Linux::Prctl - Perl extension for controlling process characteristics


  use Linux::Prctl;


The linux prctl function allows you to control specific characteristics of a process' behaviour. Usage of the function is fairly messy though, due to limitations in C and linux. This module provides a nice non-messy interface. Most of the text in this documentation is based on text from the linux manpages prctl(2) and capabilities(7)

Besides prctl, this library also wraps libcap for complete capability handling.


There are 2 export tags: :constants and :functions. These export what you think they will.


Set the state of the flag determining whether core dumps are produced for this process upon delivery of a signal whose default behavior is to produce a core dump. (Normally this flag is set for a process by default, but it is cleared when a set-user-ID or set-group-ID program is executed and also by various system calls that manipulate process UIDs and GIDs).


Return the state of the dumpable flag.


Set the endian-ness of the calling process. Valid values are ENDIAN_BIG, ENDIAN_LITTLE and ENDIAN_PPC_LITTLE (PowerPC pseudo little endian).

This function only works on PowerPC systems.


Return the endian-ness of the calling process, see set_endian


Set floating-point emulation control flag. Pass FPEMU_NOPRINT to silently emulate fp operations accesses, or FPEMU_SIGFPE to not emulate fp operations and send SIGFPE instead.

This function only works on ia64 systems.


Get floating-point emulation control flag. See set_fpemu.


Set floating-point exception mode. Pass FP_EXC_SW_ENABLE to use FPEXC for FP exception, FP_EXC_DIV for floating-point divide by zero, FP_EXC_OVF for floating-point overflow, FP_EXC_UND for floating-point underflow, FP_EXC_RES for floating-point inexact result, FP_EXC_INV for floating-point invalid operation, FP_EXC_DISABLED for FP exceptions disabled, FP_EXC_NONRECOV for async non-recoverable exception mode, FP_EXC_ASYNC for async recoverable exception mode, FP_EXC_PRECISE for precise exception mode. Modes can be combined with the | operator.

This function only works on PowerPC systems.


Return the floating-point exception mode as a bitmap of enabled modes. See set_fpexc.


Set the state of the thread's "keep capabilities" flag, which determines whether the threads's effective and permitted capability sets are cleared when a change is made to the threads's user IDs such that the threads's real UID, effective UID, and saved set-user-ID all become non-zero when at least one of them previously had the value 0. (By default, these credential sets are cleared). This value will be reset to False on subsequent calls to execve.


Return the current state of the calling threads's "keep capabilities" flag.


Set the machine check memory corruption kill policy for the current thread. The policy can be early kill (MCE_KILL_EARLY), late kill (MCE_KILL_LATE), or the system-wide default (MCE_KILL_DEFAULT). Early kill means that the task receives a SIGBUS signal as soon as hardware memory corruption is detected inside its address space. In late kill mode, the process is only killed when it accesses a corrupted page. The policy is inherited by children. use the system-wide default. The system-wide default is defined by /proc/sys/vm/memory_failure_early_kill

This function is only available for kernel 2.6.32 and newer


Return the current per-process machine check kill policy.

This function is only available for kernel 2.6.32 and newer


Set the process name for the calling process, the name can be up to 16 bytes long. This name is displayed in the output of ps and top. The initial value is the name of the executable. For perl applications this will likely be perl. As of perl 5.14, assigning to $0 also sets the process name.


Return the (first 16 bytes of) the name for the calling process.


Set the parent process death signal of the calling process (either a valid signal value from the :mod:signal module, or 0 to clear). This is the signal that the calling process will get when its parent dies. This value is cleared for the child of a fork.


Return the current value of the parent process death signal. See set_pdeathsig.


Sets the top of the process tree that is allowed to use PTRACE on the calling process, assuming other requirements are met (matching uid, wasn't setuid, etc). Use pid 0 to disallow all processes. For more details, see /etc/sysctl.d/10-ptrace.conf.

This function is only available for kernel 3.4 and newer, or Ubuntu 10.10 and newer.


Returns the top of the process tree that is allowed to use PTRACE on the calling process. See set_ptracer.

This function is only available for kernel 3.4 and newer, or Ubuntu 10.10 and newer.


Set the secure computing mode for the calling thread. In the current implementation, mode must be True. After the secure computing mode has been set to True, the only system calls that the thread is permitted to make are read, write, _exit, and sigreturn. Other system calls result in the delivery of a SIGKILL signal. Secure computing mode is useful for number-crunching applications that may need to execute untrusted byte code, perhaps obtained by reading from a pipe or socket. This operation is only available if the kernel is configured with CONFIG_SECCOMP enabled.


Return the secure computing mode of the calling thread. Not very useful for the current implementation, but may be useful for other possible future modes: if the caller is not in secure computing mode, this operation returns False; if the caller is in secure computing mode, then the prctl call will cause a SIGKILL signal to be sent to the process. This operation is only available if the kernel is configured with CONFIG_SECCOMP enabled.


Control the default "rounding" in nanoseconds that is used by select, poll and friends.

The default value of the slack is 50 microseconds; this is significantly less than the kernels average timing error but still allows the kernel to group timers somewhat to preserve power behavior.

This function is only available for kernel 2.6.28 and newer


Return the current timing slack, see get_timing_slack

This function is only available for kernel 2.6.28 and newer


Set whether to use (normal, traditional) statistical process timing or accurate timestamp based process timing, by passing TIMING_STATISTICAL or PR_TIMING_TIMESTAMP. TIMING_TIMESTAMP is not currently implemented


Return which process timing method is currently in use.


Set the state of the flag determining whether the timestamp counter can be read by the process. Pass TSC_ENABLE to allow it to be read, or TSC_SIGSEGV to generate a SIGSEGV when the process tries to read the timestamp counter.

This function only works on x86 systems.


Return the state of the flag determining whether the timestamp counter can be read, see set_tsc.


Set unaligned access control flag. Pass UNALIGN_NOPRINT to silently fix up unaligned user accesses, or UNALIGN_SIGBUS to generate SIGBUS on unaligned user access.

This function only works on ia64, parisc, PowerPC and Alpha systems.


Return unaligned access control bits, see set_unalign.


Set the "securebits" flags of the calling thread.

It is not recommended to use this function directly, use the %Linux::Prctl::securebits hash instead.


Get the "securebits" flags of the calling thread.

As with set_securebits, it is not recommended to use this function directly, use the %Linux::Prctl::securebits hash instead.


Return whether the specified capability is in the calling thread's capability bounding set. The capability bounding set dictates whether the process can receive the capability through a file's permitted capability set on a subsequent call to execve.

It is not recommended to use this function directly, use the %Linux::Prctl::cap_* hashes instead.


If the calling thread has the CAP_SETPCAP capability, then drop the specified capability specified by from the calling thread's capability bounding set. Any children of the calling thread will inherit the newly reduced bounding set.

As with capbset_read, it is not recommended to use this function directly, use the %Linux::Prctl::cap_* hashes instead.

Capabilities and the capability bounding set

For the purpose of performing permission checks, traditional Unix implementations distinguish two categories of processes: privileged processes (whose effective user ID is 0, referred to as superuser or root), and unprivileged processes (whose effective UID is non-zero). Privileged processes bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process's credentials (usually: effective UID, effective GID, and supplementary group list).

Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as capabilities, which can be independently enabled and disabled. Capabilities are a per-thread attribute.

Each thread has three capability sets containing zero or more of the capabilities described below

Permitted (the %Linux::Prctl::cap_permitted hash):

This is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabilities that may be added to the inheritable set by a thread that does not have the setpcap capability in its effective set.

If a thread drops a capability from its permitted set, it can never re-acquire that capability (unless it execve s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability).

Inheritabe (the %Linux::Prctl::cap_inheritable hash):

This is a set of capabilities preserved across an execve. It provides a mechanism for a process to assign capabilities to the permitted set of the new program during an execve.

Effective (the %Linux::Prctl::cap_effective hash):

This is the set of capabilities used by the kernel to perform permission checks for the thread.

A child created via fork inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities during :func:`execve`.

The $Linux::Prctl::capbset hash represents the current capability bounding sets of the process. The capability bounding set dictates whether the process can receive the capability through a file's permitted capability set on a subsequent call to execve. All items of this hash are true by default, unless a parent process already removed them from the bounding set.

These four hashes have a number of keys. For the capability bounding set and the effective capabilities, these can only be set to False, this drops them from the corresponding set.

All details about capabilities and capability bounding sets can be found in the capabilities(7) manpage, on which most text below is based.

These are the keys of the hashes:


Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.


Write records to kernel auditing log.


Make arbitrary changes to file UIDs and GIDs (see chown(2)).


Bypass file read, write, and execute permission checks. (DAC is an abbreviation of "discretionary access control".)

Bypass file read permission checks and directory read and execute permission checks.


Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file (e.g., chmod, utime), excluding those operations covered by dac_override and dac_read_search.
Set extended file attributes (see chattr(1)) on arbitrary files.
Set Access Control Lists (ACLs) on arbitrary files.
Ignore directory sticky bit on file deletion.
Specify O_NOATIME for arbitrary files in open and fcntl.


Don't clear set-user-ID and set-group-ID permission bits when a file is modified; set the set-group-ID bit for a file whose GID does not match the file system or any of the supplementary GIDs of the calling process.


Lock memory (mlock, mlockall, mmap, shmctl).


Bypass permission checks for operations on System V IPC objects.


Bypass permission checks for sending signals (see kill(2)). This includes use of the ioctl KDSIGACCEPT operation.


Establish leases on arbitrary files (see fcntl(2)).


Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags (see chattr(1)).


Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM).


Allow MAC configuration or state changes. Implemented for the Smack LSM.


Create special files using mknod.


Perform various network-related operations (e.g., setting privileged socket options, enabling multicasting, interface configuration, modifying routing tables).


Bind a socket to Internet domain privileged ports (port numbers less than 1024).


(Unused) Make socket broadcasts, and listen to multicasts.


Use RAW and PACKET sockets.


Make arbitrary manipulations of process GIDs and supplementary GID list; forge GID when passing socket credentials via Unix domain sockets.


Set file capabilities.


If file capabilities are not supported: grant or remove any capability in the caller's permitted capability set to or from any other process. (This property of setpcap is not available when the kernel is configured to support file capabilities, since setpcap has entirely different semantics for such kernels.)

If file capabilities are supported: add any capability from the calling thread's bounding set to its inheritable set; drop capabilities from the bounding set (via capbset_drop); make changes to the securebits flags.


Make arbitrary manipulations of process UIDs (setuid, setreuid, setresuid, setfsuid); make forged UID when passing socket credentials via Unix domain sockets.


Allow configuring the kernel's syslog (printk behaviour). Before linux 2.6.38 the sys_admin capability was needed for this.

This is only available in linux 2.6.38 and newer.


Perform a range of system administration operations including: quotactl, mount, umount, swapon, swapoff, sethostname, and setdomainname.
Perform IPC_SET and IPC_RMID operations on arbitrary System V IPC objects.
Perform operations on trusted and security Extended Attributes (see attr(5)).
Use lookup_dcookie.
Use ioprio_set to assign the IOPRIO_CLASS_RT scheduling class.
Forge UID when passing socket credentials.
Exceed /proc/sys/fs/file-max, the system-wide limit on the number of open files, in system calls that open files (e.g., accept, execve, open, pipe).
Employ CLONE_NEWNS flag with clone and unshare.
Perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl operations.


Use reboot and kexec_load.


Use chroot.


Load and unload kernel modules (see init_module(2) and delete_module(2)).


Raise process nice value (nice, setpriority) and change the nice value for arbitrary processes.
Set real-time scheduling policies for calling process, and set scheduling policies and priorities for arbitrary processes (sched_setscheduler, sched_setparam).
Set CPU affinity for arbitrary processes (sched_setaffinity)
Set I/O scheduling class and priority for arbitrary processes (ioprio_set).
Apply migrate_pages to arbitrary processes and allow processes to be migrated to arbitrary nodes.
Apply move_pages to arbitrary processes.
Use the MPOL_MF_MOVE_ALL flag with mbind and move_pages.


Use acct.


Trace arbitrary processes using ptrace.


Perform I/O port operations (iopl and ioperm); access /proc/kcore.


Use reserved space on ext2 file systems.
Make ioctl calls controlling ext3 journaling.
Override disk quota limits.
Increase resource limits (see setrlimit(2)).
Override RLIMIT_NPROC resource limit.
Raise msg_qbytes limit for a System V message queue above the limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2)).


Set system clock (settimeofday, stime, adjtimex); set real-time (hardware) clock.


Use vhangup.


Allow triggering something that will wake the system.

This is only available in linux 3.0 and newer

The four capabilities hashes also have two additional methods, to make dropping many capabilities at the same time easier:

drop(cap [, ...])

Drop all capabilities given as arguments from the set.

limit(cap [, ...])

Drop all but the given capabilities from the set.

These function accept both names of capabilities as given above and the CAP_ constants as defined in capabilities.h. These constants are available as CAP_SYS_ADMIN et cetera.

Capabilities and execve

During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm:

* P'(permitted) = (P(inheritable) & F(inheritable)) | (F(permitted) & cap_bset) * P'(effective) = F(effective) ? P'(permitted) : 0 * P'(inheritable) = P(inheritable) [i.e., unchanged]


* P denotes the value of a thread capability set before the execve * P' denotes the value of a capability set after the execve * F denotes a file capability set * cap_bset is the value of the capability bounding set

The downside of this is that you need to set file capabilities if you want to make applications capabilities-friendly via wrappers. For instance, to allow an http daemon to listen on port 80 without it needing root privileges, you could do the following:

 %Linux::Prctl.cap_inheritable{net_bind_service} = 1;
 $< = 1000;
 exec "/usr/sbin/httpd";

This only works if /usr/sbin/httpd has CAP_NET_BIND_SOCK in its inheritable and effective sets. You can do this with the setcap(8) tool shipped with libcap.

 $ sudo setcap cap_net_bind_service=ie /usr/sbin/httpd
 $ getcap /usr/sbin/httpd
 /usr/sbin/httpd = cap_net_bind_service+ei

Note that it only sets the capability in the inheritable set, so this capability is only granted if the program calling execve has it in its inheritable set too. The effective set of file capabilities does not exist in linux, it is a single bit that specifies whether capabilities in the permitted set are automatically raised in the effective set upon execve.

Establishing a capabilities-only environment with securebits

With a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flags that can be used to disable special handling of capabilities for UID 0 (root). The securebits flags are inherited by child processes. During an execve, all of the flags are preserved, except keep_caps which is always cleared.

These capabilities are available via get_securebits, but are easier accessed via the $Linux::prctl::securebits hash. This hash has keys that tell you whether specific securebits are set, or unset.

The following keys are available:


Setting this flag allows a thread that has one or more 0 UIDs to retain its capabilities when it switches all of its UIDs to a non-zero value. If this flag is not set, then such a UID switch causes the thread to lose all capabilities. This flag is always cleared on an execve.


Setting this flag stops the kernel from adjusting capability sets when the threads's effective and file system UIDs are switched between zero and non-zero values. (See the subsection Effect of User ID Changes on Capabilities in capabilities(7))


If this bit is set, then the kernel does not grant capabilities when a set-user-ID-root program is executed, or when a process with an effective or real UID of 0 calls execve. (See the subsection Capabilities and execution of programs by root in capabilities(7))


Like keep_caps, but irreversible


Like no_setuid_fixup, but irreversible


Like noroot, but irreversible


- None of the capability stuff is actually implemented at the moment


Manpages: capabilities(7) and prctl(2)

Github source:


Dennis Kaarsemaker, <>


Copyright (C) 2011 by Dennis Kaarsemaker

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.