linux-kernel - Re: [RESEND PATCH bpf-next v6 2/9] libbpf: Add support for extended bpf syscall

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Message-ID: <CAEf4Bzb_V4p7XvE6e2CUfbKhfjh96i-_mCYa-osWiSGwWObmrQ@mail.gmail.com>
Date: Fri, 23 Jan 2026 10:54:37 -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: [RESEND PATCH bpf-next v6 2/9] libbpf: Add support for extended
 bpf syscall

On Thu, Jan 22, 2026 at 5:41 PM Leon Hwang <leon.hwang@...ux.dev> wrote:
>
>
>
> On 23/1/26 08:53, Andrii Nakryiko wrote:
> > On Tue, Jan 20, 2026 at 7:26 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 BPF syscall common attributes.
> >>
> >> Signed-off-by: Leon Hwang <leon.hwang@...ux.dev>
> >> ---
> >>  tools/lib/bpf/bpf.c             | 32 ++++++++++++++++++++++++++++++++
> >>  tools/lib/bpf/features.c        |  8 ++++++++
> >>  tools/lib/bpf/libbpf_internal.h |  3 +++
> >>  3 files changed, 43 insertions(+)
> >>
> >> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> >> index 21b57a629916..ed9c6eaeb656 100644
> >> --- a/tools/lib/bpf/bpf.c
> >> +++ b/tools/lib/bpf/bpf.c
> >> @@ -69,6 +69,38 @@ 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 *attr_common,
> >> +                             unsigned int size_common)
> >> +{
> >> +       cmd = attr_common ? (cmd | BPF_COMMON_ATTRS) : (cmd & ~BPF_COMMON_ATTRS);
> >> +       return syscall(__NR_bpf, cmd, attr, size, attr_common, size_common);
> >> +}
> >> +
> >> +static inline int sys_bpf_ext_fd(enum bpf_cmd cmd, union bpf_attr *attr,
> >> +                                unsigned int size,
> >> +                                struct bpf_common_attr *attr_common,
> >> +                                unsigned int size_common)
> >> +{
> >> +       int fd;
> >> +
> >> +       fd = sys_bpf_ext(cmd, attr, size, attr_common, 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;
> >> +
> >> +       memset(&attr, 0, attr_sz);
> >> +       /* This syscall() will return error always. */
> >
> > I'll cite myself from the last review:
> >
> >> But fd should really not be >= 0, and if it is -- it's some problem,
> >> so I'd return an error in that case to keep us aware, which is why I'm
> >> saying I'd just return inside if (fd >= 0) { }
> >
> > I didn't say let's just ignore syscall return with (void) cast and
> > happily check errno no matter what, did I? Drop the comment, and
> > handle fd >= 0 case explicitly, please.
> >
>
> My mistake — sorry for the misunderstanding.
>
> You’re right; the return value should not be ignored. In the next
> revision, I’ll handle the fd >= 0 case explicitly and drop the comment.
> The logic will be updated along the lines of:
>
> fd = syscall(__NR_bpf, BPF_PROG_LOAD | BPF_COMMON_ATTRS,
>              &attr, attr_sz, NULL, sizeof(struct bpf_common_attr));
> if (fd >= 0) {
>         close(fd);
>         return 0;
> }
> return errno == EFAULT;
>

well no, it should be

fd = syscall(...);
if (fd >= 0) {
    close(fd);
    return -EINVAL;
}

return errno == EFAULT ? 1 : 0;

> Thanks,
> Leon
>
>