libbpf: Support pre-initializing .bss global variables

Remove invalid assumption in libbpf that .bss map doesn't have to be updated
in kernel. With addition of skeleton and memory-mapped initialization image,
.bss doesn't have to be all zeroes when BPF map is created, because user-code
might have initialized those variables from user-space.

Fixes: eba9c5f ("libbpf: Refactor global data map initialization")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200612194504.557844-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -3564,10 +3564,6 @@ bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
 	char *cp, errmsg[STRERR_BUFSIZE];
 	int err, zero = 0;
 
-	/* kernel already zero-initializes .bss map. */
-	if (map_type == LIBBPF_MAP_BSS)
-		return 0;
-
 	err = bpf_map_update_elem(map->fd, &zero, map->mmaped, 0);
 	if (err) {
 		err = -errno;

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

@@ -15,6 +15,8 @@ void test_skeleton(void)
 	int duration = 0, err;
 	struct test_skeleton* skel;
 	struct test_skeleton__bss *bss;
+	struct test_skeleton__data *data;
+	struct test_skeleton__rodata *rodata;
 	struct test_skeleton__kconfig *kcfg;
 
 	skel = test_skeleton__open();
@@ -24,13 +26,45 @@ void test_skeleton(void)
 	if (CHECK(skel->kconfig, "skel_kconfig", "kconfig is mmaped()!\n"))
 		goto cleanup;
 
+	bss = skel->bss;
+	data = skel->data;
+	rodata = skel->rodata;
+
+	/* validate values are pre-initialized correctly */
+	CHECK(data->in1 != -1, "in1", "got %d != exp %d\n", data->in1, -1);
+	CHECK(data->out1 != -1, "out1", "got %d != exp %d\n", data->out1, -1);
+	CHECK(data->in2 != -1, "in2", "got %lld != exp %lld\n", data->in2, -1LL);
+	CHECK(data->out2 != -1, "out2", "got %lld != exp %lld\n", data->out2, -1LL);
+
+	CHECK(bss->in3 != 0, "in3", "got %d != exp %d\n", bss->in3, 0);
+	CHECK(bss->out3 != 0, "out3", "got %d != exp %d\n", bss->out3, 0);
+	CHECK(bss->in4 != 0, "in4", "got %lld != exp %lld\n", bss->in4, 0LL);
+	CHECK(bss->out4 != 0, "out4", "got %lld != exp %lld\n", bss->out4, 0LL);
+
+	CHECK(rodata->in6 != 0, "in6", "got %d != exp %d\n", rodata->in6, 0);
+	CHECK(bss->out6 != 0, "out6", "got %d != exp %d\n", bss->out6, 0);
+
+	/* validate we can pre-setup global variables, even in .bss */
+	data->in1 = 10;
+	data->in2 = 11;
+	bss->in3 = 12;
+	bss->in4 = 13;
+	rodata->in6 = 14;
+
 	err = test_skeleton__load(skel);
 	if (CHECK(err, "skel_load", "failed to load skeleton: %d\n", err))
 		goto cleanup;
 
-	bss = skel->bss;
-	bss->in1 = 1;
-	bss->in2 = 2;
+	/* validate pre-setup values are still there */
+	CHECK(data->in1 != 10, "in1", "got %d != exp %d\n", data->in1, 10);
+	CHECK(data->in2 != 11, "in2", "got %lld != exp %lld\n", data->in2, 11LL);
+	CHECK(bss->in3 != 12, "in3", "got %d != exp %d\n", bss->in3, 12);
+	CHECK(bss->in4 != 13, "in4", "got %lld != exp %lld\n", bss->in4, 13LL);
+	CHECK(rodata->in6 != 14, "in6", "got %d != exp %d\n", rodata->in6, 14);
+
+	/* now set new values and attach to get them into outX variables */
+	data->in1 = 1;
+	data->in2 = 2;
 	bss->in3 = 3;
 	bss->in4 = 4;
 	bss->in5.a = 5;
@@ -44,14 +78,15 @@ void test_skeleton(void)
 	/* trigger tracepoint */
 	usleep(1);
 
-	CHECK(bss->out1 != 1, "res1", "got %d != exp %d\n", bss->out1, 1);
-	CHECK(bss->out2 != 2, "res2", "got %lld != exp %d\n", bss->out2, 2);
+	CHECK(data->out1 != 1, "res1", "got %d != exp %d\n", data->out1, 1);
+	CHECK(data->out2 != 2, "res2", "got %lld != exp %d\n", data->out2, 2);
 	CHECK(bss->out3 != 3, "res3", "got %d != exp %d\n", (int)bss->out3, 3);
 	CHECK(bss->out4 != 4, "res4", "got %lld != exp %d\n", bss->out4, 4);
 	CHECK(bss->handler_out5.a != 5, "res5", "got %d != exp %d\n",
 	      bss->handler_out5.a, 5);
 	CHECK(bss->handler_out5.b != 6, "res6", "got %lld != exp %d\n",
 	      bss->handler_out5.b, 6);
+	CHECK(bss->out6 != 14, "res7", "got %d != exp %d\n", bss->out6, 14);
 
 	CHECK(bss->bpf_syscall != kcfg->CONFIG_BPF_SYSCALL, "ext1",
 	      "got %d != exp %d\n", bss->bpf_syscall, kcfg->CONFIG_BPF_SYSCALL);

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

@@ -10,16 +10,26 @@ struct s {
 	long long b;
 } __attribute__((packed));
 
-int in1 = 0;
-long long in2 = 0;
+/* .data section */
+int in1 = -1;
+long long in2 = -1;
+
+/* .bss section */
 char in3 = '\0';
 long long in4 __attribute__((aligned(64))) = 0;
 struct s in5 = {};
 
-long long out2 = 0;
+/* .rodata section */
+const volatile int in6 = 0;
+
+/* .data section */
+int out1 = -1;
+long long out2 = -1;
+
+/* .bss section */
 char out3 = 0;
 long long out4 = 0;
-int out1 = 0;
+int out6 = 0;
 
 extern bool CONFIG_BPF_SYSCALL __kconfig;
 extern int LINUX_KERNEL_VERSION __kconfig;
@@ -36,6 +46,7 @@ int handler(const void *ctx)
 	out3 = in3;
 	out4 = in4;
 	out5 = in5;
+	out6 = in6;
 
 	bpf_syscall = CONFIG_BPF_SYSCALL;
 	kern_ver = LINUX_KERNEL_VERSION;