Need to intercept HID Keyboard events (and then block them)

Question

I've got a RFID USB device that registers as a HID device (A USB Keyboard more or less).

I'm looking for a way to capture this input, and block/filter it before it hits the normal keyboard event handler (and outputs the 10 digit RFID code to the console).

I would of course have to exclusively capture just this device, and leave the real keyboard input alone (or pass it along).

My initial idea was to block the device in UDEV (So the usbhid/event/kbd kernel module didn't bind to it) and write my own basic driver for this device - but I don't know where to begin, or if that'll even work.

What would be great (and I'm not sure if such a thing can be done) - is if I write a event-filter module that can sit in-line with the event driver and capture (then filter) the appropriate input from the RFID unit, but let everything else pass through. I imagine such a module wouldn't require much code, and would be the most practical.

Help?

[EDIT: I should add that Xorg is NOT installed - console only]

cat /proc/bus/input:
 I: Bus=0003 Vendor=0419 Product=0912 Version=0100
 N: Name="NewRoad Sem. NewRoad System PS2 Interface"
 P: Phys=usb-.&checktime(0000,00,1,':')d.3-2/input0
 S: Sysfs=/devices/pci0000:00/0000:00:1d.3/usb5/5-2/5-2:1.0/input/input20
 U: Uniq=
 H: Handlers=sysrq kbd mouse0 event3
 B: PROP=0
 B: EV=120017
 B: KEY=70000 0 0 e080ffdf01cfffff fffffffffffffffe
 B: REL=103
 B: MSC=10
 B: LED=1f

More info:

lsusb -d 0419:0912 -v
Bus 005 Device 019: ID 0419:0912 Samsung Info. Systems America, Inc.
Device Descriptor:
 bLength 18
 bDescriptorType 1
 bcdUSB 1.00
 bDeviceClass 0 (Defined at Interface level)
 bDeviceSubClass 0
 bDeviceProtocol 0
 bMaxPacketSize0 8
 idVendor 0x0419 Samsung Info. Systems America, Inc.
 idProduct 0x0912
 bcdDevice 0.01
 iManufacturer 1 NewRoad Sem.
 iProduct 2 NewRoad System PS2 Interface
 iSerial 0
 bNumConfigurations 1
 Configuration Descriptor:
 bLength 9
 bDescriptorType 2
 wTotalLength 34
 bNumInterfaces 1
 bConfigurationValue 1
 iConfiguration 4
 bmAttributes 0xa0
 (Bus Powered)
 Remote Wakeup
 MaxPower 100mA
 Interface Descriptor:
 bLength 9
 bDescriptorType 4
 bInterfaceNumber 0
 bAlternateSetting 0
 bNumEndpoints 1
 bInterfaceClass 3 Human Interface Device
 bInterfaceSubClass 1 Boot Interface Subclass
 bInterfaceProtocol 1 Keyboard
 iInterface 5
 HID Device Descriptor:
 bLength 9
 bDescriptorType 33
 bcdHID 1.00
 bCountryCode 0 Not supported
 bNumDescriptors 1
 bDescriptorType 34 Report
 wDescriptorLength 119
 Report Descriptors:
 ** UNAVAILABLE **
 Endpoint Descriptor:
 bLength 7
 bDescriptorType 5
 bEndpointAddress 0x81 EP 1 IN
 bmAttributes 3
 Transfer Type Interrupt
 Synch Type None
 Usage Type Data
 wMaxPacketSize 0x0008 1x 8 bytes
 bInterval 10
Device Status: 0x0000
 (Bus Powered)

• Litch: I think this SO post might have the answer I'm after...

Answer 1

So I whipped up a proof-of-concept app according to that post I found here

It does exactly what I require - just though I'd share my solution anyway.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <dirent.h>
#include <linux/input.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/select.h>
#include <sys/time.h>
#include <termios.h>
#include <signal.h>
int main(int argc, char* argv[])
{
 struct input_event ev[64];
 int fevdev = -1;
 int result = 0;
 int size = sizeof(struct input_event);
 int rd;
 int value;
 char name[256] = "Unknown";
 char *device = "/dev/input/event3";
 fevdev = open(device, O_RDONLY);
 if (fevdev == -1) {
 printf("Failed to open event device.\n");
 exit(1);
 }
 result = ioctl(fevdev, EVIOCGNAME(sizeof(name)), name);
 printf ("Reading From : %s (%s)\n", device, name);
 printf("Getting exclusive access: ");
 result = ioctl(fevdev, EVIOCGRAB, 1);
 printf("%s\n", (result == 0) ? "SUCCESS" : "FAILURE");
 while (1)
 {
 if ((rd = read(fevdev, ev, size * 64)) < size) {
 break;
 }
 value = ev[0].value;
 if (value != ' ' && ev[1].value == 1 && ev[1].type == 1) {
 printf ("Code[%d]\n", (ev[1].code));
 }
 }
 printf("Exiting.\n");
 result = ioctl(fevdev, EVIOCGRAB, 1);
 close(fevdev);
 return 0;
}

• olivervbk: should call sleep() before grabbing input, otherwise it may block the ENTER release when executing the compiled binary.
• Yuki: @Litch I did not understand how you filter events. You just blocked them, but how are you going to propagate the events?
• Svartalf: @WscriChy I don't think that Litch was trying to propagate the events. His codebase was the consumer to begin with. I am looking for a similar solution. I my case, however, I need to intercept part of the events coming off of the input feed, but propagate others...so my task is more like the one you're thinking.
• Martin L.: Is it possible to just filter a few events and forward the other events to the other listeners?

Answer 2

You can use EVIOCGRAB ioctl on an event device to capture it exclusively.

• B Furtado: How, exactly do you mean?

Answer 3

Ungrabbing requires a "false" value parameter, as below:

result = ioctl(fevdev, EVIOCGRAB, 0);