[PATCH bpf-next v4 0/4] Add prog_kfunc feature probe

[PATCH bpf-next v4 0/4] Add prog_kfunc feature probe

[Tao Chen]

More and more kfunc functions are being added to the kernel.
Different prog types have different restrictions when using kfunc.
Therefore, prog_kfunc probe is added to check whether it is supported,
and the use of this api will be added to bpftool later.

Change list:
- v3 -> v4:
  - add fd_array init for kfunc in mod btf
  - add test case for kfunc in mod btf
  - refactor common part as prog load type check for
    libbpf_probe_bpf_{helper,kfunc}
- v3
  https://lore.kernel.org/bpf/20250124144411.13468-1-chen.dylane@gmail.com

- v2 -> v3:
  - rename parameter off with btf_fd
  - extract the common part for libbpf_probe_bpf_{helper,kfunc}
- v2
  https://lore.kernel.org/bpf/20250123170555.291896-1-chen.dylane@gmail.com

- v1 -> v2:
  - check unsupported prog type like probe_bpf_helper
  - add off parameter for module btf
  - check verifier info when kfunc id invalid
- v1
  https://lore.kernel.org/bpf/20250122171359.232791-1-chen.dylane@gmail.com

Tao Chen (4):
  libbpf: Extract prog load type check from libbpf_probe_bpf_helper
  libbpf: Init fd_array when prog probe load
  libbpf: Add libbpf_probe_bpf_kfunc API
  selftests/bpf: Add libbpf_probe_bpf_kfunc API selftests

 tools/lib/bpf/libbpf.h                        |  18 ++-
 tools/lib/bpf/libbpf.map                      |   1 +
 tools/lib/bpf/libbpf_probes.c                 |  91 ++++++++++++--
 .../selftests/bpf/prog_tests/libbpf_probes.c  | 118 ++++++++++++++++++
 4 files changed, 214 insertions(+), 14 deletions(-)

-- 
2.43.0

[PATCH bpf-next v4 1/4] libbpf: Extract prog load type check from libbpf_probe_bpf_helper

[Tao Chen]

Extract prog load type check part from libbpf_probe_bpf_helper
suggested by Andrii, which will be used in both
libbpf_probe_bpf_{helper, kfunc}.

Signed-off-by: Tao Chen <chen.dylane@gmail.com>
---
 tools/lib/bpf/libbpf_probes.c | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 9dfbe7750f56..aeb4fd97d801 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -413,6 +413,23 @@ int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void *opts)
 	return libbpf_err(ret);
 }
 
+static bool can_probe_prog_type(enum bpf_prog_type prog_type)
+{
+	/* we can't successfully load all prog types to check for BPF helper
+	 * and kfunc support.
+	 */
+	switch (prog_type) {
+	case BPF_PROG_TYPE_TRACING:
+	case BPF_PROG_TYPE_EXT:
+	case BPF_PROG_TYPE_LSM:
+	case BPF_PROG_TYPE_STRUCT_OPS:
+		return false;
+	default:
+		break;
+	}
+	return true;
+}
+
 int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
 			    const void *opts)
 {
@@ -427,18 +444,8 @@ int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helpe
 	if (opts)
 		return libbpf_err(-EINVAL);
 
-	/* we can't successfully load all prog types to check for BPF helper
-	 * support, so bail out with -EOPNOTSUPP error
-	 */
-	switch (prog_type) {
-	case BPF_PROG_TYPE_TRACING:
-	case BPF_PROG_TYPE_EXT:
-	case BPF_PROG_TYPE_LSM:
-	case BPF_PROG_TYPE_STRUCT_OPS:
+	if (!can_probe_prog_type(prog_type))
 		return -EOPNOTSUPP;
-	default:
-		break;
-	}
 
 	buf[0] = '\0';
 	ret = probe_prog_load(prog_type, insns, insn_cnt, buf, sizeof(buf));
-- 
2.43.0

[PATCH bpf-next v4 2/4] libbpf: Init fd_array when prog probe load

[Tao Chen]

fd_array used to store module btf fd, which will
be used for kfunc probe in module btf.

Signed-off-by: Tao Chen <chen.dylane@gmail.com>
---
 tools/lib/bpf/libbpf_probes.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index aeb4fd97d801..e142130cb83c 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -102,12 +102,15 @@ __u32 get_kernel_version(void)
 
 static int probe_prog_load(enum bpf_prog_type prog_type,
 			   const struct bpf_insn *insns, size_t insns_cnt,
+			   int *fd_array, size_t fd_array_cnt,
 			   char *log_buf, size_t log_buf_sz)
 {
 	LIBBPF_OPTS(bpf_prog_load_opts, opts,
 		.log_buf = log_buf,
 		.log_size = log_buf_sz,
 		.log_level = log_buf ? 1 : 0,
+		.fd_array = fd_array,
+		.fd_array_cnt = fd_array_cnt,
 	);
 	int fd, err, exp_err = 0;
 	const char *exp_msg = NULL;
@@ -214,7 +217,7 @@ int libbpf_probe_bpf_prog_type(enum bpf_prog_type prog_type, const void *opts)
 	if (opts)
 		return libbpf_err(-EINVAL);
 
-	ret = probe_prog_load(prog_type, insns, insn_cnt, NULL, 0);
+	ret = probe_prog_load(prog_type, insns, insn_cnt, NULL, 0, NULL, 0);
 	return libbpf_err(ret);
 }
 
@@ -448,7 +451,7 @@ int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helpe
 		return -EOPNOTSUPP;
 
 	buf[0] = '\0';
-	ret = probe_prog_load(prog_type, insns, insn_cnt, buf, sizeof(buf));
+	ret = probe_prog_load(prog_type, insns, insn_cnt, NULL, 0, buf, sizeof(buf));
 	if (ret < 0)
 		return libbpf_err(ret);
 
-- 
2.43.0

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

[Tao Chen]

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        | 18 +++++++++++-
 tools/lib/bpf/libbpf.map      |  1 +
 tools/lib/bpf/libbpf_probes.c | 55 +++++++++++++++++++++++++++++++++++
 3 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3020ee45303a..596b27f58c58 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1680,7 +1680,23 @@ 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, 0 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 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 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);
+		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;
+	}
+
+	buf[0] = '\0';
+	ret = probe_prog_load(prog_type, insns, insn_cnt, fd_array,
+			      fd_array_cnt, buf, sizeof(buf));
+	if (ret < 0) {
+		free(fd_array);
+		return libbpf_err(ret);
+	}
+
+	free(fd_array);
+	/* 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

[PATCH bpf-next v4 4/4] selftests/bpf: Add libbpf_probe_bpf_kfunc API selftests

[Tao Chen]

Add selftests for prog_kfunc feature probing.

 ./test_progs -t libbpf_probe_kfuncs
 #153     libbpf_probe_kfuncs:OK
 Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Tao Chen <chen.dylane@gmail.com>
---
 .../selftests/bpf/prog_tests/libbpf_probes.c  | 118 ++++++++++++++++++
 1 file changed, 118 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/libbpf_probes.c b/tools/testing/selftests/bpf/prog_tests/libbpf_probes.c
index 4ed46ed58a7b..8f249ca7d5d7 100644
--- a/tools/testing/selftests/bpf/prog_tests/libbpf_probes.c
+++ b/tools/testing/selftests/bpf/prog_tests/libbpf_probes.c
@@ -126,3 +126,121 @@ void test_libbpf_probe_helpers(void)
 		ASSERT_EQ(res, d->supported, buf);
 	}
 }
+
+static int module_btf_fd(char *module)
+{
+	int fd, err;
+	__u32 id = 0, len;
+	struct bpf_btf_info info;
+	char name[64];
+
+	while (true) {
+		err = bpf_btf_get_next_id(id, &id);
+		if (err && (errno == ENOENT || errno == EPERM))
+			return 0;
+		if (err) {
+			err = -errno;
+			return err;
+		}
+		fd = bpf_btf_get_fd_by_id(id);
+		if (fd < 0) {
+			if (errno == ENOENT)
+				continue;
+			err = -errno;
+			return err;
+		}
+		len = sizeof(info);
+		memset(&info, 0, sizeof(info));
+		info.name = ptr_to_u64(name);
+		info.name_len = sizeof(name);
+		err = bpf_btf_get_info_by_fd(fd, &info, &len);
+		if (err) {
+			err = -errno;
+			goto err_out;
+		}
+		/* find target module btf */
+		if (!strcmp(name, module))
+			break;
+
+		close(fd);
+	}
+
+	return fd;
+err_out:
+	close(fd);
+	return err;
+}
+
+void test_libbpf_probe_kfuncs(void)
+{
+	int ret, kfunc_id, fd;
+	char *kfunc = "bpf_cpumask_create";
+	struct btf *vmlinux_btf = NULL;
+	struct btf *module_btf = NULL;
+
+	vmlinux_btf = btf__parse("/sys/kernel/btf/vmlinux", NULL);
+	if (!ASSERT_OK_PTR(vmlinux_btf, "btf_parse"))
+		return;
+
+	kfunc_id = btf__find_by_name_kind(vmlinux_btf, kfunc, BTF_KIND_FUNC);
+	if (!ASSERT_GT(kfunc_id, 0, kfunc))
+		goto cleanup;
+
+	/* prog BPF_PROG_TYPE_SYSCALL supports kfunc bpf_cpumask_create */
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_SYSCALL, kfunc_id, 0, NULL);
+	if (!ASSERT_EQ(ret, 1, "kfunc in vmlinux support"))
+		goto cleanup;
+
+	/* prog BPF_PROG_TYPE_KPROBE does not support kfunc bpf_cpumask_create */
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_KPROBE, kfunc_id, 0, NULL);
+	if (!ASSERT_EQ(ret, 0, "kfunc in vmlinux not suuport"))
+		goto cleanup;
+
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_KPROBE, -1, 0, NULL);
+	if (!ASSERT_EQ(ret, 0, "invalid kfunc id:-1"))
+		goto cleanup;
+
+	ret = libbpf_probe_bpf_kfunc(100000, kfunc_id, 0, NULL);
+	if (!ASSERT_ERR(ret, "invalid prog type:100000"))
+		goto cleanup;
+
+	if (!env.has_testmod)
+		goto cleanup;
+
+	module_btf = btf__load_module_btf("bpf_testmod", vmlinux_btf);
+	if (!ASSERT_OK_PTR(module_btf, "load module BTF"))
+		goto cleanup;
+
+	kfunc_id = btf__find_by_name(module_btf, "bpf_kfunc_call_test1");
+	if (!ASSERT_GT(kfunc_id, 0, "func not found"))
+		goto cleanup;
+
+	fd = module_btf_fd("bpf_testmod");
+	if (!ASSERT_GT(fd, 0, "module btf fd"))
+		goto cleanup;
+
+	/* prog BPF_PROG_TYPE_SYSCALL supports kfunc bpf_kfunc_call_test1 in bpf_testmod */
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_SYSCALL, kfunc_id, fd, NULL);
+	if (!ASSERT_EQ(ret, 1, "kfunc in module btf support"))
+		goto cleanup_fd;
+
+	/* prog BPF_PROG_TYPE_KPROBE does not support kfunc bpf_kfunc_call_test1
+	 * in bpf_testmod
+	 */
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_KPROBE, kfunc_id, fd, NULL);
+	if (!ASSERT_EQ(ret, 0, "kfunc in module btf not support"))
+		goto cleanup_fd;
+
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_SYSCALL, -1, fd, NULL);
+	if (!ASSERT_EQ(ret, 0, "invalid kfunc id in module btf"))
+		goto cleanup_fd;
+
+	ret = libbpf_probe_bpf_kfunc(BPF_PROG_TYPE_SYSCALL, kfunc_id, 100, NULL);
+	ASSERT_EQ(ret, 0, "invalid btf fd in module btf");
+
+cleanup_fd:
+	close(fd);
+cleanup:
+	btf__free(vmlinux_btf);
+	btf__free(module_btf);
+}
-- 
2.43.0

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

[Eduard Zingerman]

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.

> + * @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;
> +	}

[...]

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

[Tao Chen]

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.
> 
>> + * @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.
> 

My bad, i will rebase the repo.

>>   } 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.
> 

Good idea，thanks for your guidance，I'll make the modifications in the 
next version.

>> +		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

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

[Tao Chen]

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