slirp: fix ICMP handling on macOS hosts

On Linux, SOCK_DGRAM+IPPROTO_ICMP sockets give only the ICMP packet when
read from.  On macOS, however, the socket acts like a SOCK_RAW socket
and includes the IP header as well.

This change strips the extra IP header from the received packet on macOS
before sending it to the guest.  SOCK_DGRAM ICMP sockets aren't
supported on other BSDs, but we enable this behavior for them as well to
treat the sockets the same as raw sockets.
Signed-off-by: NAndrew Oates <aoates@google.com>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

slirp/ip_icmp.c

@@ -420,7 +420,32 @@ void icmp_receive(struct socket *so)
     icp = mtod(m, struct icmp *);
 
     id = icp->icmp_id;
-    len = qemu_recv(so->s, icp, m->m_len, 0);
+    len = qemu_recv(so->s, icp, M_ROOM(m), 0);
+    /*
+     * The behavior of reading SOCK_DGRAM+IPPROTO_ICMP sockets is inconsistent
+     * between host OSes.  On Linux, only the ICMP header and payload is
+     * included.  On macOS/Darwin, the socket acts like a raw socket and
+     * includes the IP header as well.  On other BSDs, SOCK_DGRAM+IPPROTO_ICMP
+     * sockets aren't supported at all, so we treat them like raw sockets.  It
+     * isn't possible to detect this difference at runtime, so we must use an
+     * #ifdef to determine if we need to remove the IP header.
+     */
+#ifdef CONFIG_BSD
+    if (len >= sizeof(struct ip)) {
+        struct ip *inner_ip = mtod(m, struct ip *);
+        int inner_hlen = inner_ip->ip_hl << 2;
+        if (inner_hlen > len) {
+            len = -1;
+            errno = -EINVAL;
+        } else {
+            len -= inner_hlen;
+            memmove(icp, (unsigned char *)icp + inner_hlen, len);
+        }
+    } else {
+      len = -1;
+      errno = -EINVAL;
+    }
+#endif
     icp->icmp_id = id;
 
     m->m_data -= hlen;