printk: cleanup deprecated uses of strncpy/strcpy

Cleanup some deprecated uses of strncpy() and strcpy() [1].

There doesn't seem to be any bugs with the current code but the
readability of this code could benefit from a quick makeover while
removing some deprecated stuff as a benefit.

The most interesting replacement made in this patch involves
concatenating "ttyS" with a digit-led user-supplied string. Instead of
doing two distinct string copies with carefully managed offsets and
lengths, let's use the more robust and self-explanatory scnprintf().
scnprintf will 1) respect the bounds of @buf, 2) null-terminate @buf, 3)
do the concatenation. This allows us to drop the manual NUL-byte assignment.

Also, since isdigit() is used about a dozen lines after the open-coded
version we'll replace it for uniformity's sake.

All the strcpy() --> strscpy() replacements are trivial as the source
strings are literals and much smaller than the destination size. No
behavioral change here.

Use the new 2-argument version of strscpy() introduced in Commit
e6584c39 ("string: Allow 2-argument strscpy()"). However, to make
this work fully (since the size must be known at compile time), also
update the extern-qualified declaration to have the proper size
information.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90 [2]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [3]
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240429-strncpy-kernel-printk-printk-c-v1-1-4da7926d7b69@google.com
[pmladek@suse.com: Removed obsolete brackets and added empty lines.]
Signed-off-by: Petr Mladek <pmladek@suse.com>

include/linux/printk.h

@@ -71,7 +71,7 @@ extern void console_verbose(void);
 
 /* strlen("ratelimit") + 1 */
 #define DEVKMSG_STR_MAX_SIZE 10
-extern char devkmsg_log_str[];
+extern char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE];
 struct ctl_table;
 
 extern int suppress_printk;

kernel/printk/printk.c

@@ -178,9 +178,9 @@ static int __init control_devkmsg(char *str)
 	 * Set sysctl string accordingly:
 	 */
 	if (devkmsg_log == DEVKMSG_LOG_MASK_ON)
-		strcpy(devkmsg_log_str, "on");
+		strscpy(devkmsg_log_str, "on");
 	else if (devkmsg_log == DEVKMSG_LOG_MASK_OFF)
-		strcpy(devkmsg_log_str, "off");
+		strscpy(devkmsg_log_str, "off");
 	/* else "ratelimit" which is set by default. */
 
 	/*
@@ -209,7 +209,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 			return -EINVAL;
 
 		old = devkmsg_log;
-		strncpy(old_str, devkmsg_log_str, DEVKMSG_STR_MAX_SIZE);
+		strscpy(old_str, devkmsg_log_str);
 	}
 
 	err = proc_dostring(table, write, buffer, lenp, ppos);
@@ -227,7 +227,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 
 			/* ... and restore old setting. */
 			devkmsg_log = old;
-			strncpy(devkmsg_log_str, old_str, DEVKMSG_STR_MAX_SIZE);
+			strscpy(devkmsg_log_str, old_str);
 
 			return -EINVAL;
 		}
@@ -2500,22 +2500,22 @@ static int __init console_setup(char *str)
 	/*
 	 * Decode str into name, index, options.
 	 */
-	if (str[0] >= '0' && str[0] <= '9') {
-		strcpy(buf, "ttyS");
-		strncpy(buf + 4, str, sizeof(buf) - 5);
-	} else {
-		strncpy(buf, str, sizeof(buf) - 1);
-	}
-	buf[sizeof(buf) - 1] = 0;
+	if (isdigit(str[0]))
+		scnprintf(buf, sizeof(buf), "ttyS%s", str);
+	else
+		strscpy(buf, str);
+
 	options = strchr(str, ',');
 	if (options)
 		*(options++) = 0;
+
 #ifdef __sparc__
 	if (!strcmp(str, "ttya"))
-		strcpy(buf, "ttyS0");
+		strscpy(buf, "ttyS0");
 	if (!strcmp(str, "ttyb"))
-		strcpy(buf, "ttyS1");
+		strscpy(buf, "ttyS1");
 #endif
+
 	for (s = buf; *s; s++)
 		if (isdigit(*s) || *s == ',')
 			break;