livepatch/x86: Fix incorrect use of 'strncpy'

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4UAQ1

--------------------------------

Refer to following codes, 'strncpy' would stop copying if Null character
encountered. For example, when 'code' is "53 be 00 0a 05", 'old_code' would
be "53 be 00 00 00".
  > 396 static void *klp_old_code(unsigned char *code)
  > 397 {
  > 398         static unsigned char old_code[JMP_E9_INSN_SIZE];
  > 399
  > 400         strncpy(old_code, code, JMP_E9_INSN_SIZE);
  > 401         return old_code;
  > 402 }

As a result, the instructions cannot be restored completely, and the
system becomes abnormal.

Fixes: f5a67467 ("livepatch/x86: support livepatch without ftrace")
Suggested-by: NXu Kuohai <xukuohai@huawei.com>
Signed-off-by: NZheng Yejian <zhengyejian1@huawei.com>
Reviewed-by: NKuohai Xu <xukuohai@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

arch/x86/kernel/livepatch.c

@@ -367,14 +367,6 @@ static void *klp_jmp_code(unsigned long ip, unsigned long addr)
 	return text_gen_insn(JMP32_INSN_OPCODE, (void *)ip, (void *)addr);
 }
 
-static void *klp_old_code(unsigned char *code)
-{
-	static unsigned char old_code[JMP_E9_INSN_SIZE];
-
-	strncpy(old_code, code, JMP_E9_INSN_SIZE);
-	return old_code;
-}
-
 void arch_klp_code_modify_prepare(void)
 	__acquires(&text_mutex)
 {
@@ -422,7 +414,7 @@ void arch_klp_unpatch_func(struct klp_func *func)
 	ip = (unsigned long)func_node->old_func;
 	if (list_is_singular(&func_node->func_stack)) {
 		list_del_rcu(&func->stack_node);
-		new = klp_old_code(func_node->arch_data.old_code);
+		new = func_node->arch_data.old_code;
 	} else {
 		list_del_rcu(&func->stack_node);
 		next_func = list_first_or_null_rcu(&func_node->func_stack,