Re: [PATCH bpf-next v5 3/9] libbpf: Probe percpu data feature

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

On 8/6/26 23:05, sashiko-bot@kernel.org wrote:
[...]
>> +static int probe_kern_percpu_data(int token_fd)
>> +{
>> +	struct bpf_insn insns[] = {
>> +		BPF_LD_MAP_VALUE(BPF_REG_1, 0, 0),
>> +		BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, 0),
>> +		BPF_EXIT_INSN(),
>> +	};
>> +	LIBBPF_OPTS(bpf_map_create_opts, map_opts,
>> +		.token_fd = token_fd,
>> +		.map_flags = token_fd ? BPF_F_TOKEN_FD : 0,
>> +	);
>> +	LIBBPF_OPTS(bpf_prog_load_opts, prog_opts,
>> +		.token_fd = token_fd,
>> +		.prog_flags = token_fd ? BPF_F_TOKEN_FD : 0,
>> +	);
>> +	int ret, map, insn_cnt = ARRAY_SIZE(insns);
>> +
>> +	map = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "libbpf_percpu", sizeof(int), 8, 1,
>> +			     &map_opts);
>> +	if (map < 0) {
>> +		pr_warn("Error in %s(): %s. Couldn't create simple percpu_array map.\n",
>> +			__func__, errstr(map));
>> +		return map;
>> +	}
> 
> [Severity: Low]
> Does this unconditional warning and negative return value cause log spam on
> older kernels or for unprivileged users?
> 
> When bpf_map_create() fails, returning the negative error code causes
> feat_supported() to log a second "Detection of kernel ... support failed"
> warning. Returning 0 instead would correctly signal that the feature is
> missing without the additional log output, similar to how other probes
> handle expected failures.
> 
Nope, I don't think so.

Since percpu_array map is a basic map in bpf, this map creation should
likely succeed. If it fails, the kernels are too old to use such new libbpf.

Thanks,
Leon