Re: [PATCH v4 bpf-next] bpftool: Make skeleton C++ compatible with explicit casts

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

FWIW I tested the reproducer with gcc-bpf and got no pointer conversion
warnings (not that I was expecting anything different, but just in
case):

 $ bpf-unknown-none-gcc -std=gnu11 -I./tools/include -g -O2 -c text.bpf.c -o test.bpf.o
 $ bpftool gen skeleton test.bpf.o -L > test.bpf.skel.h
 $ g++ -c test.cpp -I.

> From: WanLi Niu <niuwl1@chinatelecom.cn>
>
> Fix C++ compilation errors in generated skeleton by adding explicit
> pointer casts and using integer subtraction for offset calculation.
>
> Use struct outer::inner syntax under __cplusplus to access nested skeleton map
> structs, ensuring C++ compilation compatibility while preserving C support
>
> error: invalid conversion from 'void*' to '<obj_name>*' [-fpermissive]
>       |         skel = skel_alloc(sizeof(*skel));
>       |                ~~~~~~~~~~^~~~~~~~~~~~~~~
>       |                          |
>       |                          void*
>
> error: arithmetic on pointers to void
>       |         skel->ctx.sz = (void *)&skel->links - (void *)skel;
>       |                        ~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
>
> error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
>       |                 skel-><ident> = skel_prep_map_data((void *)data, 4096,
>       |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       |                                                 sizeof(data) - 1);
>       |                                                 ~~~~~~~~~~~~~~~~~
>
> error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
>       |         skel-><ident> = skel_finalize_map_data(&skel->maps.<ident>.initial_value,
>       |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       |                                         4096, PROT_READ | PROT_WRITE, skel->maps.<ident>.map_fd);
>       |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Minimum reproducer:
>
> 	$ cat test.bpf.c
> 	int val; // placed in .bss section
>
> 	#include "vmlinux.h"
> 	#include <bpf/bpf_helpers.h>
>
> 	SEC("raw_tracepoint/sched_wakeup_new") int handle(void *ctx) { return 0; }
>
> 	$ cat test.cpp
> 	#include <cerrno>
>
> 	extern "C" {
> 	#include "test.bpf.skel.h"
> 	}
>
> 	$ bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h
> 	$ clang -g -O2 -target bpf -c test.bpf.c -o test.bpf.o
> 	$ bpftool gen skeleton test.bpf.o -L  > test.bpf.skel.h
> 	$ g++ -c test.cpp -I.
>
> Signed-off-by: WanLi Niu <niuwl1@chinatelecom.cn>
> Co-developed-by: Menglong Dong <dongml2@chinatelecom.cn>
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
> changelog:
> v4:
> - Add a minimum reproducer to demonstrate the issue, as suggested by Yonghong Song
>
> v3: https://lore.kernel.org/all/20260104021402.2968-1-kiraskyler@163.com/
> - Fix two additional <obj_name>__<ident> type mismatches as suggested by Yonghong Song
>
> v2: https://lore.kernel.org/all/20251231102929.3843-1-kiraskyler@163.com/
> - Use generic (struct %1$s *) instead of project-specific (struct trace_bpf *)
>
> v1: https://lore.kernel.org/all/20251231092541.3352-1-kiraskyler@163.com/
> ---
>  tools/bpf/bpftool/gen.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index 993c7d9484a4..010861b7d0ea 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> @@ -731,10 +731,10 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  		{							    \n\
>  			struct %1$s *skel;				    \n\
>  									    \n\
> -			skel = skel_alloc(sizeof(*skel));		    \n\
> +			skel = (struct %1$s *)skel_alloc(sizeof(*skel));    \n\
>  			if (!skel)					    \n\
>  				goto cleanup;				    \n\
> -			skel->ctx.sz = (void *)&skel->links - (void *)skel; \n\
> +			skel->ctx.sz = (__u64)&skel->links - (__u64)skel;   \n\
>  		",
>  		obj_name, opts.data_sz);
>  	bpf_object__for_each_map(map, obj) {
> @@ -755,13 +755,17 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  		\n\
>  		\";							    \n\
>  									    \n\
> +		#ifdef __cplusplus                                          \n\
> +				skel->%1$s = (struct %3$s::%3$s__%1$s *)skel_prep_map_data((void *)data, %2$zd,\n\
> +		#else                                                       \n\
>  				skel->%1$s = skel_prep_map_data((void *)data, %2$zd,\n\
> +		#endif							    \n\
>  								sizeof(data) - 1);\n\
>  				if (!skel->%1$s)			    \n\
>  					goto cleanup;			    \n\
>  				skel->maps.%1$s.initial_value = (__u64) (long) skel->%1$s;\n\
>  			}						    \n\
> -			", ident, bpf_map_mmap_sz(map));
> +			", ident, bpf_map_mmap_sz(map), obj_name);
>  	}
>  	codegen("\
>  		\n\
> @@ -857,12 +861,16 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  
>  		codegen("\
>  		\n\
> +		#ifdef __cplusplus					    \n\
> +			skel->%1$s = (struct %4$s::%4$s__%1$s *)skel_finalize_map_data(&skel->maps.%1$s.initial_value,\n\
> +		#else							    \n\
>  			skel->%1$s = skel_finalize_map_data(&skel->maps.%1$s.initial_value,  \n\
> +		#endif							    \n\
>  							%2$zd, %3$s, skel->maps.%1$s.map_fd);\n\
>  			if (!skel->%1$s)				    \n\
>  				return -ENOMEM;				    \n\
>  			",
> -		       ident, bpf_map_mmap_sz(map), mmap_flags);
> +		       ident, bpf_map_mmap_sz(map), mmap_flags, obj_name);
>  	}
>  	codegen("\
>  		\n\