Process limits

Limits are a per-process resource enforcement mechanism. This system provides a rudimentary backstop against runaway processes. cgroup is intended for account-wide resource limits.

Process limits are applied using a PAM wrapper on PAM-aware applications, which include any login service (FTP, mail, SSH, crond).

Format

Limits may be overridden in Bootstrapper or by including a lexicographically higher file than 10-apnscp-system.conf (opens new window) in FST/siteinfo/etc/security/limits.d (see Filesystem.md. Files published within this location are inherited by all sites.

Settings take the following form:

#DOMAIN   TYPE RESOURCE VALUE
# Disable coredumps for everyone
*         -    core     0
# Enable coredumps for users under "site12"
@foobar  soft core     unlimited
# Restrict number of files opened by PHP-FPM
nobody    soft nfiles   2048
# Limit any account with a user "phil" from running more than 25 processes
phil      hard nproc    25

Domain resolution

User resolution is completed inside the virtual environment. In the above example "phil" applies to any account with a user named "phil". Often it is inappropriate to publish limits within the FST except as stopgap limits for runaway resource consumption for all users (*) or the unprivileged PHP-FPM user, apache.

A domain may be of the form:

• username, resolved within the vfs.
• group name using @group syntax
• wildcard * for default entry
• wildcard %, can be also used with %group syntax, for maxlogin limit

Resources

The following table summarizes available resource limits.

Value	Units	Remarks
core	KB†	Limits core file size. 0 disables core files.
data	KB†	Maximum data segment size.
fsize	KB†	Maximum filesize.
memlock	KB	Maximum locked-in-memory address space.
nofile	[0,2^20]†	Maximum number of open file descriptors.
rss	KB†	Ignored.
stack	KB	Maximum stack size.
cpu	minutes	Maximum CPU time.
nproc	[0,2^63)	Max number of processes.
as	KB†	Address space limit (vmem).
maxlogins	—	Max number of logins for this user.
maxsyslogins	—	Max number of global logins for system.
priority	[0,2^63)	Real-time process priority.
locks	[0,2^63)†	Max number of locks.
sigpending	[0,2^63)	Max number of pending signals.
msgqueue	B	Max memory used by POSIX message queues.
nice	[-20, 19]	Max nice priority allowed to raise.
rtprio	[0,2^63)	Max real-time priority.

† values specified accept unlimited as a value.

Hard and soft

Type may either be "hard" or "soft". A hard limit is the maximal value allowed for a resource. Only root may change this. Soft values may be changed by a user up to the hard limit.

Consider the following in site1/fst/etc/security/limits.d/nice-limit.conf:

* soft nice 0
* hard nice -5

su site1
whoami
# Reports admin
renice 0 $$
# 30307 (process ID) old priority 19, new priority 0
renice -1 $$
# renice: failed to set priority for 30307 (process ID): permission denied

Explanation

A user is able to increase process priority from 19 down to 0 but may not specify a negative process priority, which preempts other processes on the system. A priority of 0 gives equal weighting to all system processes while a value greater than 0 would deprioritize.