Re: [PATCH bpf-next v5 2/9] libbpf: Add support for extended bpf syscall

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

On 2026/1/16 08:42, Andrii Nakryiko wrote:
> On Mon, Jan 12, 2026 at 6:58 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 extended attributes.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.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?
> 

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;

Then, we can wrap all details in probe_sys_bpf_ext().

>> +       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?
> 

Ack.

I'll update it to "BPF syscall common attributes support."

>> +       },
>>  };
>>
>>  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_BPF_SYSCALL_COMMON_ATTRS seems more accurate.

Thanks,
Leon

>>         __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
>>