explicitly.

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

On 3/12/24 15:47, Andrii Nakryiko wrote:
> On Mon, Mar 11, 2024 at 6:38 PM Kui-Feng Lee <thinker.li@gmail.com> wrote:
>>
>> According to a report, skeletons fail to assign shadow pointers when being
>> compiled with C++ programs. Unlike C doing implicit casting for void
>> pointers, C++ requires an explicit casting.
>>
>> To support C++, we do explicit casting for each shadow pointer.
>>
>> Cc: yhs@meta.com
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   tools/bpf/bpftool/gen.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
>> index 4fa4ade1ce74..dedafea0c127 100644
>> --- a/tools/bpf/bpftool/gen.c
>> +++ b/tools/bpf/bpftool/gen.c
>> @@ -1131,7 +1131,7 @@ static void gen_st_ops_shadow_init(struct btf *btf, struct bpf_object *obj)
>>                          continue;
>>                  codegen("\
>>                          \n\
>> -                               obj->struct_ops.%1$s = bpf_map__initial_value(obj->maps.%1$s, NULL);\n\
>> +                               obj->struct_ops.%1$s = (typeof(obj->struct_ops.%1$s))bpf_map__initial_value(obj->maps.%1$s, NULL);\n\
> 
> Given we have a named struct type for this and we use explicit type
> names in other parts of generated skeleton code, let's maybe use
> "struct %s__%s__%s" explicitly here (passing in obj_name, ident,
> type_name)?

I have considered about this solution. But, C++ works differently. It
has nested namespaces. That means it should be referred as
"XXX_skeleton::OOO_st_ops_map" in C++. Then, we need #if #else #endif
directives to provide two separated casting.

> 
> No strong preferences, but feels like a consistent approach here would be nice.
> 
>>                          \n\
>>                          ", ident);
>>          }
>> --
>> 2.34.1
>>