[PATCH v7] /proc/sysrq-trigger: accept multiple keys at once

[Tomas Mudrunka <tomas.mudrunka@gmail.com>]

Just for convenience.
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 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>
---
 Documentation/admin-guide/sysrq.rst | 11 ++++++++++-
 drivers/tty/sysrq.c                 | 18 +++++++++++++++---
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 51906e473..fbe79d314 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/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 single character to /proc/sysrq-trigger.
+	Only the first character is processed, the rest of 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?
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 6b4a28bcf..5411351e4 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -1150,16 +1150,28 @@ 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.
+ * If first character is 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;
-- 
2.42.1