|System Calls Manual
cap_getmode — Capability
mode system calls
Standard C Library (libc, -lc)
places the current process into capability mode, a mode of execution in
which processes may only issue system calls operating on file descriptors or
reading limited global system state. Access to global name spaces, such as
file system or IPC name spaces, is prevented. If the process is already in a
capability mode sandbox, the system call is a no-op. Future process
descendants created with fork(2) or
pdfork(2) will be placed in capability mode from
When combined with
may be used to create kernel-enforced sandboxes in which
appropriately-crafted applications or application components may be run.
returns a flag indicating whether or not the process is in a capability mode
kern.trap_enotcap sysctl MIB is set
to a non-zero value, then for any process executing in a capability mode
sandbox, any syscall which results in either an
error also generates the synchronous
to the thread on the syscall return. 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
See also the
PROC_TRAPCAP_STATUS operations of the
procctl(2) function for similar per-process
Creating effective process sandboxes is a tricky process that
involves identifying the least possible rights required by the process and
then passing those rights into the process in a safe manner. Consumers of
should also be aware of other inherited rights, such as access to VM
resources, memory contents, and other process properties that should be
considered. It is advisable to use fexecve(2) to create a
runtime environment inside the sandbox that has as few implicitly acquired
rights as possible.
cap_getmode() functions return the value 0 if
successful; otherwise the value -1 is returned and the global
variable errno is set to indicate the error.
When the process is in capability mode,
cap_getmode() sets the flag to a non-zero value. A
zero value means the process is not in capability mode.
cap_getmode() system calls will fail if:
- The kernel is compiled without:
cap_getmode() system call may also
return the following error:
- Pointer modep points outside the process's allocated address space.
cap_getmode() system call first
appeared in FreeBSD 8.3. Support for capabilities
and capabilities mode was developed as part of the TrustedBSD Project.
These functions and the capability facility were created by Robert N. M. Watson at the University of Cambridge Computer Laboratory with support from a grant from Google, Inc.
|May 5, 2020