Device Input

for the Linux kernel

We want to read events from e.g. /dev/input/event0 in Ruby. For example, if you want to see what's happening "on the wire" when you press a special function key on a laptop. While this code can be used for the purpose of malicious keystroke logging, it is not well suited for it and does not provide the root privileges in order to read /dev/input. Once you've got the privilege to read /dev/input it's game over anyway.

Rationale

/dev/input/eventX is just a character device. Can't we read it with simple Unix tooling? Yes and no. First of all, a character device just means that it passes bytes (not necessarily characters or strings) from userspace into the kernel. Secondarily, the messages (defined as C structs) are in fact binary and not strings or conventional characters.

Since these are C structs (analagous to a binary message), we need to be able to delimit individual messages and decode them. We can't simply read a byte at a time and try to make sense of it. In fact, on my system, /dev/input/event0 refuses any read that is not a multiple of the struct / message size, so we need to know the message size before even attempting a read(), without even a decode().

To determine the message size, we need to know the data structure. For a long time, it was pretty simple: events are 16 bytes:

  • timestamp - 8 bytes
  • type - 1 byte
  • code - 1 byte
  • value - 2 bytes

However, this is only true for 32-bit platforms. On 64-bit platforms, event timestamps became 16 bytes, increasing events from 16 to 24 bytes. This is because a timestamp is defined as two longs, and longs are bigger on 64-bit platforms. It's easy to remember:

  • 32-bit platform: 32-bit long (4 bytes)
  • 64-bit platform: 64-bit long (8 bytes)

read(/dev/input/event0, 16) will fail on a 64-bit machine.

Your tooling must be aware of this distinction and choose the correct underlying data types just to be able to delimit messages and perform a successful read. This software does that, decodes the message, maps the encoded values to friendly strings for display, and provides both library and executable code to assist in examining kernel input events.

Installation

Install the gem:

$ gem install device_input # sudo as necessary

Or, if using Bundler, add to your Gemfile:

gem 'device_input', '~> 0.0'

Usage

Executable

$ sudo devsniff /dev/input/event0

When the f key is pressed:

Misc:ScanCode:33
Key:F:1
Sync:Sync:0

And released:

Misc:ScanCode:33
Key:F:0
Sync:Sync:0

Library

require 'device_input'

DeviceInput.read_from('/dev/input/event0') do |event|
  puts event
end

An event has:

  • #data - a Struct of ints (class name Data)
  • #time - a Time, accurate to usecs
  • #type - a String, possibly UNK-X where X is the integer from #data
  • #code - a String, possibly UNK-X-Y where X and Y are from #data
  • #value - a Fixnum (signed)

Research

Kernel docs

These events are defined as C structs with a fixed size in bytes. See more about these structs towards the end of this document.

Kernel structs

from https://www.kernel.org/doc/Documentation/input/input.txt

struct input_event {
       struct timeval time;
       unsigned short type;
       unsigned short code;
       unsigned int value;
};

from https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/input.h#n25

struct input_event {
       struct timeval time;
        __u16 type;
        __u16 code;
        __s32 value;
};

What's a timeval?

struct timeval {
       __kernel_time_t          tv_sec;      /* seconds */
       __kernel_suseconds_t     tv_usec;     /* microseconds */
};

What's a __kernel_time_t?

typedef long            __kernel_long_t;
# ...
typedef __kernel_long_t         __kernel_suseconds_t;
# ...
typedef __kernel_long_t __kernel_time_t;

What's a __u16? We're pretty sure it's an unsigned 16 bit integer. Likewise __s32 should be a signed 32-bit integer:

typedef unsigned short __u16;

typedef __signed__ int __s32;

Why is the value signed? It's meant to be able to communicate an "analog" range, say -127 to +127 as determined by the position of a joystick.

Review

input_event

  • time (timeval)
    • tv_sec (long)
    • tv_usec (long)
  • type (__u16)
  • code (__u16)
  • value (__s32)

Flattened: SEC USEC TYPE CODE VALUE

How many bytes is a long? Well, it's platform-dependent. On a 32-bit platform, you get 32 bits (4 bytes). On a 64-bit platform you get 64 bits (8 bytes). This means that the event is 16 bytes on a 32-bit machine and 24 bytes on a 64-bit machine. Software will need to accommodate.

Ruby tools

We can use RbConfig and Array#pack/String#unpack to help us read these binary structs:

FIELD      C        RbConfig  Pack
---        ---      ---       ---
tv_sec     long     long      l!
tv_usec    long     long      l!
type       __u16    uint16_t  S
code       __u16    uint16_t  S
value      __s32    int32_t   l

Acknowledgments