Re: [PATCH bpf-next v6 3/4] libbpf: Add libbpf_probe_bpf_kfunc API

[Tao Chen <chen.dylane@gmail.com>]

在 2025/2/12 06:24, Andrii Nakryiko 写道:
> On Tue, Feb 11, 2025 at 3:19 AM Tao Chen <chen.dylane@gmail.com> wrote:
>>
>> Similarly to libbpf_probe_bpf_helper, the libbpf_probe_bpf_kfunc
>> used to test the availability of the different eBPF kfuncs on the
>> current system.
>>
>> Cc: Tao Chen <dylane.chen@didiglobal.com>
>> Signed-off-by: Tao Chen <chen.dylane@gmail.com>
>> ---
>>   tools/lib/bpf/libbpf.h        | 19 +++++++++++++-
>>   tools/lib/bpf/libbpf.map      |  1 +
>>   tools/lib/bpf/libbpf_probes.c | 48 +++++++++++++++++++++++++++++++++++
>>   3 files changed, 67 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 3020ee45303a..e796e38cf255 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -1680,7 +1680,24 @@ LIBBPF_API int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void
>>    */
>>   LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
>>                                         enum bpf_func_id helper_id, const void *opts);
>> -
>> +/**
>> + * @brief **libbpf_probe_bpf_kfunc()** detects if host kernel supports the
>> + * use of a given BPF kfunc from specified BPF program type.
>> + * @param prog_type BPF program type used to check the support of BPF kfunc
>> + * @param kfunc_id The btf ID of BPF kfunc to check support for
>> + * @param btf_fd The module BTF FD, if kfunc is defined in kernel module,
>> + * btf_fd is used to point to module's BTF, which is >= 0, and -1 means kfunc
>> + * defined in vmlinux.
>> + * @param opts reserved for future extensibility, should be NULL
>> + * @return 1, if given combination of program type and kfunc is supported; 0,
>> + * if the combination is not supported; negative error code if feature
>> + * detection for provided input arguments failed or can't be performed
>> + *
>> + * Make sure the process has required set of CAP_* permissions (or runs as
>> + * root) when performing feature checking.
>> + */
>> +LIBBPF_API int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type,
>> +                                     int kfunc_id, int btf_fd, const void *opts);
>>   /**
>>    * @brief **libbpf_num_possible_cpus()** is a helper function to get the
>>    * number of possible CPUs that the host kernel supports and expects.
>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>> index b5a838de6f47..3bbfe13aeb6a 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -438,4 +438,5 @@ LIBBPF_1.6.0 {
>>                  bpf_linker__new_fd;
>>                  btf__add_decl_attr;
>>                  btf__add_type_attr;
>> +               libbpf_probe_bpf_kfunc;
>>   } LIBBPF_1.5.0;
>> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
>> index 8ed92ea922b3..ab5591c385de 100644
>> --- a/tools/lib/bpf/libbpf_probes.c
>> +++ b/tools/lib/bpf/libbpf_probes.c
>> @@ -431,6 +431,54 @@ static bool can_probe_prog_type(enum bpf_prog_type prog_type)
>>          return true;
>>   }
>>
>> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id, int btf_fd,
>> +                          const void *opts)
>> +{
>> +       struct bpf_insn insns[] = {
>> +               BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, BPF_PSEUDO_KFUNC_CALL, 1, kfunc_id),
>> +               BPF_EXIT_INSN(),
>> +       };
>> +       const size_t insn_cnt = ARRAY_SIZE(insns);
>> +       char buf[4096];
>> +       int fd_array[2] = {-1};
>> +       int ret;
>> +
>> +       if (opts)
>> +               return libbpf_err(-EINVAL);
>> +
>> +       if (!can_probe_prog_type(prog_type))
>> +               return -EOPNOTSUPP;
> 
> libbpf_err() here
> 
> pw-bot: cr
> 

Ack.

>> +
>> +       if (btf_fd >= 0) {
>> +               fd_array[1] = btf_fd;
>> +       } else if (btf_fd == -1) {
> 
> let's not hard-code the equality, use < 0 (though I'd follow
> verifier's offset == 0 convention for vmlinux BTF here as well to stay
> conceptually consistent)
> 

Ack.

>> +               /* insn.off = 0, means vmlinux btf */
>> +               insns[0].off = 0;
>> +       } else {
>> +               return libbpf_err(-EINVAL);
>> +       }
>> +
>> +       buf[0] = '\0';
>> +       ret = probe_prog_load(prog_type, insns, insn_cnt, btf_fd >= 0 ? fd_array : NULL,
>> +                             buf, sizeof(buf));
>> +       if (ret < 0)
>> +               return libbpf_err(ret);
>> +
>> +       /* If BPF verifier recognizes BPF kfunc but it's not supported for
>> +        * given BPF program type, it will emit "calling kernel function
>> +        * bpf_cpumask_create is not allowed", if the kfunc id is invalid,
> 
> bpf_cpumask_create -> <name> to keep comments generic?
> 
>> +        * it will emit "kernel btf_id 4294967295 is not a function". If btf fd
> 
> same as above, use <id> placeholder instead of specific number?
> 
> and keep BTF (all caps) use consistent, please

Ack.

> 
>> +        * invalid in module btf, it will emit "invalid module BTF fd specified" or
> 
> ditto, btf -> BTF
> 
>> +        * "negative offset disallowed for kernel module function call"
>> +        */
>> +       if (ret == 0 && (strstr(buf, "not allowed") || strstr(buf, "not a function") ||
>> +                       (strstr(buf, "invalid module BTF fd")) ||
>> +                       (strstr(buf, "negative offset disallowed"))))
> 
> stylistically, given amount of checks, I'd probably go with the
> following structure

Ack. will change it.

> 
> if (ret > 0)
>      return 1;
> 
> if (strstr(buf, "not allowed") ||
>      strstr(buf, "not a function") ||
> ...)
>      return 0;
> 
>> +               return 0;
>> +
>> +       return 1; /* assume supported */
>> +}
>> +
>>   int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
>>                              const void *opts)
>>   {
>> --
>> 2.43.0
>>


-- 
Best Regards
Dylane Chen