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Message-ID: <87eco4nuri.fsf@oracle.com>
Date: Mon, 05 Jan 2026 12:50:25 +0100
From: "Jose E. Marchesi" <jose.marchesi@...cle.com>
To: WanLi Niu <kiraskyler@....com>
Cc: Quentin Monnet <qmo@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko
<andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Eduard
Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song
<yonghong.song@...ux.dev>,
John Fastabend <john.fastabend@...il.com>,
KP
Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
Hao
Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Menglong Dong
<menglong8.dong@...il.com>, <bpf@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, WanLi Niu <niuwl1@...natelecom.cn>,
Menglong Dong <dongml2@...natelecom.cn>
Subject: Re: [PATCH v4 bpf-next] bpftool: Make skeleton C++ compatible with
explicit casts
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@...natelecom.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@...natelecom.cn>
> Co-developed-by: Menglong Dong <dongml2@...natelecom.cn>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.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\
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