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Message-ID: <7219135.9fee.19b8e385eb5.Coremail.kiraskyler@163.com>
Date: Mon, 5 Jan 2026 20:53:37 +0800 (CST)
From: "WanLi Niu" <kiraskyler@....com>
To: "Jose E. Marchesi" <jose.marchesi@...cle.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.
Thanks for testing.
The issue only occurs when bpftool gen skeleton is used in use_loader mode (bpftool gen skeleton -L|--use-loader).
Could you double-check if -L was included? I can reproduce the error reliably with it — here’s the full output:
$ rpm -qf `which bpf-unknown-none-gcc`
gcc-bpf-unknown-none-15.2.1-1.fc42.x86_64
$ bpf-unknown-none-gcc -std=gnu11 -I/usr/include -g -O2 -c test.bpf.c -o test.bpf.o
$ bpftool gen skeleton test.bpf.o -L > test.bpf.skel.h
libbpf: elf: skipping section(2) .data (size 0)
libbpf: elf: skipping unrecognized data section(7) .comment
libbpf: prog 'handle': missing .BTF.ext line info for the main program, skipping all of .BTF.ext line info.
$ g++ -c test.cpp -I.
In file included from test.cpp:4:
test.bpf.skel.h: In function ‘test_bpf* test_bpf__open()’:
test.bpf.skel.h:65:26: error: invalid conversion from ‘void*’ to ‘test_bpf*’ [-fpermissive]
65 | skel = skel_alloc(sizeof(*skel));
| ~~~~~~~~~~^~~~~~~~~~~~~~~
| |
| void*
test.bpf.skel.h:68:55: error: invalid use of ‘void’
68 | skel->ctx.sz = (void *)&skel->links - (void *)skel;
| ^~~~
test.bpf.skel.h:73:47: error: invalid conversion from ‘void*’ to ‘test_bpf::test_bpf__bss*’ [-fpermissive]
73 | skel->bss = skel_prep_map_data((void *)data, 4096,
| ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
| |
| void*
74 | sizeof(data) - 1);
| ~~~~~~~~~~~~~~~~~
test.bpf.skel.h: In function ‘int test_bpf__load(test_bpf*)’:
test.bpf.skel.h:196:43: error: invalid conversion from ‘void*’ to ‘test_bpf::test_bpf__bss*’ [-fpermissive]
196 | skel->bss = skel_finalize_map_data(&skel->maps.bss.initial_value,
| ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| void*
197 | 4096, PROT_READ | PROT_WRITE, skel->maps.bss.map_fd);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ cat test.bpf.skel.h
/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
/* THIS FILE IS AUTOGENERATED BY BPFTOOL! */
#ifndef __TEST_BPF_SKEL_H__
#define __TEST_BPF_SKEL_H__
#include <bpf/skel_internal.h>
struct test_bpf {
struct bpf_loader_ctx ctx;
struct {
struct bpf_map_desc bss;
} maps;
struct {
struct bpf_prog_desc handle;
} progs;
struct {
int handle_fd;
} links;
struct test_bpf__bss {
int val;
} *bss;
};
static inline int
test_bpf__handle__attach(struct test_bpf *skel)
{
int prog_fd = skel->progs.handle.prog_fd;
int fd = skel_raw_tracepoint_open("sched_wakeup_new", prog_fd);
if (fd > 0)
skel->links.handle_fd = fd;
return fd;
}
static inline int
test_bpf__attach(struct test_bpf *skel)
{
int ret = 0;
ret = ret < 0 ? ret : test_bpf__handle__attach(skel);
return ret < 0 ? ret : 0;
}
static inline void
test_bpf__detach(struct test_bpf *skel)
{
skel_closenz(skel->links.handle_fd);
}
static void
test_bpf__destroy(struct test_bpf *skel)
{
if (!skel)
return;
test_bpf__detach(skel);
skel_closenz(skel->progs.handle.prog_fd);
skel_free_map_data(skel->bss, skel->maps.bss.initial_value, 4096);
skel_closenz(skel->maps.bss.map_fd);
skel_free(skel);
}
static inline struct test_bpf *
test_bpf__open(void)
{
struct test_bpf *skel;
skel = skel_alloc(sizeof(*skel));
if (!skel)
goto cleanup;
skel->ctx.sz = (void *)&skel->links - (void *)skel;
{
static const char data[] __attribute__((__aligned__(8))) = "\
\0\0\0\0";
skel->bss = skel_prep_map_data((void *)data, 4096,
sizeof(data) - 1);
if (!skel->bss)
goto cleanup;
skel->maps.bss.initial_value = (__u64) (long) skel->bss;
}
return skel;
cleanup:
test_bpf__destroy(skel);
return NULL;
}
static inline int
test_bpf__load(struct test_bpf *skel)
{
struct bpf_load_and_run_opts opts = {};
int err;
static const char opts_data[] __attribute__((__aligned__(8))) = "\
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\0\0\0\x10\0\0\0\0\0\0\0";
static const char opts_insn[] __attribute__((__aligned__(8))) = "\
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\0\0\0\0\0\0\0\0\x58\x06\0\0\x18\x61\0\0\0\0\0\0\0\0\0\0\x88\x06\0\0\x7b\x01\0\
\0\0\0\0\0\x18\x60\0\0\0\0\0\0\0\0\0\0\x60\x06\0\0\x18\x61\0\0\0\0\0\0\0\0\0\0\
\x80\x06\0\0\x7b\x01\0\0\0\0\0\0\x18\x60\0\0\0\0\0\0\0\0\0\0\x70\x06\0\0\x18\
\x61\0\0\0\0\0\0\0\0\0\0\xc8\x06\0\0\x7b\x01\0\0\0\0\0\0\x18\x60\0\0\0\0\0\0\0\
\0\0\0\x78\x06\0\0\x18\x61\0\0\0\0\0\0\0\0\0\0\xd8\x06\0\0\x7b\x01\0\0\0\0\0\0\
\x18\x60\0\0\0\0\0\0\0\0\0\0\x78\x06\0\0\x18\x61\0\0\0\0\0\0\0\0\0\0\xf8\x06\0\
\0\x7b\x01\0\0\0\0\0\0\x18\x60\0\0\0\0\0\0\0\0\0\0\0\0\0\0\x18\x61\0\0\0\0\0\0\
\0\0\0\0\xf0\x06\0\0\x7b\x01\0\0\0\0\0\0\x61\x60\x08\0\0\0\0\0\x18\x61\0\0\0\0\
\0\0\0\0\0\0\x90\x06\0\0\x63\x01\0\0\0\0\0\0\x61\x60\x0c\0\0\0\0\0\x18\x61\0\0\
\0\0\0\0\0\0\0\0\x94\x06\0\0\x63\x01\0\0\0\0\0\0\x79\x60\x10\0\0\0\0\0\x18\x61\
\0\0\0\0\0\0\0\0\0\0\x98\x06\0\0\x7b\x01\0\0\0\0\0\0\x61\xa0\x78\xff\0\0\0\0\
\x18\x61\0\0\0\0\0\0\0\0\0\0\xc0\x06\0\0\x63\x01\0\0\0\0\0\0\xb7\x01\0\0\x05\0\
\0\0\x18\x62\0\0\0\0\0\0\0\0\0\0\x78\x06\0\0\xb7\x03\0\0\x8c\0\0\0\x85\0\0\0\
\xa6\0\0\0\xbf\x07\0\0\0\0\0\0\xc5\x07\x6d\xff\0\0\0\0\x63\x7a\x80\xff\0\0\0\0\
\x61\xa1\x78\xff\0\0\0\0\xd5\x01\x02\0\0\0\0\0\xbf\x19\0\0\0\0\0\0\x85\0\0\0\
\xa8\0\0\0\x61\xa0\x80\xff\0\0\0\0\x63\x06\x28\0\0\0\0\0\x18\x61\0\0\0\0\0\0\0\
\0\0\0\0\0\0\0\x61\x10\0\0\0\0\0\0\x63\x06\x18\0\0\0\0\0\xb7\0\0\0\0\0\0\0\x95\
\0\0\0\0\0\0\0";
opts.ctx = (struct bpf_loader_ctx *)skel;
opts.data_sz = sizeof(opts_data) - 1;
opts.data = (void *)opts_data;
opts.insns_sz = sizeof(opts_insn) - 1;
opts.insns = (void *)opts_insn;
err = bpf_load_and_run(&opts);
if (err < 0)
return err;
skel->bss = skel_finalize_map_data(&skel->maps.bss.initial_value,
4096, PROT_READ | PROT_WRITE, skel->maps.bss.map_fd);
if (!skel->bss)
return -ENOMEM;
return 0;
}
static inline struct test_bpf *
test_bpf__open_and_load(void)
{
struct test_bpf *skel;
skel = test_bpf__open();
if (!skel)
return NULL;
if (test_bpf__load(skel)) {
test_bpf__destroy(skel);
return NULL;
}
return skel;
}
__attribute__((unused)) static void
test_bpf__assert(struct test_bpf *s __attribute__((unused)))
{
#ifdef __cplusplus
#define _Static_assert static_assert
#endif
_Static_assert(sizeof(s->bss->val) == 4, "unexpected size of 'val'");
#ifdef __cplusplus
#undef _Static_assert
#endif
}
#endif /* __TEST_BPF_SKEL_H__ */
At 2026-01-05 19:50:25, "Jose E. Marchesi" <jose.marchesi@...cle.com> wrote:
>
>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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