libbpf: Support initialized global variables

Initialized global variables are no different in ELF from static variables,
and don't require any extra support from libbpf. But they are matching
semantics of global data (backed by BPF maps) more closely, preventing
LLVM/Clang from aggressively inlining constant values and not requiring
volatile incantations to prevent those. This patch enables global variables.
It still disables uninitialized variables, which will be put into special COM
(common) ELF section, because BPF doesn't allow uninitialized data to be
accessed.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191121070743.1309473-5-andriin@fb.com

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_arrays_output {
 	int a2;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_bitfields {
 	/* unsigned bitfields */

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_bitfields {
 	/* unsigned bitfields */

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_existence_output {
 	int a_exists;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_flavors {
 	int a;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_ints {
 	uint8_t		u8_field;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_kernel_output {
 	int valid[10];

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_misc_output {
 	int a, b, c;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_mods_output {
 	int a, b, c, d, e, f, g, h;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_nesting_substruct {
 	int a;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 enum core_reloc_primitives_enum {
 	A = 0,

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_ptr_as_arr {
 	int a;

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

@@ -8,10 +8,10 @@
 
 char _license[] SEC("license") = "GPL";
 
-static volatile struct data {
+struct {
 	char in[256];
 	char out[256];
-} data;
+} data = {};
 
 struct core_reloc_size_output {
 	int int_sz;