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Diffstat (limited to 'src/n-acd.c')
| -rw-r--r-- | src/n-acd.c | 1213 |
1 files changed, 1213 insertions, 0 deletions
diff --git a/src/n-acd.c b/src/n-acd.c new file mode 100644 index 0000000000..4f8023e896 --- /dev/null +++ b/src/n-acd.c @@ -0,0 +1,1213 @@ +/* + * IPv4 Address Conflict Detection + * + * This implements the main n-acd API. It is built around an epoll-fd to + * encapsulate a timerfd+socket. The n-acd context has quite straightforward + * lifetime rules. The parameters must be set when the engine is started, and + * they can only be changed by stopping and restartding the engine. The engine + * is started on demand and stopped when no longer needed. + * During the entire lifetime the context can be dispatched. That is, the + * dispatcher does not have to be aware of the context state. After each call + * to dispatch(), the caller must pop all pending events until -EAGAIN is + * returned. + * + * If a conflict is detected, the ACD engine reports to the caller and stops + * the engine. The caller can now modify parameters and restart the engine, if + * required. + */ + +#include <assert.h> +#include <c-list.h> +#include <c-siphash.h> +#include <endian.h> +#include <errno.h> +#include <limits.h> +#include <linux/filter.h> +#include <linux/if_ether.h> +#include <linux/if_packet.h> +#include <net/ethernet.h> +#include <netinet/if_ether.h> +#include <netinet/in.h> +#include <stddef.h> +#include <stdio.h> +#include <stdlib.h> +#include <string.h> +#include <sys/auxv.h> +#include <sys/epoll.h> +#include <sys/socket.h> +#include <sys/timerfd.h> +#include <sys/types.h> +#include <unistd.h> +#include "n-acd.h" + +#define _public_ __attribute__((__visibility__("default"))) + +/* + * These parameters and timing intervals specified in RFC-5227. The original + * values are: + * + * PROBE_NUM 3 + * PROBE_WAIT 1s + * PROBE_MIN 1s + * PROBE_MAX 3s + * ANNOUNCE_NUM 3 + * ANNOUNCE_WAIT 2s + * ANNOUNCE_INTERVAL 2s + * MAX_CONFLICTS 10 + * RATE_LIMIT_INTERVAL 60s + * DEFEND_INTERVAL 10s + * + * If we assume a best-case and worst-case scenario for non-conflicted runs, we + * end up with a runtime between 4s and 9s to finish the probe. Then it still + * takes a fixed 4s to finish the announcements. + * + * RFC 5227 section 1.1: + * [...] (Note that the values listed here are fixed constants; they are + * not intended to be modifiable by implementers, operators, or end users. + * These constants are given symbolic names here to facilitate the writing + * of future standards that may want to reference this document with + * different values for these named constants; however, at the present time + * no such future standards exist.) [...] + * + * Unfortunately, no-one ever stepped up to write a "future standard" to revise + * the timings. A 9s timeout for successful link setups is not acceptable today. + * Hence, we will just go forward and ignore the proposed values. On both + * wired and wireless local links round-trip latencies of below 3ms are common, + * while latencies above 10ms are rarely seen. We require the caller to set a + * timeout multiplier, where 1 corresponds to a total probe time of 0.5 ms and + * 1.0 ms. On modern networks a multiplier of about 100 should be a reasonable + * default. To comply with the RFC select a multiplier of 9000. + */ +#define N_ACD_RFC_PROBE_NUM (3) +#define N_ACD_RFC_PROBE_WAIT_USEC (UINT64_C(111)) /* 111us */ +#define N_ACD_RFC_PROBE_MIN_USEC (UINT64_C(111)) /* 111us */ +#define N_ACD_RFC_PROBE_MAX_USEC (UINT64_C(333)) /* 333us */ +#define N_ACD_RFC_ANNOUNCE_NUM (3) +#define N_ACD_RFC_ANNOUNCE_WAIT_USEC (UINT64_C(222)) /* 222us */ +#define N_ACD_RFC_ANNOUNCE_INTERVAL_USEC (UINT64_C(222)) /* 222us */ +#define N_ACD_RFC_MAX_CONFLICTS (10) +#define N_ACD_RFC_RATE_LIMIT_INTERVAL_USEC (UINT64_C(60000000)) /* 60s */ +#define N_ACD_RFC_DEFEND_INTERVAL_USEC (UINT64_C(10000000)) /* 10s */ + +/* + * Fake ENETDOWN error-code. We use this as replacement for known EFOOBAR error + * codes. It is explicitly chosen to be outside the known error-code range. + * Whenever we are deep down in a call-stack and notice a ENETDOWN error, we + * return this instead. It is caught by the top-level dispatcher and then + * properly handled. + * This avoids gracefully handling ENETDOWN in call-stacks, but then continuing + * with some work in the callers without noticing the soft failure. + */ +#define N_ACD_E_DOWN (INT_MAX) + +#define TIME_INFINITY ((uint64_t) -1) + +enum { + N_ACD_EPOLL_TIMER, + N_ACD_EPOLL_SOCKET, +}; + +enum { + N_ACD_STATE_INIT, + N_ACD_STATE_PROBING, + N_ACD_STATE_CONFIGURING, + N_ACD_STATE_ANNOUNCING, +}; + +typedef struct NAcdEventNode { + NAcdEvent event; + uint8_t sender[ETH_ALEN]; + CList link; +} NAcdEventNode; + +struct NAcd { + /* context */ + unsigned int seed; + int fd_epoll; + int fd_timer; + + /* configuration */ + NAcdConfig config; + uint8_t mac[ETH_ALEN]; + uint64_t timeout_multiplier; + + /* runtime */ + int fd_socket; + unsigned int state; + unsigned int n_iteration; + unsigned int n_conflicts; + unsigned int defend; + uint64_t last_defend; + uint64_t last_conflict; + + /* pending events */ + CList events; + NAcdEventNode *current; +}; + +static int n_acd_errno(void) { + /* + * Compilers continuously warn about uninitialized variables since they + * cannot deduce that `return -errno;` will always be negative. This + * small wrapper makes sure compilers figure that out. Use it as + * replacement for `errno` read access. Yes, it generates worse code, + * but only marginally and only affects slow-paths. + */ + return abs(errno) ? : EIO; +} + +static int n_acd_event_node_new(NAcdEventNode **nodep, unsigned int event) { + NAcdEventNode *node; + + node = calloc(1, sizeof(*node)); + if (!node) + return -ENOMEM; + + node->event.event = event; + node->link = (CList)C_LIST_INIT(node->link); + + *nodep = node; + + return 0; +} + +static NAcdEventNode *n_acd_event_node_free(NAcdEventNode *node) { + if (!node) + return NULL; + + c_list_unlink(&node->link); + free(node); + + return NULL; +} + +static int n_acd_get_random(unsigned int *random) { + uint8_t hash_seed[] = { 0x3a, 0x0c, 0xa6, 0xdd, 0x44, 0xef, 0x5f, 0x7a, 0x5e, 0xd7, 0x25, 0x37, 0xbf, 0x4e, 0x80, 0xa1 }; + CSipHash hash = C_SIPHASH_NULL; + struct timespec ts; + const uint8_t *p; + int r; + + /* + * We need random jitter for all timeouts when handling ARP probes. Use + * AT_RANDOM to get a seed for rand_r(3p), if available (should always + * be available on linux). See the time-out scheduler for details. + * Additionally, we include the current time in the seed. This avoids + * using the same jitter in case you run multiple ACD engines in the + * same process. Lastly, the seed is hashed with SipHash24 to avoid + * exposing the value of AT_RANDOM on the network. + */ + c_siphash_init(&hash, hash_seed); + + p = (const uint8_t *)getauxval(AT_RANDOM); + if (p) + c_siphash_append(&hash, p, 16); + + r = clock_gettime(CLOCK_BOOTTIME, &ts); + if (r < 0) + return -n_acd_errno(); + + c_siphash_append(&hash, (const uint8_t *)&ts.tv_sec, sizeof(ts.tv_sec)); + c_siphash_append(&hash, (const uint8_t *)&ts.tv_nsec, sizeof(ts.tv_nsec)); + + *random = c_siphash_finalize(&hash); + return 0; +} + +/** + * n_acd_new() - create a new ACD context + * @acdp: output argument for context + * + * Create a new ACD context and return it in @acdp. + * + * Return: 0 on success, or a negative error code on failure. + */ +_public_ int n_acd_new(NAcd **acdp) { + NAcd *acd; + int r; + + acd = calloc(1, sizeof(*acd)); + if (!acd) + return -ENOMEM; + + acd->fd_epoll = -1; + acd->fd_timer = -1; + acd->fd_socket = -1; + acd->state = N_ACD_STATE_INIT; + acd->defend = N_ACD_DEFEND_NEVER; + acd->events = (CList)C_LIST_INIT(acd->events); + acd->last_conflict = TIME_INFINITY; + + r = n_acd_get_random(&acd->seed); + if (r < 0) + return r; + + acd->fd_epoll = epoll_create1(EPOLL_CLOEXEC); + if (acd->fd_epoll < 0) { + r = -n_acd_errno(); + goto error; + } + + acd->fd_timer = timerfd_create(CLOCK_BOOTTIME, TFD_CLOEXEC | TFD_NONBLOCK); + if (acd->fd_timer < 0) { + r = -n_acd_errno(); + goto error; + } + + r = epoll_ctl(acd->fd_epoll, EPOLL_CTL_ADD, acd->fd_timer, + &(struct epoll_event){ + .events = EPOLLIN, + .data.u32 = N_ACD_EPOLL_TIMER, + }); + if (r < 0) { + r = -n_acd_errno(); + goto error; + } + + *acdp = acd; + return 0; + +error: + n_acd_free(acd); + return r; +} + +/** + * n_acd_free() - free an ACD context + * + * Frees all resources held by the context. This may be called at any time, + * but doing so invalidates all data owned by the context. + * + * Return: NULL. + */ +_public_ NAcd *n_acd_free(NAcd *acd) { + NAcdEventNode *node; + + if (!acd) + return NULL; + + n_acd_stop(acd); + + while ((node = c_list_first_entry(&acd->events, NAcdEventNode, link))) + n_acd_event_node_free(node); + + assert(acd->fd_socket < 0); + + if (acd->fd_timer >= 0) { + assert(acd->fd_epoll >= 0); + epoll_ctl(acd->fd_epoll, EPOLL_CTL_DEL, acd->fd_timer, NULL); + close(acd->fd_timer); + acd->fd_timer = -1; + } + + if (acd->fd_epoll >= 0) { + close(acd->fd_epoll); + acd->fd_epoll = -1; + } + + free(acd); + + return NULL; +} + +/** + * n_acd_get_fd() - get pollable file descriptor + * @acd: ACD context + * @fdp: output argument for file descriptor + * + * Returns a file descriptor in @fdp. This filedescriptor can be polled by + * the caller to indicate when the ACD context can be dispatched. + */ +_public_ void n_acd_get_fd(NAcd *acd, int *fdp) { + *fdp = acd->fd_epoll; +} + +static int n_acd_push_event(NAcd *acd, unsigned int event, uint16_t *operation, uint8_t (*sender)[6], uint8_t (*target)[4]) { + NAcdEventNode *node; + int r; + + r = n_acd_event_node_new(&node, event); + if (r < 0) + return r; + + switch (event) { + case N_ACD_EVENT_USED: + node->event.used.operation = be16toh(*operation); + memcpy(node->sender, sender, sizeof(node->sender)); + node->event.used.sender = node->sender; + node->event.used.n_sender = sizeof(node->sender); + memcpy(&node->event.used.target, target, sizeof(node->event.used.target)); + break; + case N_ACD_EVENT_CONFLICT: + node->event.conflict.operation = be16toh(*operation); + memcpy(node->sender, sender, sizeof(node->sender)); + node->event.used.sender = node->sender; + node->event.used.n_sender = sizeof(node->sender); + memcpy(&node->event.conflict.target, target, sizeof(node->event.conflict.target)); + break; + case N_ACD_EVENT_DEFENDED: + node->event.defended.operation = be16toh(*operation); + memcpy(node->sender, sender, sizeof(node->sender)); + node->event.used.sender = node->sender; + node->event.used.n_sender = sizeof(node->sender); + memcpy(&node->event.defended.target, target, sizeof(node->event.defended.target)); + break; + case N_ACD_EVENT_READY: + case N_ACD_EVENT_DOWN: + break; + default: + assert(0); + } + + c_list_link_tail(&acd->events, &node->link); + + return 0; +} + +static int n_acd_now(uint64_t *nowp) { + struct timespec ts; + int r; + + r = clock_gettime(CLOCK_BOOTTIME, &ts); + if (r < 0) + return -n_acd_errno(); + + *nowp = ts.tv_sec * UINT64_C(1000000) + ts.tv_nsec / UINT64_C(1000); + return 0; +} + +static int n_acd_schedule(NAcd *acd, uint64_t u_timeout, unsigned int u_jitter) { + uint64_t u_next = u_timeout; + int r; + + /* + * ACD specifies jitter values to reduce packet storms on the local + * link. This call accepts the maximum relative jitter value in + * microseconds as @u_jitter. We then use rand_r(3p) to get a + * pseudo-random jitter on top of the real timeout given as @u_timeout. + * Note that rand_r() is fine for this. Before you try to improve the + * RNG, you better spend some time securing ARP. + */ + if (u_jitter) + u_next += rand_r(&acd->seed) % u_jitter; + + /* + * Setting .it_value to 0 in timerfd_settime() disarms the timer. Avoid + * this and always schedule at least 1us. Otherwise, we'd have to + * recursively call into the time-out handler, which we really want to + * avoid. No reason to optimize performance here. + */ + if (!u_next) + u_next = 1; + + r = timerfd_settime(acd->fd_timer, 0, + &(struct itimerspec){ .it_value = { + .tv_sec = u_next / UINT64_C(1000000), + .tv_nsec = u_next % UINT64_C(1000000) * UINT64_C(1000), + } }, NULL); + if (r < 0) + return -n_acd_errno(); + + return 0; +} + +static int n_acd_send(NAcd *acd, const struct in_addr *spa) { + struct sockaddr_ll address = { + .sll_family = AF_PACKET, + .sll_protocol = htobe16(ETH_P_ARP), + .sll_ifindex = acd->config.ifindex, + .sll_halen = ETH_ALEN, + .sll_addr = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }, + }; + struct ether_arp arp = { + .ea_hdr.ar_hrd = htobe16(ARPHRD_ETHER), + .ea_hdr.ar_pro = htobe16(ETHERTYPE_IP), + .ea_hdr.ar_hln = sizeof(acd->mac), + .ea_hdr.ar_pln = sizeof(uint32_t), + .ea_hdr.ar_op = htobe16(ARPOP_REQUEST), + }; + ssize_t l; + + memcpy(arp.arp_sha, acd->mac, sizeof(acd->mac)); + memcpy(arp.arp_tpa, &acd->config.ip.s_addr, sizeof(uint32_t)); + + if (spa) + memcpy(arp.arp_spa, &spa->s_addr, sizeof(spa->s_addr)); + + l = sendto(acd->fd_socket, &arp, sizeof(arp), MSG_NOSIGNAL, (struct sockaddr *)&address, sizeof(address)); + if (l == (ssize_t)sizeof(arp)) { + /* Packet was properly sent. */ + return 0; + } else if (l >= 0) { + /* + * Ugh. The packet was truncated. This should not happen, but + * lets just pretend the packet was dropped. + */ + return 0; + } else if (errno == EAGAIN || errno == ENOBUFS) { + /* + * In case the output buffer is full, the packet is silently + * dropped. This is just as if the physical layer happened to + * drop the packet. We are not on a reliable medium, so no + * reason to pretend we are. + */ + return 0; + } else if (errno == ENETDOWN || errno == ENXIO) { + /* + * We get ENETDOWN if the network-device goes down or is + * removed. ENXIO might happen on async send-operations if the + * network-device was unplugged and thus the kernel is no + * longer aware of it. + * In any case, we do not allow proceeding with this socket. We + * stop the engine and notify the user gracefully. + */ + return -N_ACD_E_DOWN; + } + + return -n_acd_errno(); +} + +static void n_acd_remember_conflict(NAcd *acd, uint64_t now) { + if (++acd->n_conflicts >= N_ACD_RFC_MAX_CONFLICTS) { + acd->n_conflicts = N_ACD_RFC_MAX_CONFLICTS; + acd->last_conflict = now; + } +} + +static int n_acd_handle_timeout(NAcd *acd) { + int r; + + switch (acd->state) { + case N_ACD_STATE_PROBING: + /* + * We are still PROBING. We send 3 probes with a random timeout + * scheduled between each. If, after a fixed timeout, we did + * not receive any conflict we consider the probing successful. + */ + if (acd->n_iteration >= N_ACD_RFC_PROBE_NUM) { + /* + * All 3 probes succeeded and we waited enough to + * consider this address usable by now. Do not announce + * the address, yet. We must first give the caller a + * chance to configure the address (so they can answer + * ARP requests), before announcing it. But our + * callbacks are not necessarily synchronous (we want + * to allow IPC there), so just notify the caller and + * wait for further instructions, thus effectively + * increasing the probe-wait. + */ + r = n_acd_push_event(acd, N_ACD_EVENT_READY, NULL, NULL, NULL); + if (r) + return r; + + acd->state = N_ACD_STATE_CONFIGURING; + } else { + /* + * We have not sent all 3 probes, yet. A timer fired, + * so we are ready to send the next probe. If this is + * the third probe, schedule a timer for ANNOUNCE_WAIT + * to give other peers a chance to answer. If this is + * not the third probe, wait between PROBE_MIN and + * PROBE_MAX for the next probe. + */ + + r = n_acd_send(acd, NULL); + if (r < 0) + return r; + + if (++acd->n_iteration >= N_ACD_RFC_PROBE_NUM) + r = n_acd_schedule(acd, acd->timeout_multiplier * N_ACD_RFC_ANNOUNCE_WAIT_USEC, 0); + else + r = n_acd_schedule(acd, acd->timeout_multiplier * N_ACD_RFC_PROBE_MIN_USEC, + acd->timeout_multiplier * (N_ACD_RFC_PROBE_MAX_USEC - N_ACD_RFC_PROBE_MIN_USEC)); + if (r < 0) + return r; + } + + break; + + case N_ACD_STATE_ANNOUNCING: + /* + * We are ANNOUNCING, meaning the caller configured the address + * on the interface and is actively using it. We send 3 + * announcements out, in a short interval, and then just + * perform passive conflict detection. + * Note that once all 3 announcements are sent, we no longer + * schedule a timer, so this part should not trigger, anymore. + */ + + r = n_acd_send(acd, &acd->config.ip); + if (r < 0) + return r; + + if (++acd->n_iteration < N_ACD_RFC_ANNOUNCE_NUM) { + r = n_acd_schedule(acd, acd->timeout_multiplier * N_ACD_RFC_ANNOUNCE_INTERVAL_USEC, 0); + if (r < 0) + return r; + } + + break; + + case N_ACD_STATE_INIT: + case N_ACD_STATE_CONFIGURING: + default: + /* + * There are no timeouts in these states. If we trigger one, + * something is fishy. Let the caller deal with this. + */ + return -EIO; + } + + return 0; +} + +static int n_acd_handle_packet(NAcd *acd, struct ether_arp *packet) { + bool hard_conflict; + uint64_t now; + int r; + + /* + * Via BPF we discard any non-conflict packets. There are only 2 types + * that can pass: A conflict on the Sender Protocol Address, or a + * conflict on the Target Protocol Address. + * + * The former we call a hard-conflict. It implies that the sender uses + * the address already. We must always catch this and in some way react + * to it. Any kind, REQUEST or REPLY must be caught (though it is + * unlikely that we ever catch REPLIES since they tend to be unicasts). + * + * However, in case the Target Protocol Address matches, we just know + * that somebody is looking for the address. Hence, we must also check + * that the packet is an ARP-Probe (Sender Protocol Address is 0). If + * it is, it means someone else does ACD on our address. We call this a + * soft conflict. + */ + if (!memcmp(packet->arp_spa, (uint8_t[4]){ }, sizeof(packet->arp_spa)) && + !memcmp(packet->arp_tpa, &acd->config.ip.s_addr, sizeof(packet->arp_tpa)) && + packet->ea_hdr.ar_op == htobe16(ARPOP_REQUEST)) { + hard_conflict = false; + } else if (!memcmp(packet->arp_spa, &acd->config.ip.s_addr, sizeof(packet->arp_spa))) { + hard_conflict = true; + } else { + /* + * Ignore anything that is specific enough to match the BPF + * filter, but is none of the conflicts described above. + */ + return 0; + } + + r = n_acd_now(&now); + if (r < 0) + return r; + + switch (acd->state) { + case N_ACD_STATE_PROBING: + /* + * Regardless whether this is a hard or soft conflict, we must + * treat this as a probe failure. That is, notify the caller of + * the conflict and wait for further instructions. We do not + * react to this, until the caller tells us what to do. But we + * immediately stop the engine, since there is no point in + * continuing the probing. + */ + n_acd_remember_conflict(acd, now); + n_acd_stop(acd); + r = n_acd_push_event(acd, N_ACD_EVENT_USED, &packet->ea_hdr.ar_op, &packet->arp_sha, &packet->arp_tpa); + if (r) + return r; + + break; + + case N_ACD_STATE_CONFIGURING: + /* + * We are waiting for the caller to configure the interface and + * start ANNOUNCING. In this state, we cannot defend the address + * as that would indicate that it is ready to be used, and we + * cannot signal CONFLICT or USED as the caller may already have + * started to use the address (and may have configured the engine + * to always defend it, which means they should be able to rely on + * never losing it after READY). Simply drop the event, and rely + * on the anticipated ANNOUNCE to trigger it again. + */ + + break; + + case N_ACD_STATE_ANNOUNCING: + /* + * We were already instructed to announce the address, which + * means the address is configured and in use. Hence, the + * caller is responsible to serve regular ARP queries. Meaning, + * we can ignore any soft conflicts (other peers doing ACD). + * + * But if we see a hard-conflict, we either defend the address + * according to the caller's instructions, or we report the + * conflict and bail out. + */ + + if (!hard_conflict) + break; + + if (acd->defend == N_ACD_DEFEND_NEVER) { + n_acd_remember_conflict(acd, now); + n_acd_stop(acd); + r = n_acd_push_event(acd, N_ACD_EVENT_CONFLICT, &packet->ea_hdr.ar_op, &packet->arp_sha, &packet->arp_tpa); + if (r) + return r; + } else { + if (now > acd->last_defend + N_ACD_RFC_DEFEND_INTERVAL_USEC) { + r = n_acd_send(acd, &acd->config.ip); + if (r < 0) + return r; + + acd->last_defend = now; + r = n_acd_push_event(acd, N_ACD_EVENT_DEFENDED, &packet->ea_hdr.ar_op, &packet->arp_sha, &packet->arp_tpa); + if (r) + return r; + } else if (acd->defend == N_ACD_DEFEND_ONCE) { + n_acd_remember_conflict(acd, now); + n_acd_stop(acd); + r = n_acd_push_event(acd, N_ACD_EVENT_CONFLICT, &packet->ea_hdr.ar_op, &packet->arp_sha, &packet->arp_tpa); + if (r) + return r; + } else { + r = n_acd_push_event(acd, N_ACD_EVENT_DEFENDED, &packet->ea_hdr.ar_op, &packet->arp_sha, &packet->arp_tpa); + if (r) + return r; + } + } + + break; + + case N_ACD_STATE_INIT: + default: + /* + * The socket should not be dispatched in those states, since + * it is neither allocated nor added to epoll. Fail hard if we + * trigger this somehow. + */ + return -EIO; + } + + return 0; +} + +static int n_acd_dispatch_timer(NAcd *acd, struct epoll_event *event) { + uint64_t v; + int r; + + if (event->events & (EPOLLHUP | EPOLLERR)) { + /* + * There is no way to handle either gracefully. If we ignored + * them, we would busy-loop, so lets rather forward the error + * to the caller. + */ + return -EIO; + } + + if (event->events & EPOLLIN) { + for (unsigned int i = 0; i < 128; ++i) { + r = read(acd->fd_timer, &v, sizeof(v)); + if (r == sizeof(v)) { + /* + * We successfully read a timer-value. Handle it and + * return. We do NOT fall-through to EPOLLHUP handling, + * as we always must drain buffers first. + */ + return n_acd_handle_timeout(acd); + } else if (r >= 0) { + /* + * Kernel guarantees 8-byte reads; fail hard if it + * suddenly starts doing weird shit. No clue what to do + * with those values, anyway. + */ + return -EIO; + } else if (errno == EAGAIN) { + /* + * No more pending events. + */ + return 0; + } else { + /* + * Something failed. We use CLOCK_BOOTTIME, so + * ECANCELED cannot happen. Hence, there is no error + * that we could gracefully handle. Fail hard and let + * the caller deal with it. + */ + return -n_acd_errno(); + } + } + + return N_ACD_E_PREEMPTED; + } + + return 0; +} + +static int n_acd_dispatch_socket(NAcd *acd, struct epoll_event *event) { + struct ether_arp packet; + ssize_t l; + + for (unsigned int i = 0; i < 128; ++i) { + /* + * Regardless whether EPOLLIN is set in @event->events, we always + * invoke recv(2). This is a safety-net for sockets, which always fetch + * queued errors on all syscalls. That means, if anything failed on the + * socket, we will be notified via recv(2). This simplifies the code + * and avoid magic EPOLLIN/ERR/HUP juggling. + * + * Note that we must use recv(2) over read(2), since the latter cannot + * deal with empty packets properly. + */ + l = recv(acd->fd_socket, &packet, sizeof(packet), MSG_TRUNC); + if (l == (ssize_t)sizeof(packet)) { + /* + * We read a full ARP packet. We never fall-through to EPOLLHUP + * handling, as we always must drain buffers first. + */ + return n_acd_handle_packet(acd, &packet); + } else if (l >= 0) { + /* + * The BPF filter discards wrong packets, so error out + * if something slips through for any reason. Don't silently + * ignore it, since we explicitly want to know if something + * went fishy. + */ + return -EIO; + } else if (errno == ENETDOWN || errno == ENXIO) { + /* + * We get ENETDOWN if the network-device goes down or is + * removed. ENXIO might happen on async send-operations if the + * network-device was unplugged and thus the kernel is no + * longer aware of it. + * In any case, we do not allow proceeding with this socket. We + * stop the engine and notify the user gracefully. + */ + return -N_ACD_E_DOWN; + } else if (errno == EAGAIN) { + /* + * We cannot read data from the socket (we got EAGAIN). As a safety net + * check for EPOLLHUP/ERR. Those cannot be disabled with epoll, so we + * must make sure to not busy-loop by ignoring them. Note that we know + * recv(2) on sockets to return an error if either of these epoll-flags + * is set. Hence, if we did not handle it above, we have no other way + * but treating those flags as fatal errors and returning them to the + * caller. + */ + if (event->events & (EPOLLHUP | EPOLLERR)) + return -EIO; + + return 0; + } else { + /* + * Cannot dispatch the packet. This might be due to OOM, HUP, + * or something else. We cannot handle it gracefully so forward + * to the caller. + */ + return -n_acd_errno(); + } + } + + return N_ACD_E_PREEMPTED; +} + +/** + * n_acd_dispatch() - dispatch ACD context + * @acd: ACD context + * + * Return: 0 on successful dispatch of all pending events, N_ACD_E_PREEMPT in + * case there are more still more events to be dispatched, or a + * negative error code on failure. + */ +_public_ int n_acd_dispatch(NAcd *acd) { + struct epoll_event events[2]; + int n, i, r = 0; + bool preempted = false; + + n = epoll_wait(acd->fd_epoll, events, sizeof(events) / sizeof(*events), 0); + if (n < 0) { + return -n_acd_errno(); + } + + for (i = 0; i < n; ++i) { + switch (events[i].data.u32) { + case N_ACD_EPOLL_TIMER: + r = n_acd_dispatch_timer(acd, events + i); + break; + case N_ACD_EPOLL_SOCKET: + r = n_acd_dispatch_socket(acd, events + i); + break; + default: + r = 0; + break; + } + + if (r == N_ACD_E_PREEMPTED) + preempted = true; + else if (r != 0) + break; + } + + if (r == -N_ACD_E_DOWN) { + /* + * N_ACD_E_DOWN is synthesized whenever we notice + * ENETDOWN-related errors on the network interface. This + * allows bailing out of deep call-paths and then handling the + * error gracefully here. + */ + n_acd_stop(acd); + r = n_acd_push_event(acd, N_ACD_EVENT_DOWN, NULL, NULL, NULL); + if (r) + return r; + + return 0; + } + + if (preempted) + return N_ACD_E_PREEMPTED; + else + return r; +} + +/** + * n_acd_pop_event() - get the next pending event + * @acd: ACD context + * @eventp: output argument for the event + * + * Returns a pointer to the next pending event. The event is still owend by + * the context, and is only valid until the next call to n_acd_pop_event() + * or until the context is freed. + * + * The possible events are: + * * N_ACD_EVENT_READY: The configured IP address was probed successfully + * and is ready to be used. Once configured on the + * interface, the caller must call n_acd_announce() + * to announce and start defending the address. + * No further events may be received before + * n_acd_announce() has been called. + * * N_ACD_EVENT_USED: Someone is already using the IP address being + * probed. The engine was stopped, and the caller + * may restart it to try again. + * * N_ACD_EVENT_DEFENDED: A conflict was detected for the announced IP + * address, and the engine attempted to defend it. + * This is purely informational, and no action is + * required by the caller. + * * N_ACD_EVENT_CONFLICT: A conflict was detected for the announced IP + * address, and the engine was not able to defend + * it (according to the configured policy). The + * engine has stoppde, the caller must stop using + * the address immediately, and may restart the + * engine to retry. + * * N_ACD_EVENT_DOWN: A network error was detected. The engine was + * stopped and it is the responsibility of the + * caller to restart it once the network may be + * functional again. + * + * Returns: 0 on success, N_ACD_E_STOPPED if there are no more events and + * the engine has been stopped, N_ACD_E_DONE if there are no more + * events, but the engine is still running, or a negative error + * code on failure. + */ +_public_ int n_acd_pop_event(NAcd *acd, NAcdEvent **eventp) { + acd->current = n_acd_event_node_free(acd->current); + + if (c_list_is_empty(&acd->events)) { + if (acd->state == N_ACD_STATE_INIT) + return N_ACD_E_STOPPED; + else + return N_ACD_E_DONE; + } + + acd->current = c_list_first_entry(&acd->events, NAcdEventNode, link); + c_list_unlink(&acd->current->link); + + if (eventp) + *eventp = &acd->current->event; + + return 0; +} + +static int n_acd_bind_socket(NAcd *acd, int s) { + /* + * Due to strict aliasing, we cannot get uint32_t/uint16_t pointers to + * acd->config.mac, so provide a union accessor. + */ + const union { + uint8_t u8[6]; + uint16_t u16[3]; + uint32_t u32[1]; + } mac = { + .u8 = { + acd->mac[0], + acd->mac[1], + acd->mac[2], + acd->mac[3], + acd->mac[4], + acd->mac[5], + }, + }; + struct sock_filter filter[] = { + /* + * Basic ARP header validation. Make sure the packet-length, + * wire type, protocol type, and address lengths are correct. + */ + BPF_STMT(BPF_LD + BPF_W + BPF_LEN, 0), /* A <- packet length */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, sizeof(struct ether_arp), 1, 0), /* packet == arp packet ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(struct ether_arp, ea_hdr.ar_hrd)), /* A <- header */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ARPHRD_ETHER, 1, 0), /* header == ethernet ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(struct ether_arp, ea_hdr.ar_pro)), /* A <- protocol */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IP, 1, 0), /* protocol == IP ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + BPF_STMT(BPF_LD + BPF_B + BPF_ABS, offsetof(struct ether_arp, ea_hdr.ar_hln)), /* A <- hardware address length */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, sizeof(struct ether_addr), 1, 0), /* length == sizeof(ether_addr)? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + BPF_STMT(BPF_LD + BPF_B + BPF_ABS, offsetof(struct ether_arp, ea_hdr.ar_pln)), /* A <- protocol address length */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, sizeof(struct in_addr), 1, 0), /* length == sizeof(in_addr) ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(struct ether_arp, ea_hdr.ar_op)), /* A <- operation */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ARPOP_REQUEST, 2, 0), /* protocol == request ? */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ARPOP_REPLY, 1, 0), /* protocol == reply ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + + /* + * Sender hardware address must be different from ours. Note + * that BPF runs in big-endian mode, but assumes immediates are + * given in native-endian. This might look weird on 6-byte mac + * addresses, but is needed to revert the BPF magic. + */ + BPF_STMT(BPF_LD + BPF_IMM, be32toh(mac.u32[0])), /* A <- 4 bytes of client's MAC */ + BPF_STMT(BPF_MISC + BPF_TAX, 0), /* X <- A */ + BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(struct ether_arp, arp_sha)), /* A <- 4 bytes of SHA */ + BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0), /* A xor X */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 6), /* A == 0 ? */ + BPF_STMT(BPF_LD + BPF_IMM, be16toh(mac.u16[2])), /* A <- remainder of client's MAC */ + BPF_STMT(BPF_MISC + BPF_TAX, 0), /* X <- A */ + BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(struct ether_arp, arp_sha) + 4), /* A <- remainder of SHA */ + BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0), /* A xor X */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 1), /* A == 0 ? */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + + /* + * Sender protocol address or target protocol address must be + * equal to the one we care about. Again, immediates must be + * given in native-endian. + */ + BPF_STMT(BPF_LD + BPF_IMM, be32toh(acd->config.ip.s_addr)), /* A <- clients IP */ + BPF_STMT(BPF_MISC + BPF_TAX, 0), /* X <- A */ + BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(struct ether_arp, arp_spa)), /* A <- SPA */ + BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0), /* X xor A */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 1), /* A == 0 ? */ + BPF_STMT(BPF_RET + BPF_K, 65535), /* return all */ + BPF_STMT(BPF_LD + BPF_IMM, be32toh(acd->config.ip.s_addr)), /* A <- clients IP */ + BPF_STMT(BPF_MISC + BPF_TAX, 0), /* X <- A */ + BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(struct ether_arp, arp_tpa)), /* A <- TPA */ + BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0), /* X xor A */ + BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 0, 1), /* A == 0 ? */ + BPF_STMT(BPF_RET + BPF_K, 65535), /* return all */ + BPF_STMT(BPF_RET + BPF_K, 0), /* ignore */ + }; + const struct sock_fprog fprog = { + .len = sizeof(filter) / sizeof(*filter), + .filter = filter, + }; + const struct sockaddr_ll address = { + .sll_family = AF_PACKET, + .sll_protocol = htobe16(ETH_P_ARP), + .sll_ifindex = acd->config.ifindex, + .sll_halen = ETH_ALEN, + .sll_addr = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }, + }; + int r; + + /* + * Install a packet filter that matches on the ARP header and + * addresses, to reduce the number of wake-ups to a minimum. + */ + r = setsockopt(s, SOL_SOCKET, SO_ATTACH_FILTER, &fprog, sizeof(fprog)); + if (r < 0) + return -n_acd_errno(); + + /* + * Bind the packet-socket to ETH_P_ARP and the specified network + * interface. + */ + r = bind(s, (struct sockaddr *)&address, sizeof(address)); + if (r < 0) + return -n_acd_errno(); + + return 0; +} + +static int n_acd_setup_socket(NAcd *acd) { + int r, s; + + s = socket(PF_PACKET, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0); + if (s < 0) + return -n_acd_errno(); + + r = n_acd_bind_socket(acd, s); + if (r < 0) + goto error; + + r = epoll_ctl(acd->fd_epoll, EPOLL_CTL_ADD, s, + &(struct epoll_event){ + .events = EPOLLIN, + .data.u32 = N_ACD_EPOLL_SOCKET, + }); + if (r < 0) { + r = -n_acd_errno(); + goto error; + } + + acd->fd_socket = s; + return 0; + +error: + close(s); + return r; +} + +/** + * n_acd_start() - start the ACD engine + * @acd: ACD context + * @config: description of interface and desired IP address + * + * Start probing the given address on the given interface. + * + * The engine must not already be running, and there must not be + * any pending events. + * + * Returns: 0 on success, N_ACD_E_INVALID_ARGUMENT in case the configuration + * was invalid, N_ACD_E_BUSY if the engine is running or there are + * pending events, or a negative error code on failure. + */ +_public_ int n_acd_start(NAcd *acd, NAcdConfig *config) { + uint64_t now, delay; + int r; + + if (config->ifindex <= 0 || + config->transport != N_ACD_TRANSPORT_ETHERNET || + config->n_mac != ETH_ALEN || + !memcmp(config->mac, (uint8_t[ETH_ALEN]){ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }, ETH_ALEN) || + !config->ip.s_addr || + !config->timeout_msec) + return N_ACD_E_INVALID_ARGUMENT; + + if (acd->state != N_ACD_STATE_INIT || !c_list_is_empty(&acd->events)) + return N_ACD_E_BUSY; + + acd->config = *config; + memcpy(acd->mac, config->mac, config->n_mac); + acd->config.mac = acd->mac; + acd->timeout_multiplier = config->timeout_msec; + + r = n_acd_setup_socket(acd); + if (r < 0) + goto error; + + delay = 0; + if (acd->last_conflict != TIME_INFINITY) { + r = n_acd_now(&now); + if (r < 0) + goto error; + + if (now < acd->last_conflict + N_ACD_RFC_RATE_LIMIT_INTERVAL_USEC) + delay = acd->last_conflict + N_ACD_RFC_RATE_LIMIT_INTERVAL_USEC - now; + } + + r = n_acd_schedule(acd, delay, acd->timeout_multiplier * N_ACD_RFC_PROBE_WAIT_USEC); + if (r < 0) + goto error; + + acd->state = N_ACD_STATE_PROBING; + acd->defend = N_ACD_DEFEND_NEVER; + acd->n_iteration = 0; + acd->last_defend = 0; + return 0; + +error: + n_acd_stop(acd); + return r; +} + +/** + * n_acd_stop() - stop the ACD engine + * @acd: ACD context + * + * Stop the engine. No new events may be triggered, but pending events are not + * flushed. Before calling n_acd_start() again all pending events must be popped. + */ +_public_ void n_acd_stop(NAcd *acd) { + acd->state = N_ACD_STATE_INIT; + acd->defend = N_ACD_DEFEND_NEVER; + acd->n_iteration = 0; + acd->last_defend = 0; + timerfd_settime(acd->fd_timer, 0, &(struct itimerspec){}, NULL); + + if (acd->fd_socket >= 0) { + assert(acd->fd_epoll >= 0); + epoll_ctl(acd->fd_epoll, EPOLL_CTL_DEL, acd->fd_socket, NULL); + close(acd->fd_socket); + acd->fd_socket = -1; + } +} + +/** + * n_acd_announce() - announce the configured IP address + * @acd: ACD context + * @defend: defence policy + * + * Announce the IP address on the local link, and start defending it according + * to the given policy, which mut be one of N_ACD_DEFEND_ONCE, + * N_ACD_DEFEND_NEVER, or N_ACD_DEFEND_ALWAYS. + * + * This must be called after the engine in response to an N_ACD_EVENT_READY + * event, and only after the given address has been configured on the given + * interface. + * + * Return: 0 on success, N_ACD_E_INVALID_ARGUMENT in case the defence policy + * is invalid, N_ACD_E_BUSY if this is not in response to a + * N_ACD_EVENT_READY event, or a negative error code on failure. + */ +_public_ int n_acd_announce(NAcd *acd, unsigned int defend) { + uint64_t now; + int r; + + if (defend >= _N_ACD_DEFEND_N) + return N_ACD_E_INVALID_ARGUMENT; + if (acd->state != N_ACD_STATE_CONFIGURING) + return N_ACD_E_BUSY; + + /* + * Sending announcements means we finished probing and use the address + * now. We therefore reset the conflict counter in case we adhered to + * the rate-limit. Since probing is properly delayed, a well-behaving + * client will always reset the conflict counter here. However, if you + * force-use an address regardless of conflicts, then this will not + * trigger and the conflict counter stays untouched. + */ + if (acd->last_conflict != TIME_INFINITY) { + r = n_acd_now(&now); + if (r < 0) + return r; + + if (now >= acd->last_conflict + N_ACD_RFC_RATE_LIMIT_INTERVAL_USEC) + acd->n_conflicts = 0; + } + + /* + * Instead of sending the first announcement here, we schedule an idle + * timer. This avoids possibly recursing into the user callback. We + * should never trigger callbacks from arbitrary stacks, but always + * restrict them to the dispatcher. + */ + r = n_acd_schedule(acd, 0, 0); + if (r < 0) + return r; + + acd->state = N_ACD_STATE_ANNOUNCING; + acd->defend = defend; + acd->n_iteration = 0; + return 0; +} |