mm/rodata_test: verify test data is unchanged, rather than non-zero

Verify that the test variable holds the initialization value, rather than
any non-zero value.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Petr Tesarik <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Cc: Jinbum Park <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/rodata_test.c

···
 #include <linux/mm.h>
 #include <asm/sections.h>
 
-static const int rodata_test_data = 0xC3;
+#define TEST_VALUE 0xC3
+static const int rodata_test_data = TEST_VALUE;
 
 void rodata_test(void)
 {
···
 
 	/* test 1: read the value */
 	/* If this test fails, some previous testrun has clobbered the state */
-	if (!READ_ONCE(rodata_test_data)) {
+	if (unlikely(READ_ONCE(rodata_test_data) != TEST_VALUE)) {
 		pr_err("test 1 fails (start data)\n");
 		return;
 	}
···
 	}
 
 	/* test 3: check the value hasn't changed */
-	if (READ_ONCE(rodata_test_data) == zero) {
+	if (unlikely(READ_ONCE(rodata_test_data) != TEST_VALUE)) {
 		pr_err("test data was changed\n");
 		return;
 	}