[mirror_ubuntu-bionic-kernel.git] / fs / afs / addr_list.c

/* Server address list management
 *
 * Copyright (C) 2017 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/slab.h>
#include <linux/ctype.h>
#include <linux/dns_resolver.h>
#include <linux/inet.h>
#include <keys/rxrpc-type.h>
#include "internal.h"
#include "afs_fs.h"

//#define AFS_MAX_ADDRESSES
//	((unsigned int)((PAGE_SIZE - sizeof(struct afs_addr_list)) /
//			sizeof(struct sockaddr_rxrpc)))
#define AFS_MAX_ADDRESSES ((unsigned int)(sizeof(unsigned long) * 8))

/*
 * Release an address list.
 */
void afs_put_addrlist(struct afs_addr_list *alist)
{
	if (alist && refcount_dec_and_test(&alist->usage))
		call_rcu(&alist->rcu, (rcu_callback_t)kfree);
}

/*
 * Allocate an address list.
 */
struct afs_addr_list *afs_alloc_addrlist(unsigned int nr,
					 unsigned short service,
					 unsigned short port)
{
	struct afs_addr_list *alist;
	unsigned int i;

	_enter("%u,%u,%u", nr, service, port);

	alist = kzalloc(sizeof(*alist) + sizeof(alist->addrs[0]) * nr,
			GFP_KERNEL);
	if (!alist)
		return NULL;

	refcount_set(&alist->usage, 1);

	for (i = 0; i < nr; i++) {
		struct sockaddr_rxrpc *srx = &alist->addrs[i];
		srx->srx_family			= AF_RXRPC;
		srx->srx_service		= service;
		srx->transport_type		= SOCK_DGRAM;
		srx->transport_len		= sizeof(srx->transport.sin6);
		srx->transport.sin6.sin6_family	= AF_INET6;
		srx->transport.sin6.sin6_port	= htons(port);
	}

	return alist;
}

/*
 * Parse a text string consisting of delimited addresses.
 */
struct afs_addr_list *afs_parse_text_addrs(const char *text, size_t len,
					   char delim,
					   unsigned short service,
					   unsigned short port)
{
	struct afs_addr_list *alist;
	const char *p, *end = text + len;
	unsigned int nr = 0;

	_enter("%*.*s,%c", (int)len, (int)len, text, delim);

	if (!len)
		return ERR_PTR(-EDESTADDRREQ);

	if (delim == ':' && (memchr(text, ',', len) || !memchr(text, '.', len)))
		delim = ',';

	/* Count the addresses */
	p = text;
	do {
		if (!*p)
			return ERR_PTR(-EINVAL);
		if (*p == delim)
			continue;
		nr++;
		if (*p == '[') {
			p++;
			if (p == end)
				return ERR_PTR(-EINVAL);
			p = memchr(p, ']', end - p);
			if (!p)
				return ERR_PTR(-EINVAL);
			p++;
			if (p >= end)
				break;
		}

		p = memchr(p, delim, end - p);
		if (!p)
			break;
		p++;
	} while (p < end);

	_debug("%u/%u addresses", nr, AFS_MAX_ADDRESSES);
	if (nr > AFS_MAX_ADDRESSES)
		nr = AFS_MAX_ADDRESSES;

	alist = afs_alloc_addrlist(nr, service, port);
	if (!alist)
		return ERR_PTR(-ENOMEM);

	/* Extract the addresses */
	p = text;
	do {
		struct sockaddr_rxrpc *srx = &alist->addrs[alist->nr_addrs];
		const char *q, *stop;

		if (*p == delim) {
			p++;
			continue;
		}

		if (*p == '[') {
			p++;
			q = memchr(p, ']', end - p);
		} else {
			for (q = p; q < end; q++)
				if (*q == '+' || *q == delim)
					break;
		}

		if (in4_pton(p, q - p,
			     (u8 *)&srx->transport.sin6.sin6_addr.s6_addr32[3],
			     -1, &stop)) {
			srx->transport.sin6.sin6_addr.s6_addr32[0] = 0;
			srx->transport.sin6.sin6_addr.s6_addr32[1] = 0;
			srx->transport.sin6.sin6_addr.s6_addr32[2] = htonl(0xffff);
		} else if (in6_pton(p, q - p,
				    srx->transport.sin6.sin6_addr.s6_addr,
				    -1, &stop)) {
			/* Nothing to do */
		} else {
			goto bad_address;
		}

		if (stop != q)
			goto bad_address;

		p = q;
		if (q < end && *q == ']')
			p++;

		if (p < end) {
			if (*p == '+') {
				/* Port number specification "+1234" */
				unsigned int xport = 0;
				p++;
				if (p >= end || !isdigit(*p))
					goto bad_address;
				do {
					xport *= 10;
					xport += *p - '0';
					if (xport > 65535)
						goto bad_address;
					p++;
				} while (p < end && isdigit(*p));
				srx->transport.sin6.sin6_port = htons(xport);
			} else if (*p == delim) {
				p++;
			} else {
				goto bad_address;
			}
		}

		alist->nr_addrs++;
	} while (p < end && alist->nr_addrs < AFS_MAX_ADDRESSES);

	_leave(" = [nr %u]", alist->nr_addrs);
	return alist;

bad_address:
	kfree(alist);
	return ERR_PTR(-EINVAL);
}

/*
 * Compare old and new address lists to see if there's been any change.
 * - How to do this in better than O(Nlog(N)) time?
 *   - We don't really want to sort the address list, but would rather take the
 *     list as we got it so as not to undo record rotation by the DNS server.
 */
#if 0
static int afs_cmp_addr_list(const struct afs_addr_list *a1,
			     const struct afs_addr_list *a2)
{
}
#endif

/*
 * Perform a DNS query for VL servers and build a up an address list.
 */
struct afs_addr_list *afs_dns_query(struct afs_cell *cell, time64_t *_expiry)
{
	struct afs_addr_list *alist;
	char *vllist = NULL;
	int ret;

	_enter("%s", cell->name);

	ret = dns_query("afsdb", cell->name, cell->name_len,
			"ipv4", &vllist, _expiry);
	if (ret < 0)
		return ERR_PTR(ret);

	alist = afs_parse_text_addrs(vllist, strlen(vllist), ',',
				     VL_SERVICE, AFS_VL_PORT);
	if (IS_ERR(alist)) {
		kfree(vllist);
		if (alist != ERR_PTR(-ENOMEM))
			pr_err("Failed to parse DNS data\n");
		return alist;
	}

	kfree(vllist);
	return alist;
}

/*
 * Merge an IPv4 entry into a fileserver address list.
 */
void afs_merge_fs_addr4(struct afs_addr_list *alist, __be32 xdr, u16 port)
{
	struct sockaddr_in6 *a;
	__be16 xport = htons(port);
	int i;

	for (i = 0; i < alist->nr_ipv4; i++) {
		a = &alist->addrs[i].transport.sin6;
		if (xdr == a->sin6_addr.s6_addr32[3] &&
		    xport == a->sin6_port)
			return;
		if (xdr == a->sin6_addr.s6_addr32[3] &&
		    xport < a->sin6_port)
			break;
		if (xdr < a->sin6_addr.s6_addr32[3])
			break;
	}

	if (i < alist->nr_addrs)
		memmove(alist->addrs + i + 1,
			alist->addrs + i,
			sizeof(alist->addrs[0]) * (alist->nr_addrs - i));

	a = &alist->addrs[i].transport.sin6;
	a->sin6_port		  = xport;
	a->sin6_addr.s6_addr32[0] = 0;
	a->sin6_addr.s6_addr32[1] = 0;
	a->sin6_addr.s6_addr32[2] = htonl(0xffff);
	a->sin6_addr.s6_addr32[3] = xdr;
	alist->nr_ipv4++;
	alist->nr_addrs++;
}

/*
 * Merge an IPv6 entry into a fileserver address list.
 */
void afs_merge_fs_addr6(struct afs_addr_list *alist, __be32 *xdr, u16 port)
{
	struct sockaddr_in6 *a;
	__be16 xport = htons(port);
	int i, diff;

	for (i = alist->nr_ipv4; i < alist->nr_addrs; i++) {
		a = &alist->addrs[i].transport.sin6;
		diff = memcmp(xdr, &a->sin6_addr, 16);
		if (diff == 0 &&
		    xport == a->sin6_port)
			return;
		if (diff == 0 &&
		    xport < a->sin6_port)
			break;
		if (diff < 0)
			break;
	}

	if (i < alist->nr_addrs)
		memmove(alist->addrs + i + 1,
			alist->addrs + i,
			sizeof(alist->addrs[0]) * (alist->nr_addrs - i));

	a = &alist->addrs[i].transport.sin6;
	a->sin6_port		  = xport;
	a->sin6_addr.s6_addr32[0] = xdr[0];
	a->sin6_addr.s6_addr32[1] = xdr[1];
	a->sin6_addr.s6_addr32[2] = xdr[2];
	a->sin6_addr.s6_addr32[3] = xdr[3];
	alist->nr_addrs++;
}

/*
 * Get an address to try.
 */
bool afs_iterate_addresses(struct afs_addr_cursor *ac)
{
	_enter("%hu+%hd", ac->start, (short)ac->index);

	if (!ac->alist)
		return false;

	if (ac->begun) {
		ac->index++;
		if (ac->index == ac->alist->nr_addrs)
			ac->index = 0;

		if (ac->index == ac->start) {
			ac->error = -EDESTADDRREQ;
			return false;
		}
	}

	ac->begun = true;
	ac->responded = false;
	ac->addr = &ac->alist->addrs[ac->index];
	return true;
}

/*
 * Release an address list cursor.
 */
int afs_end_cursor(struct afs_addr_cursor *ac)
{
	struct afs_addr_list *alist;

	alist = ac->alist;
	if (alist) {
		if (ac->responded && ac->index != ac->start)
			WRITE_ONCE(alist->index, ac->index);
		afs_put_addrlist(alist);
	}

	ac->addr = NULL;
	ac->alist = NULL;
	ac->begun = false;
	return ac->error;
}

/*
 * Set the address cursor for iterating over VL servers.
 */
int afs_set_vl_cursor(struct afs_addr_cursor *ac, struct afs_cell *cell)
{
	struct afs_addr_list *alist;
	int ret;

	if (!rcu_access_pointer(cell->vl_addrs)) {
		ret = wait_on_bit(&cell->flags, AFS_CELL_FL_NO_LOOKUP_YET,
				  TASK_INTERRUPTIBLE);
		if (ret < 0)
			return ret;

		if (!rcu_access_pointer(cell->vl_addrs) &&
		    ktime_get_real_seconds() < cell->dns_expiry)
			return cell->error;
	}

	read_lock(&cell->vl_addrs_lock);
	alist = rcu_dereference_protected(cell->vl_addrs,
					  lockdep_is_held(&cell->vl_addrs_lock));
	if (alist->nr_addrs > 0)
		afs_get_addrlist(alist);
	else
		alist = NULL;
	read_unlock(&cell->vl_addrs_lock);

	if (!alist)
		return -EDESTADDRREQ;

	ac->alist = alist;
	ac->addr = NULL;
	ac->start = READ_ONCE(alist->index);
	ac->index = ac->start;
	ac->error = 0;
	ac->begun = false;
	return 0;
}
Commit	Line	Data
8b2a464c DH	1	/* Server address list management
	2	*
	3	* Copyright (C) 2017 Red Hat, Inc. All Rights Reserved.
	4	* Written by David Howells (dhowells@redhat.com)
	5	*
	6	* This program is free software; you can redistribute it and/or
	7	* modify it under the terms of the GNU General Public Licence
	8	* as published by the Free Software Foundation; either version
	9	* 2 of the Licence, or (at your option) any later version.
	10	*/
	11
	12	#include <linux/slab.h>
	13	#include <linux/ctype.h>
	14	#include <linux/dns_resolver.h>
	15	#include <linux/inet.h>
	16	#include <keys/rxrpc-type.h>
	17	#include "internal.h"
	18	#include "afs_fs.h"
	19
bf99a53c DH	20	//#define AFS_MAX_ADDRESSES
	21	// ((unsigned int)((PAGE_SIZE - sizeof(struct afs_addr_list)) /
	22	// sizeof(struct sockaddr_rxrpc)))
	23	#define AFS_MAX_ADDRESSES ((unsigned int)(sizeof(unsigned long) * 8))
8b2a464c DH	24
	25	/*
	26	* Release an address list.
	27	*/
	28	void afs_put_addrlist(struct afs_addr_list *alist)
	29	{
	30	if (alist && refcount_dec_and_test(&alist->usage))
	31	call_rcu(&alist->rcu, (rcu_callback_t)kfree);
	32	}
	33
	34	/*
	35	* Allocate an address list.
	36	*/
	37	struct afs_addr_list *afs_alloc_addrlist(unsigned int nr,
	38	unsigned short service,
	39	unsigned short port)
	40	{
	41	struct afs_addr_list *alist;
	42	unsigned int i;
	43
	44	_enter("%u,%u,%u", nr, service, port);
	45
	46	alist = kzalloc(sizeof(alist) + sizeof(alist->addrs[0]) nr,
	47	GFP_KERNEL);
	48	if (!alist)
	49	return NULL;
	50
	51	refcount_set(&alist->usage, 1);
	52
	53	for (i = 0; i < nr; i++) {
	54	struct sockaddr_rxrpc *srx = &alist->addrs[i];
	55	srx->srx_family = AF_RXRPC;
	56	srx->srx_service = service;
	57	srx->transport_type = SOCK_DGRAM;
	58	srx->transport_len = sizeof(srx->transport.sin6);
	59	srx->transport.sin6.sin6_family = AF_INET6;
	60	srx->transport.sin6.sin6_port = htons(port);
	61	}
	62
	63	return alist;
	64	}
	65
	66	/*
	67	* Parse a text string consisting of delimited addresses.
	68	*/
	69	struct afs_addr_list afs_parse_text_addrs(const char text, size_t len,
	70	char delim,
	71	unsigned short service,
	72	unsigned short port)
	73	{
	74	struct afs_addr_list *alist;
	75	const char p, end = text + len;
	76	unsigned int nr = 0;
	77
	78	_enter("%.s,%c", (int)len, (int)len, text, delim);
	79
	80	if (!len)
	81	return ERR_PTR(-EDESTADDRREQ);
	82
	83	if (delim == ':' && (memchr(text, ',', len) \|\| !memchr(text, '.', len)))
	84	delim = ',';
	85
	86	/* Count the addresses */
	87	p = text;
88	do {
89	if (!*p)
90	return ERR_PTR(-EINVAL);
91	if (*p == delim)
92	continue;
93	nr++;
94	if (*p == '[') {
95	p++;
96	if (p == end)
97	return ERR_PTR(-EINVAL);
98	p = memchr(p, ']', end - p);
99	if (!p)
100	return ERR_PTR(-EINVAL);
101	p++;
102	if (p >= end)
103	break;
104	}
105
106	p = memchr(p, delim, end - p);
107	if (!p)
108	break;
109	p++;
110	} while (p < end);
111
112	_debug("%u/%u addresses", nr, AFS_MAX_ADDRESSES);
113	if (nr > AFS_MAX_ADDRESSES)
114	nr = AFS_MAX_ADDRESSES;
115
116	alist = afs_alloc_addrlist(nr, service, port);
117	if (!alist)
118	return ERR_PTR(-ENOMEM);
119
120	/* Extract the addresses */
121	p = text;
122	do {
123	struct sockaddr_rxrpc *srx = &alist->addrs[alist->nr_addrs];
9de0b1ed	124	const char q, stop;
8b2a464c DH	125
	126	if (*p == delim) {
	127	p++;
	128	continue;
	129	}
	130
	131	if (*p == '[') {
	132	p++;
9de0b1ed DH	133	q = memchr(p, ']', end - p);
	134	} else {
	135	for (q = p; q < end; q++)
	136	if (q == '+' \|\| q == delim)
	137	break;
8b2a464c DH	138	}
8b2a464c DH	139
9de0b1ed	140	if (in4_pton(p, q - p,
8b2a464c	141	(u8 *)&srx->transport.sin6.sin6_addr.s6_addr32[3],
9de0b1ed	142	-1, &stop)) {
8b2a464c DH	143	srx->transport.sin6.sin6_addr.s6_addr32[0] = 0;
	144	srx->transport.sin6.sin6_addr.s6_addr32[1] = 0;
	145	srx->transport.sin6.sin6_addr.s6_addr32[2] = htonl(0xffff);
9de0b1ed	146	} else if (in6_pton(p, q - p,
8b2a464c	147	srx->transport.sin6.sin6_addr.s6_addr,
9de0b1ed	148	-1, &stop)) {
8b2a464c DH	149	/* Nothing to do */
	150	} else {
	151	goto bad_address;
	152	}
	153
9de0b1ed DH	154	if (stop != q)
	155	goto bad_address;
	156
	157	p = q;
	158	if (q < end && *q == ']')
8b2a464c	159	p++;
8b2a464c DH	160
	161	if (p < end) {
	162	if (*p == '+') {
	163	/* Port number specification "+1234" */
	164	unsigned int xport = 0;
	165	p++;
	166	if (p >= end \|\| !isdigit(*p))
	167	goto bad_address;
	168	do {
	169	xport *= 10;
	170	xport += *p - '0';
	171	if (xport > 65535)
	172	goto bad_address;
	173	p++;
	174	} while (p < end && isdigit(*p));
	175	srx->transport.sin6.sin6_port = htons(xport);
	176	} else if (*p == delim) {
	177	p++;
	178	} else {
	179	goto bad_address;
	180	}
	181	}
	182
	183	alist->nr_addrs++;
	184	} while (p < end && alist->nr_addrs < AFS_MAX_ADDRESSES);
	185
	186	_leave(" = [nr %u]", alist->nr_addrs);
	187	return alist;
	188
	189	bad_address:
	190	kfree(alist);
	191	return ERR_PTR(-EINVAL);
	192	}
	193
	194	/*
	195	* Compare old and new address lists to see if there's been any change.
	196	* - How to do this in better than O(Nlog(N)) time?
	197	* - We don't really want to sort the address list, but would rather take the
	198	* list as we got it so as not to undo record rotation by the DNS server.
	199	*/
	200	#if 0
	201	static int afs_cmp_addr_list(const struct afs_addr_list *a1,
	202	const struct afs_addr_list *a2)
	203	{
	204	}
	205	#endif
	206
	207	/*
	208	* Perform a DNS query for VL servers and build a up an address list.
	209	*/
	210	struct afs_addr_list afs_dns_query(struct afs_cell cell, time64_t *_expiry)
	211	{
	212	struct afs_addr_list *alist;
	213	char *vllist = NULL;
	214	int ret;
	215
	216	_enter("%s", cell->name);
	217
	218	ret = dns_query("afsdb", cell->name, cell->name_len,
	219	"ipv4", &vllist, _expiry);
	220	if (ret < 0)
	221	return ERR_PTR(ret);
	222
	223	alist = afs_parse_text_addrs(vllist, strlen(vllist), ',',
224	VL_SERVICE, AFS_VL_PORT);
225	if (IS_ERR(alist)) {
226	kfree(vllist);
227	if (alist != ERR_PTR(-ENOMEM))
228	pr_err("Failed to parse DNS data\n");
229	return alist;
230	}
231
232	kfree(vllist);
233	return alist;
234	}
235
d2ddc776 DH	236	/*
	237	* Merge an IPv4 entry into a fileserver address list.
	238	*/
bf99a53c	239	void afs_merge_fs_addr4(struct afs_addr_list *alist, __be32 xdr, u16 port)
d2ddc776 DH	240	{
d2ddc776 DH	241	struct sockaddr_in6 *a;
bf99a53c	242	__be16 xport = htons(port);
d2ddc776 DH	243	int i;
	244
	245	for (i = 0; i < alist->nr_ipv4; i++) {
	246	a = &alist->addrs[i].transport.sin6;
bf99a53c DH	247	if (xdr == a->sin6_addr.s6_addr32[3] &&
bf99a53c DH	248	xport == a->sin6_port)
d2ddc776	249	return;
bf99a53c DH	250	if (xdr == a->sin6_addr.s6_addr32[3] &&
	251	xport < a->sin6_port)
	252	break;
d2ddc776 DH	253	if (xdr < a->sin6_addr.s6_addr32[3])
	254	break;
	255	}
	256
	257	if (i < alist->nr_addrs)
	258	memmove(alist->addrs + i + 1,
	259	alist->addrs + i,
	260	sizeof(alist->addrs[0]) * (alist->nr_addrs - i));
	261
	262	a = &alist->addrs[i].transport.sin6;
bf99a53c	263	a->sin6_port = xport;
d2ddc776 DH	264	a->sin6_addr.s6_addr32[0] = 0;
	265	a->sin6_addr.s6_addr32[1] = 0;
	266	a->sin6_addr.s6_addr32[2] = htonl(0xffff);
	267	a->sin6_addr.s6_addr32[3] = xdr;
	268	alist->nr_ipv4++;
	269	alist->nr_addrs++;
	270	}
	271
bf99a53c DH	272	/*
	273	* Merge an IPv6 entry into a fileserver address list.
	274	*/
	275	void afs_merge_fs_addr6(struct afs_addr_list alist, __be32 xdr, u16 port)
	276	{
	277	struct sockaddr_in6 *a;
	278	__be16 xport = htons(port);
	279	int i, diff;
	280
	281	for (i = alist->nr_ipv4; i < alist->nr_addrs; i++) {
	282	a = &alist->addrs[i].transport.sin6;
	283	diff = memcmp(xdr, &a->sin6_addr, 16);
	284	if (diff == 0 &&
	285	xport == a->sin6_port)
	286	return;
	287	if (diff == 0 &&
	288	xport < a->sin6_port)
	289	break;
	290	if (diff < 0)
	291	break;
	292	}
	293
	294	if (i < alist->nr_addrs)
	295	memmove(alist->addrs + i + 1,
	296	alist->addrs + i,
	297	sizeof(alist->addrs[0]) * (alist->nr_addrs - i));
	298
	299	a = &alist->addrs[i].transport.sin6;
	300	a->sin6_port = xport;
	301	a->sin6_addr.s6_addr32[0] = xdr[0];
	302	a->sin6_addr.s6_addr32[1] = xdr[1];
	303	a->sin6_addr.s6_addr32[2] = xdr[2];
	304	a->sin6_addr.s6_addr32[3] = xdr[3];
	305	alist->nr_addrs++;
	306	}
	307
8b2a464c DH	308	/*
	309	* Get an address to try.
	310	*/
	311	bool afs_iterate_addresses(struct afs_addr_cursor *ac)
	312	{
	313	_enter("%hu+%hd", ac->start, (short)ac->index);
	314
	315	if (!ac->alist)
	316	return false;
	317
	318	if (ac->begun) {
	319	ac->index++;
	320	if (ac->index == ac->alist->nr_addrs)
	321	ac->index = 0;
	322
	323	if (ac->index == ac->start) {
	324	ac->error = -EDESTADDRREQ;
	325	return false;
	326	}
	327	}
	328
	329	ac->begun = true;
	330	ac->responded = false;
	331	ac->addr = &ac->alist->addrs[ac->index];
	332	return true;
	333	}
	334
	335	/*
	336	* Release an address list cursor.
	337	*/
	338	int afs_end_cursor(struct afs_addr_cursor *ac)
	339	{
61b3c8ca DH	340	struct afs_addr_list *alist;
	341
	342	alist = ac->alist;
	343	if (alist) {
	344	if (ac->responded && ac->index != ac->start)
	345	WRITE_ONCE(alist->index, ac->index);
	346	afs_put_addrlist(alist);
	347	}
8b2a464c	348
61b3c8ca	349	ac->addr = NULL;
8b2a464c	350	ac->alist = NULL;
61b3c8ca	351	ac->begun = false;
8b2a464c DH	352	return ac->error;
	353	}
	354
	355	/*
	356	* Set the address cursor for iterating over VL servers.
	357	*/
	358	int afs_set_vl_cursor(struct afs_addr_cursor ac, struct afs_cell cell)
	359	{
	360	struct afs_addr_list *alist;
	361	int ret;
	362
	363	if (!rcu_access_pointer(cell->vl_addrs)) {
	364	ret = wait_on_bit(&cell->flags, AFS_CELL_FL_NO_LOOKUP_YET,
	365	TASK_INTERRUPTIBLE);
	366	if (ret < 0)
	367	return ret;
	368
	369	if (!rcu_access_pointer(cell->vl_addrs) &&
	370	ktime_get_real_seconds() < cell->dns_expiry)
	371	return cell->error;
	372	}
	373
	374	read_lock(&cell->vl_addrs_lock);
	375	alist = rcu_dereference_protected(cell->vl_addrs,
	376	lockdep_is_held(&cell->vl_addrs_lock));
	377	if (alist->nr_addrs > 0)
	378	afs_get_addrlist(alist);
	379	else
	380	alist = NULL;
	381	read_unlock(&cell->vl_addrs_lock);
	382
	383	if (!alist)
	384	return -EDESTADDRREQ;
	385
	386	ac->alist = alist;
	387	ac->addr = NULL;
	388	ac->start = READ_ONCE(alist->index);
	389	ac->index = ac->start;
	390	ac->error = 0;
	391	ac->begun = false;
	392	return 0;
	393	}