watchdog-in-rust.org

Mirai’s Watchdog in rust.

Disabling the watchdog.

One of the first things that mirai does is disable the watchdog of the host to ensure that it didnt reboot if the system begins to take too long.

// Prevent watchdog from rebooting device
if ((wfd = open("/dev/watchdog", 2)) != -1 ||
    (wfd = open("/dev/misc/watchdog", 2)) != -1)
{
    int one = 1;

    ioctl(wfd, 0x80045704, &one);
    close(wfd);
    wfd = 0;
}

Lets break that down,

if ((wfd = open("/dev/watchdog", 2)) != -1 ||

It opens the watchdog device read/write (#define O_RDWR 00000002).

It issues an ‘ioctl’ with a magic number.

ioctl(wfd, 0x80045704, &one);

Then closes the opened file descriptor.

close(wfd);

While this does adhere to the spec, its also not the most supported method of disabling the watchdog. there is another…

The magic close feature.

The aptly named “magic close feature” (https://www.kernel.org/doc/Documentation/watchdog/watchdog-api.txt))

Seems to be more widely supported than the ioctl method. There are watchdog drivers in the tree that do not support ioctl manipulation, but instead support the magic close feature.

Rust doesn’t have a close() function, From the std::fs::File docs:

This would leave us with a simple function :

use std::fs;

fn disable_watchodg() -> std::io::Result<()> {
    fs::write("/dev/watchdog", "V")?
}

fn main() {
 disable_watchdog();
}

Wild right ?

Additional resources

Mirai Source code