s390/debug: Replace kmalloc() + copy_from_user() with memdup_user_nul()

Replace kmalloc() followed by copy_from_user() with memdup_user_nul() to
improve and simplify debug_get_user_string(). Remove the manual
NUL-termination.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

Author: toblux

arch/s390/kernel/debug.c

@@ -1416,18 +1416,12 @@ static inline char *debug_get_user_string(const char __user *user_buf,
 {
 	char *buffer;
 
-	buffer = kmalloc(user_len + 1, GFP_KERNEL);
-	if (!buffer)
-		return ERR_PTR(-ENOMEM);
-	if (copy_from_user(buffer, user_buf, user_len) != 0) {
-		kfree(buffer);
-		return ERR_PTR(-EFAULT);
-	}
+	buffer = memdup_user_nul(user_buf, user_len);
+	if (IS_ERR(buffer))
+		return buffer;
 	/* got the string, now strip linefeed. */
 	if (buffer[user_len - 1] == '\n')
 		buffer[user_len - 1] = 0;
-	else
-		buffer[user_len] = 0;
 	return buffer;
 }