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     cap_enter, cap_getmode — Capability mode system calls


     Standard C Library (libc, -lc)


     #include <sys/capsicum.h>


     cap_getmode(u_int *modep);


     cap_enter() 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 inception.

     When combined with cap_rights_limit(2), cap_ioctls_limit(2), cap_fcntls_limit(2),
     cap_enter() may be used to create kernel-enforced sandboxes in which appropriately-crafted
     applications or application components may be run.

     cap_getmode() returns a flag indicating whether or not the process is in a capability mode


     If the 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 ENOTCAPABLE
     or ECAPMODE error also generates the synchronous SIGTRAP signal 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 TRAP_CAP.

     See also the PROC_TRAPCAP_CTL and PROC_TRAPCAP_STATUS operations of the procctl(2) function
     for similar per-process functionality.


     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 cap_enter() 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.


     The cap_enter() and 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.


     The cap_enter() and cap_getmode() system calls will fail if:

     [ENOSYS]           The kernel is compiled without:

                        options CAPABILITY_MODE

     The cap_getmode() system call may also return the following error:

     [EFAULT]           Pointer modep points outside the process's allocated address space.


     cap_fcntls_limit(2), cap_ioctls_limit(2), cap_rights_limit(2), fexecve(2), procctl(2),
     cap_sandboxed(3), capsicum(4), sysctl(9)


     The 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.