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Message-ID: <CAEf4BzYRC+=J05C6QDwgzbJ7gO7gZD4xcEcj9ixCaJ=xaRuSsQ@mail.gmail.com>
Date: Thu, 15 Jan 2026 16:42:22 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Leon Hwang <leon.hwang@...ux.dev>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>,
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>, KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
Seth Forshee <sforshee@...nel.org>, Yuichiro Tsuji <yuichtsu@...zon.com>,
Andrey Albershteyn <aalbersh@...hat.com>, Willem de Bruijn <willemb@...gle.com>,
Jason Xing <kerneljasonxing@...il.com>, Tao Chen <chen.dylane@...ux.dev>,
Mykyta Yatsenko <yatsenko@...a.com>, Kumar Kartikeya Dwivedi <memxor@...il.com>,
Anton Protopopov <a.s.protopopov@...il.com>, Amery Hung <ameryhung@...il.com>,
Rong Tao <rongtao@...tc.cn>, linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
linux-kselftest@...r.kernel.org, kernel-patches-bot@...com
Subject: Re: [PATCH bpf-next v5 2/9] libbpf: Add support for extended bpf syscall
On Mon, Jan 12, 2026 at 6:58 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>
> To support the extended BPF syscall introduced in the previous commit,
> introduce the following internal APIs:
>
> * 'sys_bpf_ext()'
> * 'sys_bpf_ext_fd()'
> They wrap the raw 'syscall()' interface to support passing extended
> attributes.
> * 'probe_sys_bpf_ext()'
> Check whether current kernel supports the extended attributes.
>
> Signed-off-by: Leon Hwang <leon.hwang@...ux.dev>
> ---
> tools/lib/bpf/bpf.c | 34 +++++++++++++++++++++++++++++++++
> tools/lib/bpf/features.c | 8 ++++++++
> tools/lib/bpf/libbpf_internal.h | 3 +++
> 3 files changed, 45 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 21b57a629916..d44e667aaf02 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -69,6 +69,40 @@ static inline __u64 ptr_to_u64(const void *ptr)
> return (__u64) (unsigned long) ptr;
> }
>
> +static inline int sys_bpf_ext(enum bpf_cmd cmd, union bpf_attr *attr,
> + unsigned int size,
> + struct bpf_common_attr *common_attr,
nit: kernel uses consistent attr_common/size_common pattern, but here
you are inverting attr_common -> common_attr, let's not?
> + unsigned int size_common)
> +{
> + cmd = common_attr ? (cmd | BPF_COMMON_ATTRS) : (cmd & ~BPF_COMMON_ATTRS);
> + return syscall(__NR_bpf, cmd, attr, size, common_attr, size_common);
> +}
> +
> +static inline int sys_bpf_ext_fd(enum bpf_cmd cmd, union bpf_attr *attr,
> + unsigned int size,
> + struct bpf_common_attr *common_attr,
> + unsigned int size_common)
> +{
> + int fd;
> +
> + fd = sys_bpf_ext(cmd, attr, size, common_attr, size_common);
> + return ensure_good_fd(fd);
> +}
> +
> +int probe_sys_bpf_ext(void)
> +{
> + const size_t attr_sz = offsetofend(union bpf_attr, prog_token_fd);
> + union bpf_attr attr;
> + int fd;
> +
> + memset(&attr, 0, attr_sz);
> + fd = syscall(__NR_bpf, BPF_PROG_LOAD | BPF_COMMON_ATTRS, &attr, attr_sz, NULL,
> + sizeof(struct bpf_common_attr));
> + if (fd >= 0)
> + close(fd);
hm... close can change errno, this is fragile. If fd >= 0, something
is wrong with our detection, just return error right away?
> + return errno == EFAULT;
> +}
> +
> static inline int sys_bpf(enum bpf_cmd cmd, union bpf_attr *attr,
> unsigned int size)
> {
> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index b842b83e2480..d786a815f1ae 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c
> @@ -506,6 +506,11 @@ static int probe_kern_arg_ctx_tag(int token_fd)
> return probe_fd(prog_fd);
> }
>
> +static int probe_kern_extended_syscall(int token_fd)
> +{
> + return probe_sys_bpf_ext();
> +}
> +
> typedef int (*feature_probe_fn)(int /* token_fd */);
>
> static struct kern_feature_cache feature_cache;
> @@ -581,6 +586,9 @@ static struct kern_feature_desc {
> [FEAT_BTF_QMARK_DATASEC] = {
> "BTF DATASEC names starting from '?'", probe_kern_btf_qmark_datasec,
> },
> + [FEAT_EXTENDED_SYSCALL] = {
> + "Kernel supports extended syscall", probe_kern_extended_syscall,
"extended syscall" is a bit vague... We specifically detect common
attrs support, maybe say that?
> + },
> };
>
> bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index fc59b21b51b5..e2a6ef4b45ae 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -392,6 +392,8 @@ enum kern_feature_id {
> FEAT_ARG_CTX_TAG,
> /* Kernel supports '?' at the front of datasec names */
> FEAT_BTF_QMARK_DATASEC,
> + /* Kernel supports extended syscall */
> + FEAT_EXTENDED_SYSCALL,
FEAT_BPF_COMMON_ATTRS ?
> __FEAT_CNT,
> };
>
> @@ -757,4 +759,5 @@ int probe_fd(int fd);
> #define SHA256_DWORD_SIZE SHA256_DIGEST_LENGTH / sizeof(__u64)
>
> void libbpf_sha256(const void *data, size_t len, __u8 out[SHA256_DIGEST_LENGTH]);
> +int probe_sys_bpf_ext(void);
> #endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> --
> 2.52.0
>
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