linux-kernel - Re: [PATCH bpf-next v5 3/4] libbpf: Add libbpf_probe_bpf

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Message-ID: <4914fa43-650c-403e-b2a5-b5ec66d02101@gmail.com>
Date: Mon, 10 Feb 2025 23:19:13 +0800
From: Tao Chen <chen.dylane@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, haoluo@...gle.com, qmo@...nel.org, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, Tao Chen <dylane.chen@...iglobal.com>
Subject: Re: [PATCH bpf-next v5 3/4] libbpf: Add libbpf_probe_bpf_kfunc API

在 2025/2/10 21:47, Jiri Olsa 写道:
> On Mon, Feb 10, 2025 at 01:59:44PM +0800, Tao Chen wrote:
> 
> SNIP
> 
>> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
>> index e142130cb83c..53f1196394bf 100644
>> --- a/tools/lib/bpf/libbpf_probes.c
>> +++ b/tools/lib/bpf/libbpf_probes.c
>> @@ -433,6 +433,54 @@ 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;
>> +
>> +	if (btf_fd >= 0) {
>> +		fd_array[1] = btf_fd;
>> +	} else if (btf_fd == -1) {
>> +		/* 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,
>> +			      0, buf, sizeof(buf));
> 
> hum, you pass fd_array_cnt as 0, which IIUC will work properly
> 
> but I guess then we don't need to have fd_array_cnt argument in
> probe_prog_load if all callers pass 0 ?
> 
> jirka
> 

Hi, jiri
In fact, 0 is indeed used everywhere here. I was just thinking about 
whether we might need it in the future. Anyway, it seems better to 
remove it. I'll make the modifications in the next version.

>> +	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,
>> +	 * it will emit "kernel btf_id 4294967295 is not a function". If btf fd
>> +	 * invalid in module btf, it will emit "invalid module BTF fd specified" or
>> +	 * "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"))))
>> +		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