Re: [PATCH bpf-next v3 2/3] libbpf: Add libbpf_probe_bpf_kfunc API

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

在 2025/1/25 00:27, Jiri Olsa 写道:
> On Fri, Jan 24, 2025 at 10:44:10PM +0800, Tao Chen 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.
>>
>> Signed-off-by: Tao Chen <chen.dylane@gmail.com>
>> ---
>>   tools/lib/bpf/libbpf.h        | 17 ++++++++++++++++-
>>   tools/lib/bpf/libbpf.map      |  1 +
>>   tools/lib/bpf/libbpf_probes.c | 30 ++++++++++++++++++++++++++++++
>>   3 files changed, 47 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 3020ee45303a..035829e22099 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -1680,7 +1680,22 @@ 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, 0 for 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, __s16 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 a8b2936a1646..e93fae101efd 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -436,4 +436,5 @@ LIBBPF_1.6.0 {
>>   		bpf_linker__add_buf;
>>   		bpf_linker__add_fd;
>>   		bpf_linker__new_fd;
>> +		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 b73345977b4e..cd7d16c1cc49 100644
>> --- a/tools/lib/bpf/libbpf_probes.c
>> +++ b/tools/lib/bpf/libbpf_probes.c
>> @@ -446,6 +446,36 @@ static int probe_func_comm(enum bpf_prog_type prog_type, struct bpf_insn insn,
>>   	return 0;
>>   }
>>   
>> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id,
>> +			   __s16 btf_fd, const void *opts)
>> +{
>> +	struct bpf_insn insn;
>> +	int err;
>> +	char buf[4096];
>> +
>> +	if (opts)
>> +		return libbpf_err(-EINVAL);
>> +
>> +	insn.code = BPF_JMP | BPF_CALL;
>> +	insn.src_reg = BPF_PSEUDO_KFUNC_CALL;
>> +	insn.imm = kfunc_id;
>> +	insn.off = btf_fd;
> 
> nit, you could use
> 
>          struct bpf_insn insns[] = {
>          	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, BPF_PSEUDO_KFUNC_CALL, off, imm),
>                  BPF_EXIT_INSN(),
>          };
> 
> jirka
> 

Yeah, it looks more concise, i will send it in v4. Thanks.

>> +
>> +	err = probe_func_comm(prog_type, insn, buf, sizeof(buf));
>> +	if (err)
>> +		return err;
>> +
>> +	/* 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,
>> +	 * it will emit "kernel btf_id 4294967295 is not a function".
>> +	 */
>> +	if (err == 0 && (strstr(buf, "not allowed") || strstr(buf, "not a function")))
>> +		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