Re: [PATCH bpf-next v4 3/4] libbpf: Add libbpf_probe_bpf_kfunc API

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

在 2025/2/8 06:35, Eduard Zingerman 写道:
> On Thu, 2025-02-06 at 13:15 +0800, Tao Chen wrote:
> 
> [...]
> 
>>   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, 0 means kfunc defined in vmlinux.
> 
> Regarding '0' as special value:
> in general FD is considered invalid only if it's negative, 0 is a valid FD.
> Andrii, I remember there was a lengthy discussion about FD==0 and BPF,
> but I don't remember the conclusion.
> 

Hi Eduard,
As you said, so what about "-1 means kfunc defined in vmlinux", -1 just 
used to distinguish whether it is vmlinux，then processing in 
libbpf_probe_bpf_kfunc like:

offset = 0;
// vmlinux btf offset default is 0
insn.off = offset;
if (btf_fd >= 0) {
	offset = 1;
	insn.off = offset;
	fd_array[offset] = btf_fd;
}
What do you think?

>> + * @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 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;
> 
> This is now in conflict with bpf-next.
> 
>>   } LIBBPF_1.5.0;
>> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
>> index e142130cb83c..c7f2b2dfbcf1 100644
>> --- a/tools/lib/bpf/libbpf_probes.c
>> +++ b/tools/lib/bpf/libbpf_probes.c
>> @@ -433,6 +433,61 @@ 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, btf_fd, kfunc_id),
>> +		BPF_EXIT_INSN(),
>> +	};
>> +	const size_t insn_cnt = ARRAY_SIZE(insns);
>> +	char buf[4096];
>> +	int *fd_array = NULL;
>> +	size_t fd_array_cnt = 0, fd_array_cap = fd_array_cnt;
>> +	int ret;
>> +
>> +	if (opts)
>> +		return libbpf_err(-EINVAL);
>> +
>> +	if (!can_probe_prog_type(prog_type))
>> +		return -EOPNOTSUPP;
>> +
>> +	if (btf_fd) {
>> +		ret = libbpf_ensure_mem((void **)&fd_array, &fd_array_cap,
>> +					sizeof(int), fd_array_cnt + btf_fd);
> 
> Please take a look at the tools/testing/selftests/bpf/prog_tests/fd_array.c,
> e.g. test case check_fd_array_cnt__fd_array_ok(). The offset field of the
> call instruction does not have to be an fd (as it only has 16 bits),
> instead it's an offset inside the fd_array.
> Here it would be sufficient to allocate a small array on stack.
> 
>> +		if (ret)
>> +			return ret;
>> +
>> +		/* In kernel, obtain the btf fd by means of the offset of
>> +		 * the fd_array, and the offset is the btf fd.
>> +		 */
>> +		fd_array[btf_fd] = btf_fd;
>> +	}
> 
> [...]
> 


-- 
Best Regards
Dylane Chen