1 files changed, 115 insertions, 0 deletions
diff --git a/examples/bpf/bpf_tailcall.c b/examples/bpf/bpf_tailcall.c
new file mode 100644
index 0000000..f186e57
--- /dev/null
+++ b/examples/bpf/bpf_tailcall.c
@@ -0,0 +1,115 @@
+#include <linux/bpf.h>
+
+#include "bpf_funcs.h"
+
+#define ENTRY_INIT	3
+#define ENTRY_0		0
+#define ENTRY_1		1
+#define MAX_JMP_SIZE	2
+
+#define FOO		42
+#define BAR		43
+
+/* This example doesn't really do anything useful, but it's purpose is to
+ * demonstrate eBPF tail calls on a very simple example.
+ *
+ * cls_entry() is our classifier entry point, from there we jump based on
+ * skb->hash into cls_case1() or cls_case2(). They are both part of the
+ * program array jmp_tc. Indicated via __section_tail(), the tc loader
+ * populates the program arrays with the loaded file descriptors already.
+ *
+ * To demonstrate nested jumps, cls_case2() jumps within the same jmp_tc
+ * array to cls_case1(). And whenever we arrive at cls_case1(), we jump
+ * into cls_exit(), part of the jump array jmp_ex.
+ *
+ * Also, to show it's possible, all programs share map_sh and dump the value
+ * that the entry point incremented. The sections that are loaded into a
+ * program array can be atomically replaced during run-time, e.g. to change
+ * classifier behaviour.
+ */
+struct bpf_elf_map __section("maps") map_sh = {
+	.type		= BPF_MAP_TYPE_ARRAY,
+	.size_key	= sizeof(int),
+	.size_value	= sizeof(int),
+	.pinning	= PIN_OBJECT_NS,
+	.max_elem	= 1,
+};
+
+struct bpf_elf_map __section("maps") jmp_tc = {
+	.type		= BPF_MAP_TYPE_PROG_ARRAY,
+	.id		= FOO,
+	.size_key	= sizeof(int),
+	.size_value	= sizeof(int),
+	.pinning	= PIN_OBJECT_NS,
+	.max_elem	= MAX_JMP_SIZE,
+};
+
+struct bpf_elf_map __section("maps") jmp_ex = {
+	.type		= BPF_MAP_TYPE_PROG_ARRAY,
+	.id		= BAR,
+	.size_key	= sizeof(int),
+	.size_value	= sizeof(int),
+	.pinning	= PIN_OBJECT_NS,
+	.max_elem	= 1,
+};
+
+__section_tail(FOO, ENTRY_0) int cls_case1(struct __sk_buff *skb)
+{
+	char fmt[] = "case1: map-val: %d from:%u\n";
+	int key = 0, *val;
+
+	val = bpf_map_lookup_elem(&map_sh, &key);
+	if (val)
+		bpf_printk(fmt, sizeof(fmt), *val, skb->cb[0]);
+
+	skb->cb[0] = ENTRY_0;
+	bpf_tail_call(skb, &jmp_ex, ENTRY_0);
+	return 0;
+}
+
+__section_tail(FOO, ENTRY_1) int cls_case2(struct __sk_buff *skb)
+{
+	char fmt[] = "case2: map-val: %d from:%u\n";
+	int key = 0, *val;
+
+	val = bpf_map_lookup_elem(&map_sh, &key);
+	if (val)
+		bpf_printk(fmt, sizeof(fmt), *val, skb->cb[0]);
+
+	skb->cb[0] = ENTRY_1;
+	bpf_tail_call(skb, &jmp_tc, ENTRY_0);
+	return 0;
+}
+
+__section_tail(BAR, ENTRY_0) int cls_exit(struct __sk_buff *skb)
+{
+	char fmt[] = "exit: map-val: %d from:%u\n";
+	int key = 0, *val;
+
+	val = bpf_map_lookup_elem(&map_sh, &key);
+	if (val)
+		bpf_printk(fmt, sizeof(fmt), *val, skb->cb[0]);
+
+	/* Termination point. */
+	return -1;
+}
+
+__section("classifier") int cls_entry(struct __sk_buff *skb)
+{
+	char fmt[] = "fallthrough\n";
+	int key = 0, *val;
+
+	/* For transferring state, we can use skb->cb[0] ... skb->cb[4]. */
+	val = bpf_map_lookup_elem(&map_sh, &key);
+	if (val) {
+		__sync_fetch_and_add(val, 1);
+
+		skb->cb[0] = ENTRY_INIT;
+		bpf_tail_call(skb, &jmp_tc, skb->hash & (MAX_JMP_SIZE - 1));
+	}
+
+	bpf_printk(fmt, sizeof(fmt));
+	return 0;
+}
+
+char __license[] __section("license") = "GPL";