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

[Leon Hwang <leon.hwang@linux.dev>]

On 23/1/26 08:53, Andrii Nakryiko wrote:
> On Tue, Jan 20, 2026 at 7:26 AM Leon Hwang <leon.hwang@linux.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@linux.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;

Thanks,
Leon