x86/platform/uv: Refactor code using deprecated strcpy()/strncpy() interfaces to use strscpy()

Both `strncpy` and `strcpy` are deprecated for use on NUL-terminated
destination strings [1].

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ the case for `strncpy` or `strcpy`!

In this case, we can drop both the forced NUL-termination and the `... -1` from:
|       strncpy(arg, val, ACTION_LEN - 1);
as `strscpy` implicitly has this behavior.

Also include slight refactor to code removing possible new-line chars as
per Yang Yang's work at [3]. This reduces code size and complexity by
using more robust and better understood interfaces.

Co-developed-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dimitri Sivanich <sivanich@hpe.com>
Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://lore.kernel.org/all/202212091545310085328@zte.com.cn/ [3]
Link: KSPP#90
Link: https://lore.kernel.org/r/20230824-strncpy-arch-x86-platform-uv-uv_nmi-v2-1-e16d9a3ec570@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: JustinStitt

arch/x86/platform/uv/uv_nmi.c

@@ -202,21 +202,17 @@ static int param_set_action(const char *val, const struct kernel_param *kp)
 {
 	int i;
 	int n = ARRAY_SIZE(valid_acts);
-	char arg[ACTION_LEN], *p;
+	char arg[ACTION_LEN];
 
 	/* (remove possible '\n') */
-	strncpy(arg, val, ACTION_LEN - 1);
-	arg[ACTION_LEN - 1] = '\0';
-	p = strchr(arg, '\n');
-	if (p)
-		*p = '\0';
+	strscpy(arg, val, strnchrnul(val, sizeof(arg)-1, '\n') - val + 1);
 
 	for (i = 0; i < n; i++)
 		if (!strcmp(arg, valid_acts[i].action))
 			break;
 
 	if (i < n) {
-		strcpy(uv_nmi_action, arg);
+		strscpy(uv_nmi_action, arg, sizeof(uv_nmi_action));
 		pr_info("UV: New NMI action:%s\n", uv_nmi_action);
 		return 0;
 	}
@@ -959,7 +955,7 @@ static int uv_handle_nmi(unsigned int reason, struct pt_regs *regs)
 
 		/* Unexpected return, revert action to "dump" */
 		if (master)
-			strncpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
+			strscpy(uv_nmi_action, "dump", sizeof(uv_nmi_action));
 	}
 
 	/* Pause as all CPU's enter the NMI handler */