On Fri, Jan 16, 2026 at 5:58 AM Leon Hwang <[email protected]> wrote:
>
>
>
> On 2026/1/16 08:42, Andrii Nakryiko wrote:
> > On Mon, Jan 12, 2026 at 6:58 AM Leon Hwang <[email protected]> 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 <[email protected]>
> >> ---
> >> 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?
> >
>
> Ack.
>
> I'll keep the same pattern.
>
> >> + 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?
> >
>
> How about capture errno before closing?
>
> err = errno;
> if (fd >= 0)
> close(fd);
> return err = EFAULT;
not sure what this code is trying to do, but yes, preserving errno is
one way to fix an immediate problem.
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) { }
>
> Then, we can wrap all details in probe_sys_bpf_ext().
>
> >> + return errno == EFAULT;
> >> +}
> >> +
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