network: Accept loopback that is already configured

network.c

@@ -25,6 +25,7 @@
 #include <linux/loop.h>
 #include <linux/netlink.h>
 #include <linux/rtnetlink.h>
+#include <sys/ioctl.h>
 
 #include "utils.h"
 #include "network.h"
@@ -133,6 +134,31 @@ rtnl_setup_request (char  *buffer,
   return header;
 }
 
+static bool
+loopback_is_configured (void)
+{
+  cleanup_fd int fd = socket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+  struct ifreq flags = { .ifr_name = "lo" };
+  struct ifreq address_query = { .ifr_name = "lo", .ifr_addr.sa_family = AF_INET };
+  struct ifreq mask_query = { .ifr_name = "lo", .ifr_netmask.sa_family = AF_INET };
+  struct sockaddr_in *address;
+
+  if (fd < 0 || ioctl (fd, SIOCGIFFLAGS, &flags) != 0 ||
+      (flags.ifr_flags & (IFF_UP | IFF_LOOPBACK)) != (IFF_UP | IFF_LOOPBACK))
+    return false;
+
+  if (ioctl (fd, SIOCGIFADDR, &address_query) != 0 || address_query.ifr_addr.sa_family != AF_INET)
+    return false;
+  address = (struct sockaddr_in *) &address_query.ifr_addr;
+  if (address->sin_addr.s_addr != htonl (INADDR_LOOPBACK))
+    return false;
+
+  if (ioctl (fd, SIOCGIFNETMASK, &mask_query) != 0 || mask_query.ifr_netmask.sa_family != AF_INET)
+    return false;
+  address = (struct sockaddr_in *) &mask_query.ifr_netmask;
+  return address->sin_addr.s_addr == htonl (0xff000000);
+}
+
 void
 loopback_setup (void)
 {
@@ -145,6 +171,10 @@ loopback_setup (void)
   struct ifinfomsg *infomsg;
   struct in_addr *ip_addr;
 
+  /* Some runtimes, such as gVisor, bring up lo in a fresh network namespace. */
+  if (loopback_is_configured ())
+    return;
+
   src_addr.nl_pid = getpid ();
 
   if_loopback = (int) if_nametoindex ("lo");

tests/meson.build

@@ -1,4 +1,15 @@
 test_programs = [
+  ['test-network', executable(
+    'test-network',
+    'test-network.c',
+    '../network.c',
+    include_directories : common_include_directories,
+    link_args : [
+      '-Wl,--wrap=socket',
+      '-Wl,--wrap=ioctl',
+      '-Wl,--wrap=if_nametoindex',
+    ],
+  )],
   ['test-utils', executable(
     'test-utils',
     'test-utils.c',

tests/test-network.c

@@ -0,0 +1,172 @@
+/* SPDX-License-Identifier: LGPL-2.0-or-later */
+
+#include <arpa/inet.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+
+#include "network.h"
+#include "utils.h"
+
+enum condition
+{
+  READY,
+  SOCKET_ERROR,
+  FLAGS_ERROR,
+  DOWN,
+  NOT_LOOPBACK,
+  ADDRESS_ERROR,
+  WRONG_ADDRESS_FAMILY,
+  WRONG_ADDRESS,
+  MISSING_ADDRESS,
+  MASK_ERROR,
+  WRONG_MASK_FAMILY,
+  WRONG_MASK,
+  MISSING_MASK
+};
+
+static const char * const case_names[] = {
+  "already configured",
+  "socket error",
+  "flags query error",
+  "interface down",
+  "not a loopback interface",
+  "address query error",
+  "wrong address family",
+  "wrong address",
+  "missing address",
+  "netmask query error",
+  "wrong netmask family",
+  "wrong netmask",
+  "missing netmask"
+};
+
+static enum condition current;
+static int control = -1;
+
+int __wrap_socket (int domain, int type, int protocol);
+int __wrap_ioctl (int fd, unsigned long operation, ...);
+unsigned int __wrap_if_nametoindex (const char *name);
+
+static bool
+closed (void)
+{
+  return control == -1 || (fcntl (control, F_GETFD) == -1 && errno == EBADF);
+}
+
+int
+__wrap_socket (int domain, int type, int protocol)
+{
+  (void) protocol;
+  /* Stop at the original setup path: errors and mismatches must reach it. */
+  if (domain == AF_NETLINK)
+    _exit (current != READY && closed () ? 0 : 1);
+  if (domain != AF_INET || type != (SOCK_DGRAM | SOCK_CLOEXEC))
+    _exit (2);
+  if (current == SOCKET_ERROR)
+    {
+      errno = EMFILE;
+      return -1;
+    }
+  control = open ("/dev/null", O_RDONLY | O_CLOEXEC);
+  if (control < 0)
+    _exit (3);
+  return control;
+}
+
+static int
+address_query (struct ifreq *query, bool mask)
+{
+  struct sockaddr_in *address = (struct sockaddr_in *) &query->ifr_addr;
+  if (address->sin_family != AF_INET || address->sin_addr.s_addr != 0)
+    _exit (4);
+  if (current == (mask ? MASK_ERROR : ADDRESS_ERROR))
+    {
+      errno = EIO;
+      return -1;
+    }
+  if (current == (mask ? MISSING_MASK : MISSING_ADDRESS))
+    return 0;
+  if (current == (mask ? WRONG_MASK_FAMILY : WRONG_ADDRESS_FAMILY))
+    address->sin_family = AF_INET6;
+  /* gVisor can write only the address, preserving the input family. */
+  address->sin_addr.s_addr = htonl (mask ? 0xff000000 : INADDR_LOOPBACK);
+  if (current == (mask ? WRONG_MASK : WRONG_ADDRESS))
+    address->sin_addr.s_addr = htonl (mask ? 0xffffff00 : 0x7f000002);
+  return 0;
+}
+
+int
+__wrap_ioctl (int fd, unsigned long operation, ...)
+{
+  va_list arguments;
+  struct ifreq *query;
+  if (fd != control)
+    _exit (5);
+  va_start (arguments, operation);
+  query = va_arg (arguments, struct ifreq *);
+  va_end (arguments);
+  if (strcmp (query->ifr_name, "lo") != 0)
+    _exit (6);
+  if (operation == SIOCGIFADDR || operation == SIOCGIFNETMASK)
+    return address_query (query, operation == SIOCGIFNETMASK);
+  if (operation != SIOCGIFFLAGS)
+    _exit (7);
+  if (current == FLAGS_ERROR)
+    {
+      errno = EIO;
+      return -1;
+    }
+  query->ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
+  if (current == DOWN)
+    query->ifr_flags &= ~IFF_UP;
+  if (current == NOT_LOOPBACK)
+    query->ifr_flags &= ~IFF_LOOPBACK;
+  return 0;
+}
+
+unsigned int
+__wrap_if_nametoindex (const char *name)
+{
+  return strcmp (name, "lo") == 0 ? 1 : 0;
+}
+
+void
+die_with_error (const char *format, ...)
+{
+  (void) format;
+  _exit (8);
+}
+
+int
+main (void)
+{
+  puts ("1..13");
+  fflush (stdout);
+
+  for (current = READY; current <= MISSING_MASK; current++)
+    {
+      pid_t child = fork ();
+      int status;
+      if (child == -1)
+        {
+          perror ("fork");
+          return 9;
+        }
+      if (child == 0)
+        {
+          loopback_setup ();
+          _exit (current == READY && closed () ? 0 : 10);
+        }
+      if (waitpid (child, &status, 0) != child || !WIFEXITED (status) || WEXITSTATUS (status) != 0)
+        {
+          printf ("not ok %u - %s\n", (unsigned int) current + 1, case_names[current]);
+          return 11;
+        }
+      printf ("ok %u - %s; query descriptor closed\n", (unsigned int) current + 1, case_names[current]);
+      fflush (stdout);
+    }
+  return 0;
+}