kmemcheck: add opcode self-testing at boot

We've had some troubles in the past with weird instructions. This
patch adds a self-test framework which can be used to verify that
a certain set of opcodes are decoded correctly. Of course, the
opcodes which are not tested can still give the wrong results.

In short, this is just a safeguard to catch unintentional changes
in the opcode decoder. It does not mean that errors can't still
occur!

[rebased for mainline inclusion]
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

arch/x86/mm/kmemcheck/selftest.c

@@ -0,0 +1,69 @@
+#include <linux/kernel.h>
+
+#include "opcode.h"
+#include "selftest.h"
+
+struct selftest_opcode {
+	unsigned int expected_size;
+	const uint8_t *insn;
+	const char *desc;
+};
+
+static const struct selftest_opcode selftest_opcodes[] = {
+	/* REP MOVS */
+	{1, "\xf3\xa4", 		"rep movsb <mem8>, <mem8>"},
+	{4, "\xf3\xa5",			"rep movsl <mem32>, <mem32>"},
+
+	/* MOVZX / MOVZXD */
+	{1, "\x66\x0f\xb6\x51\xf8",	"movzwq <mem8>, <reg16>"},
+	{1, "\x0f\xb6\x51\xf8",		"movzwq <mem8>, <reg32>"},
+
+	/* MOVSX / MOVSXD */
+	{1, "\x66\x0f\xbe\x51\xf8",	"movswq <mem8>, <reg16>"},
+	{1, "\x0f\xbe\x51\xf8",		"movswq <mem8>, <reg32>"},
+
+#ifdef CONFIG_X86_64
+	/* MOVZX / MOVZXD */
+	{1, "\x49\x0f\xb6\x51\xf8",	"movzbq <mem8>, <reg64>"},
+	{2, "\x49\x0f\xb7\x51\xf8",	"movzbq <mem16>, <reg64>"},
+
+	/* MOVSX / MOVSXD */
+	{1, "\x49\x0f\xbe\x51\xf8",	"movsbq <mem8>, <reg64>"},
+	{2, "\x49\x0f\xbf\x51\xf8",	"movsbq <mem16>, <reg64>"},
+	{4, "\x49\x63\x51\xf8",		"movslq <mem32>, <reg64>"},
+#endif
+};
+
+static bool selftest_opcode_one(const struct selftest_opcode *op)
+{
+	unsigned size;
+
+	kmemcheck_opcode_decode(op->insn, &size);
+
+	if (size == op->expected_size)
+		return true;
+
+	printk(KERN_WARNING "kmemcheck: opcode %s: expected size %d, got %d\n",
+		op->desc, op->expected_size, size);
+	return false;
+}
+
+static bool selftest_opcodes_all(void)
+{
+	bool pass = true;
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(selftest_opcodes); ++i)
+		pass = pass && selftest_opcode_one(&selftest_opcodes[i]);
+
+	return pass;
+}
+
+bool kmemcheck_selftest(void)
+{
+	bool pass = true;
+
+	pass = pass && selftest_opcodes_all();
+
+	return pass;
+}