CAP_ENTER(2)                 BSD System Calls Manual                CAP_ENTER(2)

     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 sandbox.

     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.

     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

     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.

     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.

BSD                                May 5, 2020                               BSD