networkd: FIONREAD is not reliable on some sockets

Fixes: #2457

src/basic/socket-util.c

@@ -936,3 +936,37 @@ int receive_one_fd(int transport_fd, int flags) {
 
         return *(int*) CMSG_DATA(found);
 }
+
+ssize_t next_datagram_size_fd(int fd) {
+        ssize_t l;
+        int k;
+
+        /* This is a bit like FIONREAD/SIOCINQ, however a bit more powerful. The difference being: recv(MSG_PEEK) will
+         * actually cause the next datagram in the queue to be validated regarding checksums, which FIONREAD dosn't
+         * do. This difference is actually of major importance as we need to be sure that the size returned here
+         * actually matches what we will read with recvmsg() next, as otherwise we might end up allocating a buffer of
+         * the wrong size. */
+
+        l = recv(fd, NULL, 0, MSG_PEEK|MSG_TRUNC);
+        if (l < 0) {
+                if (errno == EOPNOTSUPP)
+                        goto fallback;
+
+                return -errno;
+        }
+        if (l == 0)
+                goto fallback;
+
+        return l;
+
+fallback:
+        k = 0;
+
+        /* Some sockets (AF_PACKET) do not support null-sized recv() with MSG_TRUNC set, let's fall back to FIONREAD
+         * for them. Checksums don't matter for raw sockets anyway, hence this should be fine. */
+
+        if (ioctl(fd, FIONREAD, &k) < 0)
+                return -errno;
+
+        return (ssize_t) k;
+}

src/basic/socket-util.h

@@ -133,5 +133,7 @@ int send_one_fd_sa(int transport_fd,
 #define send_one_fd(transport_fd, fd, flags) send_one_fd_sa(transport_fd, fd, NULL, 0, flags)
 int receive_one_fd(int transport_fd, int flags);
 
+ssize_t next_datagram_size_fd(int fd);
+
 #define CMSG_FOREACH(cmsg, mh)                                          \
         for ((cmsg) = CMSG_FIRSTHDR(mh); (cmsg); (cmsg) = CMSG_NXTHDR((mh), (cmsg)))

src/libsystemd-network/sd-dhcp-client.c

@@ -1525,20 +1525,17 @@ static int client_receive_message_udp(sd_event_source *s, int fd,
                                       uint32_t revents, void *userdata) {
         sd_dhcp_client *client = userdata;
         _cleanup_free_ DHCPMessage *message = NULL;
-        int buflen = 0, len, r;
         const struct ether_addr zero_mac = { { 0, 0, 0, 0, 0, 0 } };
         const struct ether_addr *expected_chaddr = NULL;
         uint8_t expected_hlen = 0;
+        ssize_t len, buflen;
 
         assert(s);
         assert(client);
 
-        r = ioctl(fd, FIONREAD, &buflen);
-        if (r < 0)
-                return -errno;
-        else if (buflen < 0)
-                /* this can't be right */
-                return -EIO;
+        buflen = next_datagram_size_fd(fd);
+        if (buflen < 0)
+                return buflen;
 
         message = malloc0(buflen);
         if (!message)
@@ -1616,17 +1613,15 @@ static int client_receive_message_raw(sd_event_source *s, int fd,
         };
         struct cmsghdr *cmsg;
         bool checksum = true;
-        int buflen = 0, len, r;
+        ssize_t buflen, len;
+        int r;
 
         assert(s);
         assert(client);
 
-        r = ioctl(fd, FIONREAD, &buflen);
-        if (r < 0)
-                return -errno;
-        else if (buflen < 0)
-                /* this can't be right */
-                return -EIO;
+        buflen = next_datagram_size_fd(fd);
+        if (buflen < 0)
+                return buflen;
 
         packet = malloc0(buflen);
         if (!packet)

src/libsystemd-network/sd-dhcp-server.c

@@ -955,14 +955,13 @@ static int server_receive_message(sd_event_source *s, int fd,
                 .msg_controllen = sizeof(cmsgbuf),
         };
         struct cmsghdr *cmsg;
-        int buflen = 0, len;
+        ssize_t buflen, len;
 
         assert(server);
 
-        if (ioctl(fd, FIONREAD, &buflen) < 0)
-                return -errno;
-        else if (buflen < 0)
-                return -EIO;
+        buflen = next_datagram_size_fd(fd);
+        if (buflen < 0)
+                return buflen;
 
         message = malloc(buflen);
         if (!message)

src/libsystemd-network/sd-dhcp6-client.c

@@ -33,6 +33,7 @@
 #include "in-addr-util.h"
 #include "network-internal.h"
 #include "random-util.h"
+#include "socket-util.h"
 #include "string-table.h"
 #include "util.h"
 
@@ -891,18 +892,16 @@ static int client_receive_message(sd_event_source *s, int fd, uint32_t revents,
         sd_dhcp6_client *client = userdata;
         DHCP6_CLIENT_DONT_DESTROY(client);
         _cleanup_free_ DHCP6Message *message = NULL;
-        int r, buflen, len;
+        ssize_t buflen, len;
+        int r = 0;
 
         assert(s);
         assert(client);
         assert(client->event);
 
-        r = ioctl(fd, FIONREAD, &buflen);
-        if (r < 0)
-                return -errno;
-        else if (buflen < 0)
-                /* This really should not happen */
-                return -EIO;
+        buflen = next_datagram_size_fd(fd);
+        if (buflen < 0)
+                return buflen;
 
         message = malloc(buflen);
         if (!message)

src/libsystemd-network/sd-ndisc.c

@@ -491,19 +491,16 @@ static int ndisc_router_advertisment_recv(sd_event_source *s, int fd, uint32_t r
         struct cmsghdr *cmsg;
         struct in6_addr *gw;
         unsigned lifetime;
-        ssize_t len;
-        int r, pref, stateful, buflen = 0;
+        ssize_t len, buflen;
+        int r, pref, stateful;
 
         assert(s);
         assert(nd);
         assert(nd->event);
 
-        r = ioctl(fd, FIONREAD, &buflen);
-        if (r < 0)
-                return -errno;
-        else if (buflen < 0)
-                /* This really should not happen */
-                return -EIO;
+        buflen = next_datagram_size_fd(fd);
+        if (buflen < 0)
+                return buflen;
 
         iov.iov_len = buflen;
 

src/resolve/resolved-manager.c

@@ -617,18 +617,16 @@ int manager_recv(Manager *m, int fd, DnsProtocol protocol, DnsPacket **ret) {
         struct msghdr mh = {};
         struct cmsghdr *cmsg;
         struct iovec iov;
-        int ms = 0, r;
-        ssize_t l;
+        ssize_t ms, l;
+        int r;
 
         assert(m);
         assert(fd >= 0);
         assert(ret);
 
-        r = ioctl(fd, FIONREAD, &ms);
-        if (r < 0)
-                return -errno;
+        ms = next_datagram_size_fd(fd);
         if (ms < 0)
-                return -EIO;
+                return ms;
 
         r = dns_packet_new(&p, protocol, ms);
         if (r < 0)