/proc/sysrq-trigger: accept multiple keys at once

This way we can do:
`echo _reisub > /proc/sysrq-trigger`
Instead of:
`for i in r e i s u b; do echo "$i" > /proc/sysrq-trigger; done;`

This can be very useful when trying to execute sysrq combo remotely
or from userspace. When sending keys in multiple separate writes,
userspace (eg. bash or ssh) can be killed before whole combo is completed.
Therefore putting all keys in single write is more robust approach.
Signed-off-by: Tomas Mudrunka <tomas.mudrunka@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20231120111451.527952-1-tomas.mudrunka@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/sysrq.rst

@@ -75,10 +75,19 @@ On other
 	submit a patch to be included in this section.
 
 On all
-	Write a character to /proc/sysrq-trigger.  e.g.::
+	Write a single character to /proc/sysrq-trigger.
+	Only the first character is processed, the rest of the string is
+	ignored. However, it is not recommended to write any extra characters
+	as the behavior is undefined and might change in the future versions.
+	E.g.::
 
 		echo t > /proc/sysrq-trigger
 
+	Alternatively, write multiple characters prepended by underscore.
+	This way, all characters will be processed. E.g.::
+
+		echo _reisub > /proc/sysrq-trigger
+
 The :kbd:`<command key>` is case sensitive.
 
 What are the 'command' keys?

drivers/tty/sysrq.c

@@ -1150,16 +1150,29 @@ EXPORT_SYMBOL(unregister_sysrq_key);
 #ifdef CONFIG_PROC_FS
 /*
  * writing 'C' to /proc/sysrq-trigger is like sysrq-C
+ * Normally, only the first character written is processed.
+ * However, if the first character is an underscore,
+ * all characters are processed.
  */
 static ssize_t write_sysrq_trigger(struct file *file, const char __user *buf,
 				   size_t count, loff_t *ppos)
 {
-	if (count) {
+	bool bulk = false;
+	size_t i;
+
+	for (i = 0; i < count; i++) {
 		char c;
 
-		if (get_user(c, buf))
+		if (get_user(c, buf + i))
 			return -EFAULT;
-		__handle_sysrq(c, false);
+
+		if (c == '_')
+			bulk = true;
+		else
+			__handle_sysrq(c, false);
+
+		if (!bulk)
+			break;
 	}
 
 	return count;