[PATCH v3] input/uinput: add UI_GET_SYSNAME ioctl to retrieve the sysfs path

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

Evemu [1] uses uinput to replay devices traces it has recorded. However,
the way evemu uses uinput is slightly different from how uinput is
supposed to be used.
Evemu relies on libevdev, which creates the device node through uinput.
It then injects events through the input device node directly (and it
completely skips the uinput node).

Currently, libevdev relies on an heuristic to guess which input node was
created. The problem is that is heuristic is subjected to races between
different uinput devices or even with physical devices. Having a way
to retrieve the sysfs path allows us to find the event node without
having to rely on this heuristic.

[1] http://www.freedesktop.org/wiki/Evemu/

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
---

Ok, I am resurrecting this. The original patch was sent last July here:
https://patchwork.kernel.org/patch/2827524/

changes since v2:
- the ioctl returns only the device name, thus I renamed the ioctl to UI_GET_SYSNAME
- reordered uinput_str_to_user() arguments
- be sure to terminate the user string we send by \0
- abort if udev->state is not UIST_CREATED
- dropped the patch 1/2 (adding resolution to uinput) because I think David has
  already it in one of his queues (ABS2 IIRC)

I also posted the corresponding libevdev here:
http://lists.freedesktop.org/archives/input-tools/2014-January/000757.html

Cheers,
Benjamin

 drivers/input/misc/uinput.c | 46 ++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/uinput.h      |  2 ++
 include/uapi/linux/uinput.h | 13 ++++++++++++-
 3 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index 7728359..0203219 100644
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -20,6 +20,8 @@
  * Author: Aristeu Sergio Rozanski Filho <aris@cathedrallabs.org>
  *
  * Changes/Revisions:
+ *	0.4	01/09/2014 (Benjamin Tissoires <benjamin.tissoires@redhat.com>)
+ *		- add UI_GET_SYSNAME ioctl
  *	0.3	09/04/2006 (Anssi Hannula <anssi.hannula@gmail.com>)
  *		- updated ff support for the changes in kernel interface
  *		- added MODULE_VERSION
@@ -670,6 +672,27 @@ static int uinput_ff_upload_from_user(const char __user *buffer,
 	__ret;						\
 })
 
+static int uinput_str_to_user(void __user *dest, const char *str,
+			      unsigned int maxlen)
+{
+	int len, ret;
+
+	if (!str)
+		return -ENOENT;
+
+	len = strlen(str) + 1;
+	if (len > maxlen)
+		len = maxlen;
+
+	ret = copy_to_user(dest, str, len);
+	if (ret)
+		return -EFAULT;
+
+	/* force terminating '\0' */
+	ret = copy_to_user(dest + len - 1, "\0", 1);
+	return ret ? -EFAULT : len;
+}
+
 static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 				 unsigned long arg, void __user *p)
 {
@@ -679,6 +702,8 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 	struct uinput_ff_erase  ff_erase;
 	struct uinput_request   *req;
 	char			*phys;
+	const char		*name;
+	unsigned int		size;
 
 	retval = mutex_lock_interruptible(&udev->mutex);
 	if (retval)
@@ -831,7 +856,26 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 			break;
 
 		default:
-			retval = -EINVAL;
+			retval = -EAGAIN;
+	}
+
+	if (retval == -EAGAIN) {
+		size = _IOC_SIZE(cmd);
+
+		/* Now check variable-length commands */
+		switch (cmd & ~IOCSIZE_MASK) {
+			case UI_GET_SYSNAME(0):
+				if (udev->state != UIST_CREATED) {
+					retval = -ENOENT;
+					goto out;
+				}
+				name = dev_name(&udev->dev->dev);
+				retval = uinput_str_to_user(p, name, size);
+				break;
+
+			default:
+				retval = -EINVAL;
+		}
 	}
 
  out:
diff --git a/include/linux/uinput.h b/include/linux/uinput.h
index 0a4487d..0994c0d 100644
--- a/include/linux/uinput.h
+++ b/include/linux/uinput.h
@@ -20,6 +20,8 @@
  * Author: Aristeu Sergio Rozanski Filho <aris@cathedrallabs.org>
  *
  * Changes/Revisions:
+ *	0.4	01/09/2014 (Benjamin Tissoires <benjamin.tissoires@redhat.com>)
+ *		- add UI_GET_SYSNAME ioctl
  *	0.3	24/05/2006 (Anssi Hannula <anssi.hannulagmail.com>)
  *		- update ff support for the changes in kernel interface
  *		- add UINPUT_VERSION
diff --git a/include/uapi/linux/uinput.h b/include/uapi/linux/uinput.h
index fe46431..0389b48 100644
--- a/include/uapi/linux/uinput.h
+++ b/include/uapi/linux/uinput.h
@@ -20,6 +20,8 @@
  * Author: Aristeu Sergio Rozanski Filho <aris@cathedrallabs.org>
  *
  * Changes/Revisions:
+ *	0.4	01/09/2014 (Benjamin Tissoires <benjamin.tissoires@redhat.com>)
+ *		- add UI_GET_SYSNAME ioctl
  *	0.3	24/05/2006 (Anssi Hannula <anssi.hannulagmail.com>)
  *		- update ff support for the changes in kernel interface
  *		- add UINPUT_VERSION
@@ -35,7 +37,7 @@
 #include <linux/types.h>
 #include <linux/input.h>
 
-#define UINPUT_VERSION		3
+#define UINPUT_VERSION		4
 
 
 struct uinput_ff_upload {
@@ -73,6 +75,15 @@ struct uinput_ff_erase {
 #define UI_BEGIN_FF_ERASE	_IOWR(UINPUT_IOCTL_BASE, 202, struct uinput_ff_erase)
 #define UI_END_FF_ERASE		_IOW(UINPUT_IOCTL_BASE, 203, struct uinput_ff_erase)
 
+/**
+ * UI_GET_SYSNAME - get the sysfs name of the created uinput device
+ *
+ * @return the sysfs name of the created virtual input device.
+ * The complete sysfs path is then /sys/devices/virtual/input/--NAME--
+ * Usually, it is in the form "inputN"
+ */
+#define UI_GET_SYSNAME(len)	_IOC(_IOC_READ, UINPUT_IOCTL_BASE, 300, len)
+
 /*
  * To write a force-feedback-capable driver, the upload_effect
  * and erase_effect callbacks in input_dev must be implemented.
-- 
1.8.3.1