procfs (process filesystem) is a virtual filesystem (virtual means it takes up no disk space) to output 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.
Activate the following kernel options:
File systems ---> Pseudo filesystems ---> <*> /proc file system support
/proc is mounted by OpenRC's /lib/rc/sh/init.sh
mount | grep proc
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
The init service /etc/init.d/procfs goes in runlevel boot.
/sbin/rc-update | grep procfs
procfs | boot
Restricting access to PID directories
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.
||The file located in /proc/<pid>/* will be world readable. This is the default behavior.|
||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.|
|| Same as |
proc /proc proc nosuid,nodev,noexec,hidepid=2,gid=wheel 0 0
See the Wikipedia article for the file's purpose.
Use cat to read the information, e.g. output the version of the currently running kernel:
To set values at runtime - if possible - use echo, e.g. to enable the Magic SysRQ keys:
echo 1 > /proc/sys/kernel.sysrq
sysctl is a mechanism to modify certain kernel parameters at runtime. It is part of procfs and the files are located at /proc/sys/. /usr/src/linux/Documentation/sysctl/ contains documentation of most files.
sysctl work with keys, which can be assembled from the file path by removing the /proc/sys prefix and replace the backsplash with a point. So, e.g. /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 key:
To show all keys and their current values:
To configure kernel parameters at boot, add them to a .conf file in /etc/sysctl.d/. 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:
The sysctl service reads the files at boot and executes settings. The service is enabled by default.
systemd knows beside /etc/sysctl.d/ also /usr/lib/sysctl.d/ for package-provided config files.
The systemd-sysctl service is enabled by default.
- 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.
- Vasiliy Kulikov. procfs: add hidepid= and gid= mount options, Linux kernel source tree, January 10th, 2012. Retrieved on July 31st, 2015.