selftests/bpf: Adjust BPF selftest for xdp_adjust_tail

Current selftest for BPF-helper xdp_adjust_tail only shrink tail.
Make it more clear that this is a shrink test case.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158945350058.97035.17280775016196207372.stgit@firesoul

tools/testing/selftests/bpf/prog_tests/xdp_adjust_tail.c

@@ -2,9 +2,9 @@
 #include <test_progs.h>
 #include <network_helpers.h>
 
-void test_xdp_adjust_tail(void)
+void test_xdp_adjust_tail_shrink(void)
 {
-	const char *file = "./test_adjust_tail.o";
+	const char *file = "./test_xdp_adjust_tail_shrink.o";
 	struct bpf_object *obj;
 	char buf[128];
 	__u32 duration, retval, size;
@@ -28,3 +28,8 @@ void test_xdp_adjust_tail(void)
 	      err, errno, retval, size);
 	bpf_object__close(obj);
 }
+
+void test_xdp_adjust_tail(void)
+{
+	test_xdp_adjust_tail_shrink();
+}

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

@@ -1,5 +1,5 @@
-/* SPDX-License-Identifier: GPL-2.0
- * Copyright (c) 2018 Facebook
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2018 Facebook
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of version 2 of the GNU General Public
@@ -11,15 +11,15 @@
 
 int _version SEC("version") = 1;
 
-SEC("xdp_adjust_tail")
-int _xdp_adjust_tail(struct xdp_md *xdp)
+SEC("xdp_adjust_tail_shrink")
+int _xdp_adjust_tail_shrink(struct xdp_md *xdp)
 {
 	void *data_end = (void *)(long)xdp->data_end;
 	void *data = (void *)(long)xdp->data;
 	int offset = 0;
 
-	if (data_end - data == 54)
-		offset = 256;
+	if (data_end - data == 54) /* sizeof(pkt_v4) */
+		offset = 256; /* shrink too much */
 	else
 		offset = 20;
 	if (bpf_xdp_adjust_tail(xdp, 0 - offset))