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

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Message-ID: <f3ab8c9f-3bb9-4427-8b43-c1ed9d488970@kernel.org>
Date: Mon, 5 Jan 2026 10:39:59 +0000
From: Quentin Monnet <qmo@...nel.org>
To: WanLi Niu <kiraskyler@....com>
Cc: 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

On 05/01/2026 07:12, WanLi Niu wrote:
> 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.


I tried it and could confirm that the warnings are gone.


> Signed-off-by: WanLi Niu <niuwl1@...natelecom.cn>
> Co-developed-by: Menglong Dong <dongml2@...natelecom.cn>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>


Acked-by: Quentin Monnet <qmo@...nel.org>

Thank you!