libbpf: Add ring__producer_pos, ring__consumer_pos

Add APIs to get the producer and consumer position for 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/20230925215045.2375758-6-martin.kelly@crowdstrike.com

src/libbpf.h

@@ -1264,6 +1264,24 @@ LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);
 LIBBPF_API struct ring *ring_buffer__ring(struct ring_buffer *rb,
 					  unsigned int idx);
 
+/**
+ * @brief **ring__consumer_pos()** returns the current consumer position in the
+ * given ringbuffer.
+ *
+ * @param r A ringbuffer object.
+ * @return The current consumer position.
+ */
+LIBBPF_API unsigned long ring__consumer_pos(const struct ring *r);
+
+/**
+ * @brief **ring__producer_pos()** returns the current producer position in the
+ * given ringbuffer.
+ *
+ * @param r A ringbuffer object.
+ * @return The current producer position.
+ */
+LIBBPF_API unsigned long ring__producer_pos(const struct ring *r);
+
 struct user_ring_buffer_opts {
 	size_t sz; /* size of this struct, for forward/backward compatibility */
 };

src/libbpf.map

@@ -400,5 +400,7 @@ LIBBPF_1.3.0 {
 		bpf_program__attach_netfilter;
 		bpf_program__attach_tcx;
 		bpf_program__attach_uprobe_multi;
+		ring__consumer_pos;
+		ring__producer_pos;
 		ring_buffer__ring;
 } LIBBPF_1.2.0;

src/ringbuf.c

@@ -338,6 +338,20 @@ struct ring *ring_buffer__ring(struct ring_buffer *rb, unsigned int idx)
 	return rb->rings[idx];
 }
 
+unsigned long ring__consumer_pos(const struct ring *r)
+{
+	/* Synchronizes with smp_store_release() in ringbuf_process_ring(). */
+	return smp_load_acquire(r->consumer_pos);
+}
+
+unsigned long ring__producer_pos(const struct ring *r)
+{
+	/* Synchronizes with smp_store_release() in __bpf_ringbuf_reserve() in
+	 * the kernel.
+	 */
+	return smp_load_acquire(r->producer_pos);
+}
+
 static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
 {
 	if (rb->consumer_pos) {