experiment01-tailgrow: add xdp_prog_kern4.c that use ctx_store_bytes

The code works when 'len' is a constant, but doesn't work when
we calculate the 'len' value.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Author: netoptimizer

experiment01-tailgrow/Makefile

@@ -2,6 +2,7 @@
 
 XDP_TARGETS  := xdp_prog_kern xdp_prog_kern2
 XDP_TARGETS  += xdp_prog_kern3
+XDP_TARGETS  += xdp_prog_kern4
 XDP_TARGETS  += xdp_prog_fail1
 XDP_TARGETS  += xdp_prog_fail2
 

experiment01-tailgrow/xdp_prog_kern4.c

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+#include "xdp_data_access_helpers.h"
+
+SEC("xdp_test1")
+int _xdp_test1(struct xdp_md *ctx)
+{
+//	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+	unsigned int len;
+//	len = (data_end - data) - 2 ; // Not working, due to verifier
+	len = 12;
+
+	unsigned int offset = len - 2;
+
+	if (ctx_store_bytes(ctx, offset, data, 2, 0) < 0)
+		return XDP_ABORTED;
+
+	return XDP_PASS;
+}
+