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 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));
+	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

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

> +	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
>

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