xnu-6153.141.1.tar.gz

[apple/xnu.git] / bsd / security / audit / audit_bsm_klib.c

/*-
 * Copyright (c) 1999-2009 Apple Inc.
 * Copyright (c) 2005 Robert N. M. Watson
 * All rights reserved.
 *
 * @APPLE_BSD_LICENSE_HEADER_START@
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 * 3.  Neither the name of Apple Inc. ("Apple") nor the names of
 *     its contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR
 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 * @APPLE_BSD_LICENSE_HEADER_END@
 */
/*
 * NOTICE: This file was modified by SPARTA, Inc. in 2005 to introduce
 * support for mandatory and extensible security protections.  This notice
 * is included in support of clause 2.2 (b) of the Apple Public License,
 * Version 2.0.
 */

#include <sys/systm.h>
#include <sys/sysent.h>
#include <sys/types.h>
#include <sys/proc_internal.h>
#include <sys/vnode_internal.h>
#include <sys/fcntl.h>
#include <sys/filedesc.h>
#include <sys/sem.h>
#include <sys/syscall.h>

#include <bsm/audit.h>
#include <bsm/audit_kevents.h>
#include <security/audit/audit.h>
#include <security/audit/audit_bsd.h>
#include <security/audit/audit_private.h>
#include <IOKit/IOBSD.h>

#if CONFIG_AUDIT
/*
 * Hash table functions for the audit event number to event class mask
 * mapping.
 */
#define EVCLASSMAP_HASH_TABLE_SIZE      251
struct evclass_elem {
	au_event_t event;
	au_class_t class;
	LIST_ENTRY(evclass_elem) entry;
};
struct evclass_list {
	LIST_HEAD(, evclass_elem) head;
};

static MALLOC_DEFINE(M_AUDITEVCLASS, "audit_evclass", "Audit event class");
static struct rwlock            evclass_lock;
static struct evclass_list      evclass_hash[EVCLASSMAP_HASH_TABLE_SIZE];

#define EVCLASS_LOCK_INIT()     rw_init(&evclass_lock, "evclass_lock")
#define EVCLASS_RLOCK()         rw_rlock(&evclass_lock)
#define EVCLASS_RUNLOCK()       rw_runlock(&evclass_lock)
#define EVCLASS_WLOCK()         rw_wlock(&evclass_lock)
#define EVCLASS_WUNLOCK()       rw_wunlock(&evclass_lock)

/*
 * Look up the class for an audit event in the class mapping table.
 */
au_class_t
au_event_class(au_event_t event)
{
	struct evclass_list *evcl;
	struct evclass_elem *evc;
	au_class_t class;

	EVCLASS_RLOCK();
	evcl = &evclass_hash[event % EVCLASSMAP_HASH_TABLE_SIZE];
	class = 0;
	LIST_FOREACH(evc, &evcl->head, entry) {
		if (evc->event == event) {
			class = evc->class;
			goto out;
		}
	}
out:
	EVCLASS_RUNLOCK();
	return class;
}

/*
 * Return a new class mask that allows changing the reserved class bit
 * only if the current task is entitled to do so or if this is being done
 * from the kernel task. If the current task is not allowed to make the
 * change, the reserved bit is reverted to its previous state and the rest
 * of the mask is left intact.
 */
static au_class_t
au_class_protect(au_class_t old_class, au_class_t new_class)
{
	au_class_t result = new_class;

	/* Check if the reserved class bit has been flipped */
	if ((old_class & AU_CLASS_MASK_RESERVED) !=
	    (new_class & AU_CLASS_MASK_RESERVED)) {
		task_t task = current_task();
		if (task != kernel_task &&
		    !IOTaskHasEntitlement(task, AU_CLASS_RESERVED_ENTITLEMENT)) {
			/*
			 * If the caller isn't entitled, revert the class bit:
			 * - First remove the reserved bit from the new_class mask
			 * - Next get the state of the old_class mask's reserved bit
			 * - Finally, OR the result from the first two operations
			 */
			result = (new_class & ~AU_CLASS_MASK_RESERVED) |
			    (old_class & AU_CLASS_MASK_RESERVED);
		}
	}

	return result;
}

/*
 * Insert a event to class mapping. If the event already exists in the
 * mapping, then replace the mapping with the new one.
 *
 * IMPORTANT: This function should only be called from the kernel during
 * initialization (e.g. during au_evclassmap_init). Calling afterwards can
 * have adverse effects on other system components that rely on event/class
 * map state.
 *
 * XXX There is currently no constraints placed on the number of mappings.
 * May want to either limit to a number, or in terms of memory usage.
 */
void
au_evclassmap_insert(au_event_t event, au_class_t class)
{
	struct evclass_list *evcl;
	struct evclass_elem *evc, *evc_new;

	/*
	 * If this event requires auditing a system call then add it to  our
	 * audit kernel event mask.  We use audit_kevent_mask to check to see
	 * if the audit syscalls flag needs to be set when preselection masks
	 * are set.
	 */
	if (AUE_IS_A_KEVENT(event)) {
		audit_kevent_mask |= class;
	}

	/*
	 * Pessimistically, always allocate storage before acquiring mutex.
	 * Free if there is already a mapping for this event.
	 */
	evc_new = malloc(sizeof(*evc), M_AUDITEVCLASS, M_WAITOK);

	EVCLASS_WLOCK();
	evcl = &evclass_hash[event % EVCLASSMAP_HASH_TABLE_SIZE];
	LIST_FOREACH(evc, &evcl->head, entry) {
		if (evc->event == event) {
			evc->class = au_class_protect(evc->class, class);
			EVCLASS_WUNLOCK();
			free(evc_new, M_AUDITEVCLASS);
			return;
		}
	}
	evc = evc_new;
	evc->event = event;
	/*
	 * Mappings that require a new element must use 0 as the "old_class" since
	 * there is no previous state.
	 */
	evc->class = au_class_protect(0, class);
	LIST_INSERT_HEAD(&evcl->head, evc, entry);
	EVCLASS_WUNLOCK();
}

void
au_evclassmap_init(void)
{
	unsigned int i;

	EVCLASS_LOCK_INIT();
	for (i = 0; i < EVCLASSMAP_HASH_TABLE_SIZE; i++) {
		LIST_INIT(&evclass_hash[i].head);
	}

	/*
	 * Set up the initial event to class mapping for system calls.
	 */
	for (i = 0; i < nsysent; i++) {
		if (sys_au_event[i] != AUE_NULL) {
			au_evclassmap_insert(sys_au_event[i], 0);
		}
	}

	/*
	 *  Add the Mach system call events. These are not in sys_au_event[].
	 */
	au_evclassmap_insert(AUE_TASKFORPID, 0);
	au_evclassmap_insert(AUE_PIDFORTASK, 0);
	au_evclassmap_insert(AUE_SWAPON, 0);
	au_evclassmap_insert(AUE_SWAPOFF, 0);
	au_evclassmap_insert(AUE_MAPFD, 0);
	au_evclassmap_insert(AUE_INITPROCESS, 0);
}

/*
 * Check whether an event is aditable by comparing the mask of classes this
 * event is part of against the given mask.
 */
int
au_preselect(__unused au_event_t event, au_class_t class, au_mask_t *mask_p,
    int sorf)
{
	au_class_t effmask = 0;

	if (mask_p == NULL) {
		return -1;
	}

	/*
	 * Perform the actual check of the masks against the event.
	 */
	if (sorf & AU_PRS_SUCCESS) {
		effmask |= (mask_p->am_success & class);
	}

	if (sorf & AU_PRS_FAILURE) {
		effmask |= (mask_p->am_failure & class);
	}

	if (effmask) {
		return 1;
	} else {
		return 0;
	}
}

/*
 * Convert sysctl names and present arguments to events.
 */
au_event_t
audit_ctlname_to_sysctlevent(int name[], uint64_t valid_arg)
{
	/* can't parse it - so return the worst case */
	if ((valid_arg & (ARG_CTLNAME | ARG_LEN)) != (ARG_CTLNAME | ARG_LEN)) {
		return AUE_SYSCTL;
	}

	switch (name[0]) {
	/* non-admin "lookups" treat them special */
	case KERN_OSTYPE:
	case KERN_OSRELEASE:
	case KERN_OSREV:
	case KERN_VERSION:
	case KERN_ARGMAX:
	case KERN_CLOCKRATE:
	case KERN_BOOTTIME:
	case KERN_POSIX1:
	case KERN_NGROUPS:
	case KERN_JOB_CONTROL:
	case KERN_SAVED_IDS:
	case KERN_OSRELDATE:
	case KERN_NETBOOT:
	case KERN_SYMFILE:
	case KERN_SHREG_PRIVATIZABLE:
	case KERN_OSVERSION:
		return AUE_SYSCTL_NONADMIN;

	/* only treat the changeable controls as admin */
	case KERN_MAXVNODES:
	case KERN_MAXPROC:
	case KERN_MAXFILES:
	case KERN_MAXPROCPERUID:
	case KERN_MAXFILESPERPROC:
	case KERN_HOSTID:
	case KERN_AIOMAX:
	case KERN_AIOPROCMAX:
	case KERN_AIOTHREADS:
	case KERN_COREDUMP:
	case KERN_SUGID_COREDUMP:
	case KERN_NX_PROTECTION:
		return (valid_arg & ARG_VALUE32) ?
		       AUE_SYSCTL : AUE_SYSCTL_NONADMIN;

	default:
		return AUE_SYSCTL;
	}
	/* NOTREACHED */
}

/*
 * Convert an open flags specifier into a specific type of open event for
 * auditing purposes.
 */
au_event_t
audit_flags_and_error_to_openevent(int oflags, int error)
{
	au_event_t aevent;

	/*
	 * Need to check only those flags we care about.
	 */
	oflags = oflags & (O_RDONLY | O_CREAT | O_TRUNC | O_RDWR | O_WRONLY);

	/*
	 * These checks determine what flags are on with the condition that
	 * ONLY that combination is on, and no other flags are on.
	 */
	switch (oflags) {
	case O_RDONLY:
		aevent = AUE_OPEN_R;
		break;

	case (O_RDONLY | O_CREAT):
		aevent = AUE_OPEN_RC;
		break;

	case (O_RDONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_RTC;
		break;

	case (O_RDONLY | O_TRUNC):
		aevent = AUE_OPEN_RT;
		break;

	case O_RDWR:
		aevent = AUE_OPEN_RW;
		break;

	case (O_RDWR | O_CREAT):
		aevent = AUE_OPEN_RWC;
		break;

	case (O_RDWR | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_RWTC;
		break;

	case (O_RDWR | O_TRUNC):
		aevent = AUE_OPEN_RWT;
		break;

	case O_WRONLY:
		aevent = AUE_OPEN_W;
		break;

	case (O_WRONLY | O_CREAT):
		aevent = AUE_OPEN_WC;
		break;

	case (O_WRONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_WTC;
		break;

	case (O_WRONLY | O_TRUNC):
		aevent = AUE_OPEN_WT;
		break;

	default:
		aevent = AUE_OPEN;
		break;
	}

	/*
	 * Convert chatty errors to better matching events.  Failures to
	 * find a file are really just attribute events -- so recast them as
	 * such.
	 *
	 * XXXAUDIT: Solaris defines that AUE_OPEN will never be returned, it
	 * is just a placeholder.  However, in Darwin we return that in
	 * preference to other events.
	 *
	 * XXXRW: This behavior differs from FreeBSD, so possibly revise this
	 * code or this comment.
	 */
	switch (aevent) {
	case AUE_OPEN_R:
	case AUE_OPEN_RT:
	case AUE_OPEN_RW:
	case AUE_OPEN_RWT:
	case AUE_OPEN_W:
	case AUE_OPEN_WT:
		if (error == ENOENT) {
			aevent = AUE_OPEN;
		}
	}
	return aevent;
}

/*
 * Convert an open flags specifier into a specific type of open_extended event
 * for auditing purposes.
 */
au_event_t
audit_flags_and_error_to_openextendedevent(int oflags, int error)
{
	au_event_t aevent;

	/*
	 * Need to check only those flags we care about.
	 */
	oflags = oflags & (O_RDONLY | O_CREAT | O_TRUNC | O_RDWR | O_WRONLY);

	/*
	 * These checks determine what flags are on with the condition that
	 * ONLY that combination is on, and no other flags are on.
	 */
	switch (oflags) {
	case O_RDONLY:
		aevent = AUE_OPEN_EXTENDED_R;
		break;

	case (O_RDONLY | O_CREAT):
		aevent = AUE_OPEN_EXTENDED_RC;
		break;

	case (O_RDONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_RTC;
		break;

	case (O_RDONLY | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_RT;
		break;

	case O_RDWR:
		aevent = AUE_OPEN_EXTENDED_RW;
		break;

	case (O_RDWR | O_CREAT):
		aevent = AUE_OPEN_EXTENDED_RWC;
		break;

	case (O_RDWR | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_RWTC;
		break;

	case (O_RDWR | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_RWT;
		break;

	case O_WRONLY:
		aevent = AUE_OPEN_EXTENDED_W;
		break;

	case (O_WRONLY | O_CREAT):
		aevent = AUE_OPEN_EXTENDED_WC;
		break;

	case (O_WRONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_WTC;
		break;

	case (O_WRONLY | O_TRUNC):
		aevent = AUE_OPEN_EXTENDED_WT;
		break;

	default:
		aevent = AUE_OPEN_EXTENDED;
		break;
	}

	/*
	 * Convert chatty errors to better matching events.  Failures to
	 * find a file are really just attribute events -- so recast them as
	 * such.
	 *
	 * XXXAUDIT: Solaris defines that AUE_OPEN will never be returned, it
	 * is just a placeholder.  However, in Darwin we return that in
	 * preference to other events.
	 *
	 * XXXRW: This behavior differs from FreeBSD, so possibly revise this
	 * code or this comment.
	 */
	switch (aevent) {
	case AUE_OPEN_EXTENDED_R:
	case AUE_OPEN_EXTENDED_RT:
	case AUE_OPEN_EXTENDED_RW:
	case AUE_OPEN_EXTENDED_RWT:
	case AUE_OPEN_EXTENDED_W:
	case AUE_OPEN_EXTENDED_WT:
		if (error == ENOENT) {
			aevent = AUE_OPEN_EXTENDED;
		}
	}
	return aevent;
}

/*
 * Convert an open flags specifier into a specific type of open_extended event
 * for auditing purposes.
 */
au_event_t
audit_flags_and_error_to_openatevent(int oflags, int error)
{
	au_event_t aevent;

	/*
	 * Need to check only those flags we care about.
	 */
	oflags = oflags & (O_RDONLY | O_CREAT | O_TRUNC | O_RDWR | O_WRONLY);

	/*
	 * These checks determine what flags are on with the condition that
	 * ONLY that combination is on, and no other flags are on.
	 */
	switch (oflags) {
	case O_RDONLY:
		aevent = AUE_OPENAT_R;
		break;

	case (O_RDONLY | O_CREAT):
		aevent = AUE_OPENAT_RC;
		break;

	case (O_RDONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPENAT_RTC;
		break;

	case (O_RDONLY | O_TRUNC):
		aevent = AUE_OPENAT_RT;
		break;

	case O_RDWR:
		aevent = AUE_OPENAT_RW;
		break;

	case (O_RDWR | O_CREAT):
		aevent = AUE_OPENAT_RWC;
		break;

	case (O_RDWR | O_CREAT | O_TRUNC):
		aevent = AUE_OPENAT_RWTC;
		break;

	case (O_RDWR | O_TRUNC):
		aevent = AUE_OPENAT_RWT;
		break;

	case O_WRONLY:
		aevent = AUE_OPENAT_W;
		break;

	case (O_WRONLY | O_CREAT):
		aevent = AUE_OPENAT_WC;
		break;

	case (O_WRONLY | O_CREAT | O_TRUNC):
		aevent = AUE_OPENAT_WTC;
		break;

	case (O_WRONLY | O_TRUNC):
		aevent = AUE_OPENAT_WT;
		break;

	default:
		aevent = AUE_OPENAT;
		break;
	}

	/*
	 * Convert chatty errors to better matching events.  Failures to
	 * find a file are really just attribute events -- so recast them as
	 * such.
	 *
	 * XXXAUDIT: Solaris defines that AUE_OPENAT will never be returned, it
	 * is just a placeholder.  However, in Darwin we return that in
	 * preference to other events.
	 *
	 * XXXRW: This behavior differs from FreeBSD, so possibly revise this
	 * code or this comment.
	 */
	switch (aevent) {
	case AUE_OPENAT_R:
	case AUE_OPENAT_RT:
	case AUE_OPENAT_RW:
	case AUE_OPENAT_RWT:
	case AUE_OPENAT_W:
	case AUE_OPENAT_WT:
		if (error == ENOENT) {
			aevent = AUE_OPENAT;
		}
	}
	return aevent;
}

/*
 * Convert an open flags specifier into a specific type of openbyid event
 * for auditing purposes.
 */
au_event_t
audit_flags_and_error_to_openbyidevent(int oflags, int error)
{
	au_event_t aevent;

	/*
	 * Need to check only those flags we care about.
	 */
	oflags = oflags & (O_RDONLY | O_TRUNC | O_RDWR | O_WRONLY);

	/*
	 * These checks determine what flags are on with the condition that
	 * ONLY that combination is on, and no other flags are on.
	 */
	switch (oflags) {
	case O_RDONLY:
		aevent = AUE_OPENBYID_R;
		break;

	case (O_RDONLY | O_TRUNC):
		aevent = AUE_OPENBYID_RT;
		break;

	case O_RDWR:
		aevent = AUE_OPENBYID_RW;
		break;

	case (O_RDWR | O_TRUNC):
		aevent = AUE_OPENBYID_RWT;
		break;

	case O_WRONLY:
		aevent = AUE_OPENBYID_W;
		break;

	case (O_WRONLY | O_TRUNC):
		aevent = AUE_OPENBYID_WT;
		break;

	default:
		aevent = AUE_OPENBYID;
		break;
	}

	/*
	 * Convert chatty errors to better matching events.  Failures to
	 * find a file are really just attribute events -- so recast them as
	 * such.
	 */
	switch (aevent) {
	case AUE_OPENBYID_R:
	case AUE_OPENBYID_RT:
	case AUE_OPENBYID_RW:
	case AUE_OPENBYID_RWT:
	case AUE_OPENBYID_W:
	case AUE_OPENBYID_WT:
		if (error == ENOENT) {
			aevent = AUE_OPENBYID;
		}
	}
	return aevent;
}

/*
 * Convert a MSGCTL command to a specific event.
 */
au_event_t
audit_msgctl_to_event(int cmd)
{
	switch (cmd) {
	case IPC_RMID:
		return AUE_MSGCTL_RMID;

	case IPC_SET:
		return AUE_MSGCTL_SET;

	case IPC_STAT:
		return AUE_MSGCTL_STAT;

	default:
		/* We will audit a bad command. */
		return AUE_MSGCTL;
	}
}

/*
 * Convert a SEMCTL command to a specific event.
 */
au_event_t
audit_semctl_to_event(int cmd)
{
	switch (cmd) {
	case GETALL:
		return AUE_SEMCTL_GETALL;

	case GETNCNT:
		return AUE_SEMCTL_GETNCNT;

	case GETPID:
		return AUE_SEMCTL_GETPID;

	case GETVAL:
		return AUE_SEMCTL_GETVAL;

	case GETZCNT:
		return AUE_SEMCTL_GETZCNT;

	case IPC_RMID:
		return AUE_SEMCTL_RMID;

	case IPC_SET:
		return AUE_SEMCTL_SET;

	case SETALL:
		return AUE_SEMCTL_SETALL;

	case SETVAL:
		return AUE_SEMCTL_SETVAL;

	case IPC_STAT:
		return AUE_SEMCTL_STAT;

	default:
		/* We will audit a bad command. */
		return AUE_SEMCTL;
	}
}

/*
 * Convert a command for the auditon() system call to a audit event.
 */
au_event_t
auditon_command_event(int cmd)
{
	switch (cmd) {
	case A_GETPOLICY:
		return AUE_AUDITON_GPOLICY;

	case A_SETPOLICY:
		return AUE_AUDITON_SPOLICY;

	case A_GETKMASK:
		return AUE_AUDITON_GETKMASK;

	case A_SETKMASK:
		return AUE_AUDITON_SETKMASK;

	case A_GETQCTRL:
		return AUE_AUDITON_GQCTRL;

	case A_SETQCTRL:
		return AUE_AUDITON_SQCTRL;

	case A_GETCWD:
		return AUE_AUDITON_GETCWD;

	case A_GETCAR:
		return AUE_AUDITON_GETCAR;

	case A_GETSTAT:
		return AUE_AUDITON_GETSTAT;

	case A_SETSTAT:
		return AUE_AUDITON_SETSTAT;

	case A_SETUMASK:
		return AUE_AUDITON_SETUMASK;

	case A_SETSMASK:
		return AUE_AUDITON_SETSMASK;

	case A_GETCOND:
		return AUE_AUDITON_GETCOND;

	case A_SETCOND:
		return AUE_AUDITON_SETCOND;

	case A_GETCLASS:
		return AUE_AUDITON_GETCLASS;

	case A_SETCLASS:
		return AUE_AUDITON_SETCLASS;

	case A_GETPINFO:
	case A_SETPMASK:
	case A_SETFSIZE:
	case A_GETFSIZE:
	case A_GETPINFO_ADDR:
	case A_GETKAUDIT:
	case A_SETKAUDIT:
	case A_GETSINFO_ADDR:
	default:
		return AUE_AUDITON;   /* No special record */
	}
}

/*
 * For darwin we rewrite events generated by fcntl(F_OPENFROM,...) and
 * fcntl(F_UNLINKFROM,...) system calls to AUE_OPENAT_* and AUE_UNLINKAT audit
 * events.
 */
au_event_t
audit_fcntl_command_event(int cmd, int oflags, int error)
{
	switch (cmd) {
	case F_OPENFROM:
		return audit_flags_and_error_to_openatevent(oflags, error);

	case F_UNLINKFROM:
		return AUE_UNLINKAT;

	default:
		return AUE_FCNTL;   /* Don't change from AUE_FCNTL. */
	}
}

/*
 * Create a canonical path from given path by prefixing either the root
 * directory, or the current working directory.
 */
int
audit_canon_path(struct vnode *cwd_vp, char *path, char *cpath)
{
	int len;
	int ret;
	char *bufp = path;

	/*
	 * Convert multiple leading '/' into a single '/' if the cwd_vp is
	 * NULL (i.e. an absolute path), and strip them entirely if the
	 * cwd_vp represents a chroot directory (i.e. the caller checked for
	 * an initial '/' character itself, saw one, and passed fdp->fd_rdir).
	 * Somewhat complicated, but it places the onus for locking structs
	 * involved on the caller, and makes proxy operations explicit rather
	 * than implicit.
	 */
	if (*(path) == '/') {
		while (*(bufp) == '/') {
			bufp++;         /* skip leading '/'s */
		}
		if (cwd_vp == NULL) {
			bufp--;         /* restore one '/' */
		}
	}
	if (cwd_vp != NULL) {
		len = MAXPATHLEN;
		ret = vn_getpath(cwd_vp, cpath, &len);
		if (ret != 0) {
			cpath[0] = '\0';
			return ret;
		}
		if (len < MAXPATHLEN) {
			cpath[len - 1] = '/';
		}
		strlcpy(cpath + len, bufp, MAXPATHLEN - len);
	} else {
		strlcpy(cpath, bufp, MAXPATHLEN);
	}
	return 0;
}
#endif /* CONFIG_AUDIT */