bpftool: Make skeleton C++ compatible with explicit casts

Fix C++ compilation errors in generated skeleton by adding explicit
pointer casts and use char * subtraction for offset calculation

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.

Co-developed-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: WanLi Niu <niuwl1@chinatelecom.cn>
Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260106023123.2928-1-kiraskyler@163.com

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 993c7d9484a463a5cf05aded325a0a689b0e9e02..2f9e10752e288d1c6ab9f5f40da82099aeb69a6f 100644 (file)
--- 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 = (char *)&skel->links - (char *)skel; \n\
                ",
                obj_name, opts.data_sz);
        bpf_object__for_each_map(map, obj) {
@@ -755,7 +755,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
                \n\
                \";                                                         \n\
                                                                            \n\
-                               skel->%1$s = skel_prep_map_data((void *)data, %2$zd,\n\
+                               skel->%1$s = (__typeof__(skel->%1$s))skel_prep_map_data((void *)data, %2$zd,\n\
                                                                sizeof(data) - 1);\n\
                                if (!skel->%1$s)                            \n\
                                        goto cleanup;                       \n\
@@ -857,7 +857,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 
                codegen("\
                \n\
-                       skel->%1$s = skel_finalize_map_data(&skel->maps.%1$s.initial_value,  \n\
+                       skel->%1$s = (__typeof__(skel->%1$s))skel_finalize_map_data(&skel->maps.%1$s.initial_value,\n\
                                                        %2$zd, %3$s, skel->maps.%1$s.map_fd);\n\
                        if (!skel->%1$s)                                    \n\
                                return -ENOMEM;                             \n\