Re: [PATCH v2 bpf-next] bpftool: Make skeleton C++ compatible with explicit casts

[Yonghong Song]

On 12/31/25 2:29 AM, WanLi Niu wrote:
> From: WanLi Niu <niuwl1@xxxxxxxxxxxxxxx>
>
> Fix C++ compilation errors in generated skeleton by adding explicit
> pointer casts and using integer subtraction for offset calculation.
>
> error: invalid conversion from 'void*' to 'trace_bpf*' [-fpermissive]
>        |         skel = skel_alloc(sizeof(*skel));
>        |                ~~~~~~~~~~^~~~~~~~~~~~~~~
>        |                          |
>        |                          void*
>
> error: invalid use of 'void'
>        |         skel->ctx.sz = (void *)&skel->links - (void *)skel;
>
> Signed-off-by: WanLi Niu <niuwl1@xxxxxxxxxxxxxxx>
> Co-developed-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>

For llvm22, I hacked with core_kern_overflow.lskel.h and has the following
warning/errors:

In file included from prog_tests/core_kern_overflow.cc:4:
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/core_kern_overflow.lskel.h:65:9: error: assigning to
      'struct core_kern_overflow_lskel *' from incompatible type 'void *'
   65 |         skel = skel_alloc(sizeof(*skel));
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/core_kern_overflow.lskel.h:68:38: error: arithmetic on pointers to void
   68 |         skel->ctx.sz = (void *)&skel->links - (void *)skel;
      |                        ~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~

Your patch fixed the issue.

But for llvm side, I got the following two more errors:

/home/yhs/work/bpf-next/tools/testing/selftests/bpf/core_kern_overflow.lskel.h:73:15: error: assigning to
      'struct core_kern_overflow_lskel__bss *' from incompatible type 'void *'
   73 |                 skel->bss = skel_prep_map_data((void *)data, 4096,
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   74 |                                                 sizeof(data) - 1);
      |                                                 ~~~~~~~~~~~~~~~~~
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/core_kern_overflow.lskel.h:223:14: error: assigning to
      'struct core_kern_overflow_lskel__bss *' from incompatible type 'void *'
  223 |         skel->bss = skel_finalize_map_data(&skel->maps.bss.initial_value,
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  224 |                                         4096, PROT_READ | PROT_WRITE, skel->maps.bss.map_fd);
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So these two issues can be fixed similar to above skel_alloc() case.

Could you fix both of them?

> ---
>   tools/bpf/bpftool/gen.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index 993c7d9484a4..71446a776130 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> @@ -731,10 +731,10 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>   		{							    \n\
>   			struct %1$s *skel;				    \n\
>   									    \n\
> -			skel = skel_alloc(sizeof(*skel));		    \n\
> +			skel = (struct %1$s *)skel_alloc(sizeof(*skel));    \n\
>   			if (!skel)					    \n\
>   				goto cleanup;				    \n\
> -			skel->ctx.sz = (void *)&skel->links - (void *)skel; \n\
> +			skel->ctx.sz = (__u64)&skel->links - (__u64)skel;   \n\
>   		",
>   		obj_name, opts.data_sz);
>   	bpf_object__for_each_map(map, obj) {