Reference the selinux kernel status without invocation of system calls
#include <selinux/avc.h>
int selinux_status_open(int fallback);
void selinux_status_close(void);
int selinux_status_updated(void);
int selinux_status_getenforce(void);
int selinux_status_policyload(void);
int selinux_status_deny_unknown(void);
Linux 2.6.37 or later provides a SELinux kernel status page; being mostly placed on /selinux/status entry. It enables userspace applications to mmap this page with read-only mode, then it informs some status without system call invocations.
In some cases that a userspace application tries to apply heavy frequent access control; such as row-level security in databases, it will face unignorable cost to communicate with kernel space to check invalidation of userspace avc.
These functions provides applications a way to know some kernel events without system-call invocation or worker thread for monitoring.
selinux_status_open() tries to open(2) /selinux/status and mmap(2) it in read-only mode. The file-descriptor and pointer to the page shall be stored internally; Don't touch them directly. Set 1 on the fallback argument to handle a case of older kernels without kernel status page support. In this case, this function tries to open a netlink socket using avc_netlink_open(3) and overwrite corresponding callbacks ( setenforce and policyload). Thus, we need to pay attention to the interaction with these interfaces, when fallback mode is enabled.
selinux_status_close() unmap the kernel status page and close its file descriptor, or close the netlink socket if fallbacked.
selinux_status_updated() informs us whether something has been updated since the last call. It returns 0 if nothing was happened, however, 1 if something has been updated in this duration, or -1 on error.
selinux_status_getenforce() returns 0 if SELinux is running in permissive mode, 1 if enforcing mode, or -1 on error. Same as security_getenforce(3) except with or without system call invocation.
selinux_status_policyload() returns times of policy reloaded on the running system, or -1 on error. Note that it is not a reliable value on fallback-mode until it receive the first event message via netlink socket. Thus, don't use this value to know actual times of policy reloaded.
selinux_status_deny_unknown() returns 0 if SELinux treats policy queries on undefined object classes or permissions as being allowed, 1 if such queries are denied, or -1 on error.
Also note that these interfaces are not thread-safe, so you have to protect them from concurrent calls using exclusive locks when multiple threads are performing.
selinux_status_open() returns 0 or 1 on success. 1 means we are ready to use these interfaces, but netlink socket was opened as fallback instead of the kernel status page. On error, -1 shall be returned.
Any other functions with a return value shall return its characteristic value as described above, or -1 on errors.