[PATCH v3 bpf-next] bpftool: Make skeleton C++ compatible with explicit casts

[WanLi Niu]

From: WanLi Niu <niuwl1@xxxxxxxxxxxxxxx>

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);
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: WanLi Niu <niuwl1@xxxxxxxxxxxxxxx>
Co-developed-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
---
changelog:
v3:
- Fix two additional <obj_name>__<ident> type mismatches as suggested by Yonghong Song

v2: https://lore.kernel.org/all/20251231102929.3843-1-kiraskyler@xxxxxxx/
- Use generic (struct %1$s *) instead of project-specific (struct trace_bpf *)

v1: https://lore.kernel.org/all/20251231092541.3352-1-kiraskyler@xxxxxxx/
---
 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\
-- 
2.39.1