avc_init

Section: SE Linux API documentation (3)
Updated: 27 May 2004
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NAME

avc_init, avc_destroy, avc_reset, avc_cleanup - userspace AVC setup and teardown.  

SYNOPSIS

#include <selinux/selinux.h>
#include <selinux/avc.h>

int avc_init(const char *msgprefix, const struct avc_memory_callback *mem_callbacks,
const struct avc_log_callback *log_callbacks,
const struct avc_thread_callback *thread_callbacks,
const struct avc_lock_callback *lock_callbacks);

void avc_destroy(void);

int avc_reset(void);

void avc_cleanup(void);  

DESCRIPTION

avc_init initializes the userspace AVC and must be called before any other AVC operation can be performed. A non-NULL msgprefix will be prepended to all audit messages produced by the userspace AVC. The default is `uavc'. The remaining arguments, if non-NULL, specify callbacks to be used by the userspace AVC. See CALLBACKS below.

avc_destroy destroys the userspace AVC, freeing all internal memory structures. After this call has been made, avc_init must be called again before any AVC operations can be performed.

avc_reset flushes the userspace AVC, causing it to forget any cached access decisions. The userspace AVC normally calls this function automatically when needed, see NETLINK NOTIFICATION below.

avc_cleanup forces the userspace AVC to search for and free all unused SID's and any access decision entries that refer to them. Normally, the userspace AVC lazily reclaims unused SID's.

 

CALLBACKS

The userspace AVC can be directed how to perform memory allocation, logging, thread creation, and locking via callback functions passed to avc_init. The purpose of this functionality is to allow the userspace AVC to be smoothly integrated into existing userspace object managers.

Use an avc_memory_callback structure to specify alternate functions for dynamic memory allocation.

struct avc_memory_callback {
    void  *(*func_malloc)(size_t size);
    void  (*func_free)(void *ptr);
};

The two fields of the structure should be pointers to functions which behave as malloc(3) and free(3), which are used by default.

Use an avc_log_callback structure to specify alternate functions for logging.

struct avc_log_callback {
    void  (*func_log)(const char *fmt, ...);
    void  (*func_audit)(void *auditdata,
                        security_class_t class,
                        char *msgbuf, size_t msgbufsize);
};

The func_log callback should accept a printf(3) style format and arguments and log them as desired. The default behavior prints the message on the standard error. The func_audit callback should interpret the auditdata parameter for the given class, printing a human-readable interpretation to msgbuf using no more than msgbufsize characters. The default behavior is to ignore auditdata.

Use an avc_thread_callback structure to specify functions for starting and manipulating threads.

struct avc_thread_callback {
    void  *(*func_create_thread)(void (*run)(void));
    void  (*func_stop_thread)(void *thread);
};

The func_create_thread callback should create a new thread and return a pointer which references it. The thread should execute the run argument, which does not return under normal conditions. The func_stop_thread callback should cancel the running thread referenced by thread. By default, threading is not used; see NETLINK NOTIFICATION below.

Use an avc_lock_callback structure to specify functions to create, obtain, and release locks for use by threads.

struct avc_lock_callback {
    void  *(*func_alloc_lock)(void);
    void  (*func_get_lock)(void *lock);
    void  (*func_release_lock)(void *lock);
    void  (*func_free_lock)(void *lock);
};

The func_alloc_lock callback should create a new lock, returning a pointer which references it. The func_get_lock callback should obtain lock, blocking if necessary. The func_release_lock callback should release lock. The func_free_lock callback should destroy lock, freeing any resources associated with it. The default behavior is not to perform any locking. Note that undefined behavior may result if threading is used without appropriate locking.

 

NETLINK NOTIFICATION

Beginning with version 2.6.4, the Linux kernel supports SELinux status change notification via netlink. Two message types are currently implemented, indicating changes to the enforcing mode and to the loaded policy in the kernel, respectively. The userspace AVC listens for these messages and takes the appropriate action, modifying the behavior of avc_has_perm(3) to reflect the current enforcing mode and flushing the cache on receipt of a policy load notification. Audit messages are produced when netlink notifications are processed.

In the default single-threaded mode, the userspace AVC checks for new netlink messages at the start of each permission query. If threading and locking callbacks are passed to avc_init however, a dedicated thread will be started to listen on the netlink socket. This may increase performance and will ensure that log messages are generated immediately rather than at the time of the next permission query.

 

RETURN VALUE

Functions with a return value return zero on success. On error, -1 is returned and errno is set appropriately.

 

NOTES

The msgprefix argument to avc_init currently has a length limit of 15 characters and will be truncated if necessary.

If a provided func_malloc callback does not set errno appropriately on error, userspace AVC calls may exhibit the same behavior.

If a netlink thread has been created and an error occurs on the socket (such as an access error), the thread may terminate and cause the userspace AVC to return EINVAL on all further permission checks until avc_destroy is called.

 

AUTHOR

Eamon Walsh <ewalsh@epoch.ncsc.mil>

 

SEE ALSO

avc_has_perm(3), avc_context_to_sid(3), avc_cache_stats(3), avc_add_callback(3), security_compute_av(3)


 

Index

NAME
SYNOPSIS
DESCRIPTION
CALLBACKS
NETLINK NOTIFICATION
RETURN VALUE
NOTES
AUTHOR
SEE ALSO

linux.jgfs.net manual pages