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NAME
procctl — control processes
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/procctl.h>
int
procctl(idtype_t idtype, id_t id, int cmd, void *data);
DESCRIPTION
The procctl() system call provides for control over processes. The idtype and id arguments specify the
set of processes to control. If multiple processes match the identifier, procctl will make a “best
effort” to control as many of the selected processes as possible. An error is only returned if no
selected processes successfully complete the request. The following identifier types are supported:
P_PID Control the process with the process ID id.
P_PGID Control processes belonging to the process group with the ID id.
The control request to perform is specified by the cmd argument. The following commands are supported:
PROC_ASLR_CTL Controls the Address Space Layout Randomization (ASLR) in the program images created
by execve(2) in the specified process or its descendants that did not changed the
control nor modified it by other means. The data parameter must point to the
integer variable holding one of the following values:
PROC_ASLR_FORCE_ENABLE Request that ASLR is enabled after execution, even if it is
disabled system-wide. The image flag and set-uid might
prevent ASLR enablement still.
PROC_ASLR_FORCE_DISABLE Request that ASLR is disabled after execution. Same notes
as for PROC_ASLR_FORCE_ENABLE apply.
PROC_ASLR_NOFORCE Use system-wide configured policy for ASLR.
PROC_ASLR_STATUS Returns the current status of ASLR enablement for the target process. The data
parameter must point to the integer variable, where one of the following values is
written:
PROC_ASLR_FORCE_ENABLE
PROC_ASLR_FORCE_DISABLE
PROC_ASLR_NOFORCE
If the currently executed image in the process itself has ASLR enabled, the
PROC_ASLR_ACTIVE flag is or-ed with the value listed above.
PROC_SPROTECT Set process protection state. This is used to mark a process as protected from
being killed if the system exhausts the available memory and swap. The data
parameter must point to an integer containing an operation and zero or more optional
flags. The following operations are supported:
PPROT_SET Mark the selected processes as protected.
PPROT_CLEAR Clear the protected state of selected processes.
The following optional flags are supported:
PPROT_DESCEND Apply the requested operation to all child processes of each selected
process in addition to each selected process.
PPROT_INHERIT When used with PPROT_SET, mark all future child processes of each
selected process as protected. Future child processes will also mark
all of their future child processes.
PROC_REAP_ACQUIRE Acquires the reaper status for the current process. Reaper status means that
children orphaned by the reaper's descendants that were forked after the acquisition
of reaper status are reparented to the reaper process. After system initialization,
init(8) is the default reaper.
PROC_REAP_RELEASE Release the reaper state for the current process. The reaper of the current process
becomes the new reaper of the current process's descendants.
PROC_REAP_STATUS Provides information about the reaper of the specified process, or the process
itself when it is a reaper. The data argument must point to a procctl_reaper_status
structure which is filled in by the syscall on successful return.
struct procctl_reaper_status {
u_int rs_flags;
u_int rs_children;
u_int rs_descendants;
pid_t rs_reaper;
pid_t rs_pid;
};
The rs_flags may have the following flags returned:
REAPER_STATUS_OWNED The specified process has acquired reaper status and has not
released it. When the flag is returned, the specified
process id, pid, identifies the reaper, otherwise the
rs_reaper field of the structure is set to the pid of the
reaper for the specified process id.
REAPER_STATUS_REALINIT The specified process is the root of the reaper tree, i.e.,
init(8).
The rs_children field returns the number of children of the reaper among the
descendants. It is possible to have a child whose reaper is not the specified
process, since the reaper for any existing children is not reset on the
PROC_REAP_ACQUIRE operation. The rs_descendants field returns the total number of
descendants of the reaper(s), not counting descendants of the reaper in the subtree.
The rs_reaper field returns the reaper pid. The rs_pid returns the pid of one
reaper child if there are any descendants.
PROC_REAP_GETPIDS Queries the list of descendants of the reaper of the specified process. The request
takes a pointer to a procctl_reaper_pids structure in the data parameter.
struct procctl_reaper_pids {
u_int rp_count;
struct procctl_reaper_pidinfo *rp_pids;
};
When called, the rp_pids field must point to an array of procctl_reaper_pidinfo
structures, to be filled in on return, and the rp_count field must specify the size
of the array, into which no more than rp_count elements will be filled in by the
kernel.
The struct procctl_reaper_pidinfo structure provides some information about one of
the reaper's descendants. Note that for a descendant that is not a child, it may be
incorrectly identified because of a race in which the original child process exited
and the exited process's pid was reused for an unrelated process.
struct procctl_reaper_pidinfo {
pid_t pi_pid;
pid_t pi_subtree;
u_int pi_flags;
};
The pi_pid field is the process id of the descendant. The pi_subtree field provides
the pid of the child of the reaper, which is the (grand-)parent of the process. The
pi_flags field returns the following flags, further describing the descendant:
REAPER_PIDINFO_VALID Set to indicate that the procctl_reaper_pidinfo structure was
filled in by the kernel. Zero-filling the rp_pids array and
testing the REAPER_PIDINFO_VALID flag allows the caller to
detect the end of the returned array.
REAPER_PIDINFO_CHILD The pi_pid field identifies the direct child of the reaper.
REAPER_PIDINFO_REAPER The reported process is itself a reaper. The descendants of
the subordinate reaper are not reported.
PROC_REAP_KILL Request to deliver a signal to some subset of the descendants of the reaper. The
data parameter must point to a procctl_reaper_kill structure, which is used both for
parameters and status return.
struct procctl_reaper_kill {
int rk_sig;
u_int rk_flags;
pid_t rk_subtree;
u_int rk_killed;
pid_t rk_fpid;
};
The rk_sig field specifies the signal to be delivered. Zero is not a valid signal
number, unlike for kill(2). The rk_flags field further directs the operation. It
is or-ed from the following flags:
REAPER_KILL_CHILDREN Deliver the specified signal only to direct children of the
reaper.
REAPER_KILL_SUBTREE Deliver the specified signal only to descendants that were
forked by the direct child with pid specified in the
rk_subtree field.
If neither the REAPER_KILL_CHILDREN nor the REAPER_KILL_SUBTREE flags are specified,
all current descendants of the reaper are signalled.
If a signal was delivered to any process, the return value from the request is zero.
In this case, the rk_killed field identifies the number of processes signalled. The
rk_fpid field is set to the pid of the first process for which signal delivery
failed, e.g., due to permission problems. If no such process exists, the rk_fpid
field is set to -1.
PROC_TRACE_CTL Enable or disable tracing of the specified process(es), according to the value of
the integer argument. Tracing includes attachment to the process using the
ptrace(2) and ktrace(2), debugging sysctls, hwpmc(4), dtrace(1), and core dumping.
Possible values for the data argument are:
PROC_TRACE_CTL_ENABLE Enable tracing, after it was disabled by
PROC_TRACE_CTL_DISABLE. Only allowed for self.
PROC_TRACE_CTL_DISABLE Disable tracing for the specified process. Tracing is
re-enabled when the process changes the executing
program with the execve(2) syscall. A child inherits
the trace settings from the parent on fork(2).
PROC_TRACE_CTL_DISABLE_EXEC Same as PROC_TRACE_CTL_DISABLE, but the setting
persists for the process even after execve(2).
PROC_TRACE_STATUS Returns the current tracing status for the specified process in the integer variable
pointed to by data. If tracing is disabled, data is set to -1. If tracing is
enabled, but no debugger is attached by the ptrace(2) syscall, data is set to 0. If
a debugger is attached, data is set to the pid of the debugger process.
PROC_TRAPCAP_CTL Controls the capability mode sandbox actions for the specified sandboxed processes,
on a return from any syscall which gives either a ENOTCAPABLE or ECAPMODE error. If
the control is enabled, such errors from the syscalls cause delivery of the
synchronous SIGTRAP signal to the thread immediately before returning from the
syscalls.
Possible values for the data argument are:
PROC_TRAPCAP_CTL_ENABLE Enable the SIGTRAP signal delivery on capability mode
access violations. The enabled mode is inherited by the
children of the process, and is kept after fexecve(2)
calls.
PROC_TRAPCAP_CTL_DISABLE Disable the signal delivery on capability mode access
violations. Note that the global sysctl kern.trap_enotcap
might still cause the signal to be delivered. See
capsicum(4).
On signal delivery, the si_errno member of the siginfo signal handler parameter is
set to the syscall error value, and the si_code member is set to TRAP_CAP.
See capsicum(4) for more information about the capability mode.
PROC_TRAPCAP_STATUS Return the current status of signalling capability mode access violations for the
specified process. The integer value pointed to by the data argument is set to the
PROC_TRAPCAP_CTL_ENABLE value if the process control enables signal delivery, and to
PROC_TRAPCAP_CTL_DISABLE otherwise.
See the note about sysctl kern.trap_enotcap above, which gives independent global
control of signal delivery.
PROC_PDEATHSIG_CTL Request the delivery of a signal when the parent of the calling process exits.
idtype must be P_PID and id must be the either caller's pid or zero, with no
difference in effect. The value is cleared for child processes and when executing
set-user-ID or set-group-ID binaries. arg must point to a value of type int
indicating the signal that should be delivered to the caller. Use zero to cancel a
previously requested signal delivery.
PROC_PDEATHSIG_STATUS
Query the current signal number that will be delivered when the parent of the
calling process exits. idtype must be P_PID and id must be the either caller's pid
or zero, with no difference in effect. arg must point to a memory location that can
hold a value of type int. If signal delivery has not been requested, it will
contain zero on return.
PROC_STACKGAP_CTL Controls the stack gaps in the specified process. A stack gap is the part of the
growth area for a MAP_STACK mapped region that is reserved and never filled by
memory. Instead, the process is guaranteed to receive a SIGSEGV signal on accessing
pages in the gap. Gaps protect against stack overflow corrupting memory adjacent to
the stack.
The data argument must point to an integer variable containing flags. The following
flags are allowed:
PROC_STACKGAP_ENABLE This flag is only accepted for consistency with
PROC_STACKGAP_STATUS. If stack gaps are enabled, the
flag is ignored. If disabled, the flag causes an EINVAL
error to be returned. After gaps are disabled in a
process, they can only be re-enabled when an execve(2)
is performed.
PROC_STACKGAP_DISABLE Disable stack gaps for the process. For existing
stacks, the gap is no longer a reserved part of the
growth area and can be filled by memory on access.
PROC_STACKGAP_ENABLE_EXEC Enable stack gaps for programs started after an
execve(2) by the specified process.
PROC_STACKGAP_DISABLE_EXEC Inherit disabled stack gaps state after execve(2). In
other words, if the currently executing program has
stack gaps disabled, they are kept disabled on exec. If
gaps were enabled, they are kept enabled after exec.
The stack gap state is inherited from the parent on fork(2).
PROC_STACKGAP_STATUS
Returns the current stack gap state for the specified process. data must point to
an integer variable, which is used to return a bitmask consisting of the following
flags:
PROC_STACKGAP_ENABLE Stack gaps are enabled.
PROC_STACKGAP_DISABLE Stack gaps are disabled.
PROC_STACKGAP_ENABLE_EXEC Stack gaps are enabled in the process after execve(2).
PROC_STACKGAP_DISABLE_EXEC Stack gaps are disabled in the process after execve(2).
x86 MACHINE-SPECIFIC REQUESTS
PROC_KPTI_CTL AMD64 only. Controls the Kernel Page Table Isolation (KPTI) option for the children of
the specified process. For the command to work, the vm.pmap.kpti tunable must be
enabled on boot. It is not possible to change the KPTI setting for a running process,
except at the execve(2), where the address space is reinitialized.
The data parameter must point to an integer variable containing one of the following
commands:
PROC_KPTI_CTL_ENABLE_ON_EXEC Enable KPTI after execve(2).
PROC_KPTI_CTL_DISABLE_ON_EXEC Disable KPTI after execve(2). Only root or a process
having the PRIV_IO privilege might use this option.
PROC_KPTI_STATUS Returns the current KPTI status for the specified process. data must point to the
integer variable, which returns the following statuses:
PROC_KPTI_CTL_ENABLE_ON_EXEC
PROC_KPTI_CTL_DISABLE_ON_EXEC
The status is or-ed with the PROC_KPTI_STATUS_ACTIVE in case KPTI is active for the
current address space of the process.
NOTES
Disabling tracing on a process should not be considered a security feature, as it is bypassable both by
the kernel and privileged processes, and via other system mechanisms. As such, it should not be utilized
to reliably protect cryptographic keying material or other confidential data.
RETURN VALUES
If an error occurs, a value of -1 is returned and errno is set to indicate the error.
ERRORS
The procctl() system call will fail if:
[EFAULT] The arg parameter points outside the process's allocated address space.
[EINVAL] The cmd argument specifies an unsupported command.
The idtype argument specifies an unsupported identifier type.
[EPERM] The calling process does not have permission to perform the requested operation on any
of the selected processes.
[ESRCH] No processes matched the requested idtype and id.
[EINVAL] An invalid operation or flag was passed in arg for a PROC_SPROTECT command.
[EPERM] The idtype argument is not equal to P_PID, or id is not equal to the pid of the
calling process, for PROC_REAP_ACQUIRE or PROC_REAP_RELEASE requests.
[EINVAL] Invalid or undefined flags were passed to a PROC_REAP_KILL request.
[EINVAL] An invalid or zero signal number was requested for a PROC_REAP_KILL request.
[EINVAL] The PROC_REAP_RELEASE request was issued by the init(8) process.
[EBUSY] The PROC_REAP_ACQUIRE request was issued by a process that had already acquired reaper
status and has not yet released it.
[EBUSY] The PROC_TRACE_CTL request was issued for a process already being traced.
[EPERM] The PROC_TRACE_CTL request to re-enable tracing of the process
(PROC_TRACE_CTL_ENABLE), or to disable persistence of PROC_TRACE_CTL_DISABLE on
execve(2) was issued for a non-current process.
[EINVAL] The value of the integer data parameter for the PROC_TRACE_CTL or PROC_TRAPCAP_CTL
request is invalid.
[EINVAL] The PROC_PDEATHSIG_CTL or PROC_PDEATHSIG_STATUS request referenced an unsupported id,
idtype or invalid signal number.
SEE ALSO
dtrace(1), cap_enter(2), kill(2), ktrace(2), ptrace(2), wait(2), capsicum(4), hwpmc(4), init(8)
HISTORY
The procctl() function appeared in FreeBSD 10.0.
The reaper facility is based on a similar feature of Linux and DragonflyBSD, and first appeared in
FreeBSD 10.2.
The PROC_PDEATHSIG_CTL facility is based on the prctl(PR_SET_PDEATHSIG, ...) feature of Linux, and first
appeared in FreeBSD 11.2.
The ASLR support was added to system for the checklists compliance in FreeBSD 13.0.
Debian June 13, 2020 PROCCTL(2)