libbpf: Add ring__avail_data_size

Add ring__avail_data_size for querying the currently available data in
the ringbuffer, similar to the BPF_RB_AVAIL_DATA flag in
bpf_ringbuf_query. This is racy during ongoing operations but is still
useful for overall information on how a ringbuffer is behaving.

Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230925215045.2375758-8-martin.kelly@crowdstrike.com

src/libbpf.h

@@ -1282,6 +1282,17 @@ LIBBPF_API unsigned long ring__consumer_pos(const struct ring *r);
  */
 LIBBPF_API unsigned long ring__producer_pos(const struct ring *r);
 
+/**
+ * @brief **ring__avail_data_size()** returns the number of bytes in the
+ * ringbuffer not yet consumed. This has no locking associated with it, so it
+ * can be inaccurate if operations are ongoing while this is called. However, it
+ * should still show the correct trend over the long-term.
+ *
+ * @param r A ringbuffer object.
+ * @return The number of bytes not yet consumed.
+ */
+LIBBPF_API size_t ring__avail_data_size(const struct ring *r);
+
 struct user_ring_buffer_opts {
 	size_t sz; /* size of this struct, for forward/backward compatibility */
 };

src/libbpf.map

@@ -400,6 +400,7 @@ LIBBPF_1.3.0 {
 		bpf_program__attach_netfilter;
 		bpf_program__attach_tcx;
 		bpf_program__attach_uprobe_multi;
+		ring__avail_data_size;
 		ring__consumer_pos;
 		ring__producer_pos;
 		ring_buffer__ring;

src/ringbuf.c

@@ -352,6 +352,15 @@ unsigned long ring__producer_pos(const struct ring *r)
 	return smp_load_acquire(r->producer_pos);
 }
 
+size_t ring__avail_data_size(const struct ring *r)
+{
+	unsigned long cons_pos, prod_pos;
+
+	cons_pos = ring__consumer_pos(r);
+	prod_pos = ring__producer_pos(r);
+	return prod_pos - cons_pos;
+}
+
 static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
 {
 	if (rb->consumer_pos) {