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Last Updated: [2024-03-30 Sat 01:00]
Sarcastic Title: “/dev/null is already 98% full”
I logged into a box the other day and was greeted with a rather odd
error message: -bash: /dev/null: Permission denied
This struck me as a bit odd, as /dev/null is a device file which should be
accessible by all. So, I popped open DF-SHOW (shameless self-plug) at the
/dev directory, and had a look at the
null file.
Looking at the file in DF-SHOW, I could see that the null file has
been somehow replaced with an actual file rather than a character
special file (represented by a - in the
first character, rather than a c, of the
Attrs column (this would be the first column in the output of ls -l as well), the other giveaway is that the
size of the file is reported rather than a driver denoted by a major and
minor number. (In the case of /dev/null
this should be 1, 3)
To fix this, we need to recreate the null file. For this, we’ll first need to remove
the bad file and use the mknod command,
which is used to create block or character special files, to recreate
null and using chmod to give it the correct permissions.
The caveat here is that between removing the old file, and creating
the new one, if any application tries to write to the null file, it will
be recreated as a regular file. To get around this, we'll be creating a
new null with a different name, and
forcefully moving it to the correct location.
Finally, if the system we're using has SELinux, we need to give the
new null the correct security context
(even though we might have disabled SELinux on that particular box, it’s
just good practice when dealing with Red Hat based systems like CentOS)
for this we’ll be using the chcon
command:
sudo mknod /dev/null-new c 1 3
sudo chmod 666 /dev/null-new
sudo mv -f /dev/null-new /dev/null
sudo chcon system_u:object_r:null_device_t:s0 /dev/nullIf all goes to plan, the /dev/null
device should be working correctly.
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