[PATCH v2 bpf-next 2/4] libbpf: Add helpers to extract perf fd from bpf_link

[Daniel Xu <dxu@dxuuu.xyz>]

It is sometimes necessary to perform ioctl's on the underlying perf fd.
There is not currently a way to extract the fd given a bpf_link, so add a
a pair of casting and getting helpers.

The casting and getting helpers are nice because they let us define
broad categories of links that makes it clear to users what they can
expect to extract from what type of link.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 tools/lib/bpf/libbpf.c   | 19 +++++++++++++++++++
 tools/lib/bpf/libbpf.h   |  8 ++++++++
 tools/lib/bpf/libbpf.map |  6 ++++++
 3 files changed, 33 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ead915aec349..f4d750863abd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4004,6 +4004,25 @@ static int bpf_link__destroy_perf_event(struct bpf_link *link)
 	return err;
 }
 
+const struct bpf_link_fd *bpf_link__as_fd(const struct bpf_link *link)
+{
+	if (!link)
+		return NULL;
+
+	if (link->destroy != &bpf_link__destroy_perf_event)
+		return NULL;
+
+	return (struct bpf_link_fd *)link;
+}
+
+int bpf_link_fd__fd(const struct bpf_link_fd *link)
+{
+	if (!link)
+		return -1;
+
+	return link->fd;
+}
+
 struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
 						int pfd)
 {
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 8a9d462a6f6d..4498b6ae459a 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -166,6 +166,14 @@ LIBBPF_API int bpf_program__unpin(struct bpf_program *prog, const char *path);
 LIBBPF_API void bpf_program__unload(struct bpf_program *prog);
 
 struct bpf_link;
+struct bpf_link_fd;
+
+/* casting APIs */
+LIBBPF_API const struct bpf_link_fd *
+bpf_link__as_fd(const struct bpf_link *link);
+
+/* getters APIs */
+LIBBPF_API int bpf_link_fd__fd(const struct bpf_link_fd *link);
 
 LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
 
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index f9d316e873d8..b58dd0f0259c 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -184,3 +184,9 @@ LIBBPF_0.0.4 {
 		perf_buffer__new_raw;
 		perf_buffer__poll;
 } LIBBPF_0.0.3;
+
+LIBBPF_0.0.5 {
+	global:
+		bpf_link__as_fd;
+		bpf_link_fd__fd;
+} LIBBPF_0.0.4;
-- 
2.20.1