cap_rights_limit2

Usage

Library

Standard C Library (libc, -lc)

Overview

When a file descriptor is created by a function such as fhopen(2), kqueue(2), mq_open(2), open(2), pdfork(2), pipe(2), shm_open(2), socket(2) or socketpair(2), it is assigned all capability rights; for accept(2), accept4(2) or openat(2), it inherits capability rights from the "parent" file descriptor. Those rights can be reduced (but never expanded) by using the cap_rights_limit() system call. Once capability rights are reduced, operations on the file descriptor will be limited to those permitted by rights.

The rights argument should be prepared using cap_rights_init(3) family of functions.

Capability rights assigned to a file descriptor can be obtained with the cap_rights_get(3) function.

The complete list of the capability rights can be found in the rights(4) manual page.

Return values

The cap_rights_limit() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

Examples

The following example demonstrates how to limit file descriptor capability rights to allow reading only.

cap_rights_t setrights;
char buf[1];
int fd;
fd = open("/tmp/foo", O_RDWR);
if (fd < 0)
	err(1, "open() failed");
if (cap_enter() < 0)
	err(1, "cap_enter() failed");
cap_rights_init(&setrights, CAP_READ);
if (cap_rights_limit(fd, &setrights) < 0)
	err(1, "cap_rights_limit() failed");
buf[0] = 'X';
if (write(fd, buf, sizeof(buf)) > 0)
	errx(1, "write() succeeded!");
if (read(fd, buf, sizeof(buf)) < 0)
	err(1, "read() failed");

Errors

cap_rights_limit() succeeds unless:

[EBADF]

The fd argument is not a valid active descriptor.

[EINVAL]

An invalid right has been requested in rights.

[ENOSYS]

The running kernel was compiled without options CAPABILITY_MODE.

[ENOTCAPABLE]

The rights argument contains capability rights not present for the given file descriptor. Capability rights list can only be reduced, never expanded.