libbpf: Add ring__map_fd

Add ring__map_fd to get the file descriptor underlying a given
ringbuffer.
Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230925215045.2375758-12-martin.kelly@crowdstrike.com

tools/lib/bpf/libbpf.h

@@ -1303,6 +1303,15 @@ LIBBPF_API size_t ring__avail_data_size(const struct ring *r);
  */
 LIBBPF_API size_t ring__size(const struct ring *r);
 
+/**
+ * @brief **ring__map_fd()** returns the file descriptor underlying the given
+ * ringbuffer.
+ *
+ * @param r A ringbuffer object.
+ * @return The underlying ringbuffer file descriptor
+ */
+LIBBPF_API int ring__map_fd(const struct ring *r);
+
 struct user_ring_buffer_opts {
 	size_t sz; /* size of this struct, for forward/backward compatibility */
 };

tools/lib/bpf/libbpf.map

@@ -402,6 +402,7 @@ LIBBPF_1.3.0 {
 		bpf_program__attach_uprobe_multi;
 		ring__avail_data_size;
 		ring__consumer_pos;
+		ring__map_fd;
 		ring__producer_pos;
 		ring__size;
 		ring_buffer__ring;

tools/lib/bpf/ringbuf.c

@@ -366,6 +366,11 @@ size_t ring__size(const struct ring *r)
 	return r->mask + 1;
 }
 
+int ring__map_fd(const struct ring *r)
+{
+	return r->map_fd;
+}
+
 static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
 {
 	if (rb->consumer_pos) {