selftests/bpf: Additional test for CO-RE in the kernel.

Add a test where randmap() function is appended to three different bpf
programs. That action checks struct bpf_core_relo replication logic
and offset adjustment in gen loader part of libbpf.

Fourth bpf program has 360 CO-RE relocations from vmlinux, bpf_testmod,
and non-existing type. It tests candidate cache logic.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/Makefile

···
 
 LSKELS := kfunc_call_test.c fentry_test.c fexit_test.c fexit_sleep.c \
 	test_ringbuf.c atomics.c trace_printk.c trace_vprintk.c \
-	map_ptr_kern.c
+	map_ptr_kern.c core_kern.c
 # Generate both light skeleton and libbpf skeleton for these
 LSKELS_EXTRA := test_ksyms_module.c test_ksyms_weak.c kfunc_call_test_subprog.c
 SKEL_BLACKLIST += $$(LSKELS)

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include "test_progs.h"
+#include "core_kern.lskel.h"
+
+void test_core_kern_lskel(void)
+{
+	struct core_kern_lskel *skel;
+
+	skel = core_kern_lskel__open_and_load();
+	ASSERT_OK_PTR(skel, "open_and_load");
+	core_kern_lskel__destroy(skel);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+#define ATTR __always_inline
+#include "test_jhash.h"
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, u32);
+	__type(value, u32);
+	__uint(max_entries, 256);
+} array1 SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, u32);
+	__type(value, u32);
+	__uint(max_entries, 256);
+} array2 SEC(".maps");
+
+static __noinline int randmap(int v, const struct net_device *dev)
+{
+	struct bpf_map *map = (struct bpf_map *)&array1;
+	int key = bpf_get_prandom_u32() & 0xff;
+	int *val;
+
+	if (bpf_get_prandom_u32() & 1)
+		map = (struct bpf_map *)&array2;
+
+	val = bpf_map_lookup_elem(map, &key);
+	if (val)
+		*val = bpf_get_prandom_u32() + v + dev->mtu;
+
+	return 0;
+}
+
+SEC("tp_btf/xdp_devmap_xmit")
+int BPF_PROG(tp_xdp_devmap_xmit_multi, const struct net_device
+	     *from_dev, const struct net_device *to_dev, int sent, int drops,
+	     int err)
+{
+	return randmap(from_dev->ifindex, from_dev);
+}
+
+SEC("fentry/eth_type_trans")
+int BPF_PROG(fentry_eth_type_trans, struct sk_buff *skb,
+	     struct net_device *dev, unsigned short protocol)
+{
+	return randmap(dev->ifindex + skb->len, dev);
+}
+
+SEC("fexit/eth_type_trans")
+int BPF_PROG(fexit_eth_type_trans, struct sk_buff *skb,
+	     struct net_device *dev, unsigned short protocol)
+{
+	return randmap(dev->ifindex + skb->len, dev);
+}
+
+volatile const int never;
+
+struct __sk_bUfF /* it will not exist in vmlinux */ {
+	int len;
+} __attribute__((preserve_access_index));
+
+struct bpf_testmod_test_read_ctx /* it exists in bpf_testmod */ {
+	size_t len;
+} __attribute__((preserve_access_index));
+
+SEC("tc")
+int balancer_ingress(struct __sk_buff *ctx)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+	void *ptr;
+	int ret = 0, nh_off, i = 0;
+
+	nh_off = 14;
+
+	/* pragma unroll doesn't work on large loops */
+#define C do { \
+	ptr = data + i; \
+	if (ptr + nh_off > data_end) \
+		break; \
+	ctx->tc_index = jhash(ptr, nh_off, ctx->cb[0] + i++); \
+	if (never) { \
+		/* below is a dead code with unresolvable CO-RE relo */ \
+		i += ((struct __sk_bUfF *)ctx)->len; \
+		/* this CO-RE relo may or may not resolve
+		 * depending on whether bpf_testmod is loaded.
+		 */ \
+		i += ((struct bpf_testmod_test_read_ctx *)ctx)->len; \
+	} \
+	} while (0);
+#define C30 C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;C;
+	C30;C30;C30; /* 90 calls */
+	return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";