Procfs

procfs (process filesystem) is a virtual filesystem (i.e. it takes up no disk space) that can be used to show and change system and process information. It is generated by the kernel and mounted at /proc. The information in the sysfs filesystem replaces more and more system information of procfs.

Installation[edit | edit source]

Kernel[edit | edit source]

Activate the following kernel options:

KERNEL Enable procfs support
File systems  --->
    Pseudo filesystems  --->
        <*> /proc file system support

Configuration[edit | edit source]

OpenRC[edit | edit source]

/proc is mounted by OpenRC's /lib/rc/sh/init.sh[1]

user $mount | grep proc
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)

The init service /etc/init.d/procfs goes in runlevel boot.

user $/sbin/rc-update | grep procfs
procfs | boot

Restricting access to PID directories[edit | edit source]

procfs provides the hidepid mount option to restrict access to the /proc/<pid> directories by other users. This is a hardening technique that can make it more difficult for malicious local users to gather information about the processes of other users[2].

Value Description
hidepid=0 The file located in /proc/<pid>/* will be world readable. This is the default behavior.
hidepid=1 The /proc/<pid> directories are visible by all users, but users can only access the /proc/<pid> directories they own. This will protect files such as /proc/<pid>/cmdline, which may contain sensitive information.
hidepid=2 Same as hidepid=1 but the /proc/<pid> directories of other users are hidden. Since this will hide process information from all users except root, it may be useful to allow access to privileged users such as those belonging to the wheel group.
FILE /etc/fstabHiding process information from other users except those in the wheel group
proc     /proc     proc     nosuid,nodev,noexec,hidepid=2,gid=wheel     0 0

Usage[edit | edit source]

See the Wikipedia article for a description of each file's purpose.

Use cat to read information. For example, users can get the version of the currently running kernel with the following command:

user $cat /proc/version

Use echo to set values at runtime (if possible). For example, users can enable the Magic SysRq keys with the following command:

root #echo 1 > /proc/sys/kernel.sysrq

sysctl[edit | edit source]

sysctl is a mechanism to modify certain kernel parameters at runtime. It is part of procfs and its files are located at /proc/sys/. These articles contain documentation of most files.

sysctl works with keys value pairs. The keys can be assembled from the file path by removing the /proc/sys prefix and replacing the forward slash with a dot. For example /proc/sys/kernel/sysrq becomes kernel.sysrq.

It is certainly possible to modify files in /proc like performed above, however the sysctl tool (part of sys-process/procps) can modify /proc information in a much more structured way:

To enable the magic SysRq keys:

root #sysctl kernel.sysrq=1

To show all sysctl keys and their current values:

root #sysctl -a

To configure kernel parameters at system boot, add them to a configuration file with a .conf suffix in the /etc/sysctl.d/ directory. The recommended location for local settings is /etc/sysctl.d/local.conf. The legacy file /etc/sysctl.conf is also supported. To enable the magic SysRq key at boot:

FILE /etc/sysctl.d/local.conf
kernel.sysrq=1

OpenRC[edit | edit source]

The sysctl service reads the files at boot and executes settings. The service is enabled by default.

systemd[edit | edit source]

Besides the /etc/sysctl.d/ directory systemd also knows the /usr/lib/sysctl.d/ directory. This second directory is for package-provided configuration files.

The systemd-sysctl service is enabled by default.

See also[edit | edit source]

References[edit | edit source]

  1. ↑ Bug 406263 – Remove the two lines concerning proc and shm since they don't reflect the /etc/fstab file that is default in current state3 installation, Gentoo's Bugzilla Main Page, (Last modified) April 29th, 2012. Retrieved on October 23rd, 2015.
  2. ↑ Vasiliy Kulikov. procfs: add hidepid= and gid= mount options, Linux kernel source tree, January 10th, 2012. Retrieved on July 31st, 2015.
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