selftests/bpf: Xdp_adjust_tail add grow tail tests

Extend BPF selftest xdp_adjust_tail with grow tail tests, which is added
as subtest's. The first grow test stays in same form as original shrink
test. The second grow test use the newer bpf_prog_test_run_xattr() calls,
and does extra checking of data contents.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158945350567.97035.9632611946765811876.stgit@firesoul

tools/testing/selftests/bpf/progs/test_xdp_adjust_tail_grow.c

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("xdp_adjust_tail_grow")
+int _xdp_adjust_tail_grow(struct xdp_md *xdp)
+{
+	void *data_end = (void *)(long)xdp->data_end;
+	void *data = (void *)(long)xdp->data;
+	unsigned int data_len;
+	int offset = 0;
+
+	/* Data length determine test case */
+	data_len = data_end - data;
+
+	if (data_len == 54) { /* sizeof(pkt_v4) */
+		offset = 4096; /* test too large offset */
+	} else if (data_len == 74) { /* sizeof(pkt_v6) */
+		offset = 40;
+	} else if (data_len == 64) {
+		offset = 128;
+	} else if (data_len == 128) {
+		offset = 4096 - 256 - 320 - data_len; /* Max tail grow 3520 */
+	} else {
+		return XDP_ABORTED; /* No matching test */
+	}
+
+	if (bpf_xdp_adjust_tail(xdp, offset))
+		return XDP_DROP;
+	return XDP_TX;
+}
+
+char _license[] SEC("license") = "GPL";