tap_manager: work around flexible array members in uapi

Recently the kernel replaced depracted zero-length arrays with flexible
arrays [1]. Unfortunately this causes the compilation to fail, since now
G++ rejects anything in structs behind the flexible array member:

Feb 16 11:42:33 | In file included from ../git/src/netlink/tap_manager.cc:11:
Feb 16 11:42:33 | /tmp/generic-x86-64/work/corei7-64-poky-linux/baseboxd/2.1.0-r0/recipe-sysroot/usr/include/linux/ethtool.h: In member function 'virtual int basebox::tap_manager::set_port_speed(std::string, uint32_t, uint8_t)':
Feb 16 11:42:33 | /tmp/generic-x86-64/work/corei7-64-poky-linux/baseboxd/2.1.0-r0/recipe-sysroot/usr/include/linux/ethtool.h:2199:17: error: flexible array member 'ethtool_link_settings::link_mode_masks' not at end of 'struct basebox::tap_manager::set_port_speed(std::string, uint32_t, uint8_t)::<unnamed>'
Feb 16 11:42:33 |  2199 |         __u32   link_mode_masks[];
Feb 16 11:42:33 |       |                 ^~~~~~~~~~~~~~~
Feb 16 11:42:33 | ../git/src/netlink/tap_manager.cc:394:11: note: next member '__u32 basebox::tap_manager::set_port_speed(std::string, uint32_t, uint8_t)::<unnamed struct>::link_mode_data [381]' declared here
Feb 16 11:42:33 |   394 |     __u32 link_mode_data[3 * ETHTOOL_LINK_MODE_MASK_MAX_KERNEL_NU32];
Feb 16 11:42:33 |       |           ^~~~~~~~~~~~~~
Feb 16 11:42:33 | ../git/src/netlink/tap_manager.cc:392:10: note: in the definition of 'struct basebox::tap_manager::set_port_speed(std::string, uint32_t, uint8_t)::<unnamed>'
Feb 16 11:42:33 |   392 |   struct {
Feb 16 11:42:33 |       |          ^

Work around this by switching the order of the elements in our own
struct so that the flexible array member is last, but the struct still
is sized correctly. Then use a ethtool_link_settings pointer pointing to
the beginning of this struct for any further accesses.

[1] torvalds/linux@94dfc73

Signed-off-by: Jonas Gorski <[email protected]>

src/netlink/tap_manager.cc

@@ -390,31 +390,33 @@ int tap_manager::set_port_speed(const std::string name, uint32_t speed,
   // mode bitmaps.
 
   struct {
-    struct ethtool_link_settings req;
     __u32 link_mode_data[3 * ETHTOOL_LINK_MODE_MASK_MAX_KERNEL_NU32];
-  } link_settings;
+    struct ethtool_link_settings req;
+  } link_settings_request;
+  auto *link_settings =
+      reinterpret_cast<struct ethtool_link_settings *>(&link_settings_request);
 
-  memset(&link_settings, 0, sizeof(link_settings));
-  link_settings.req.cmd = ETHTOOL_GLINKSETTINGS;
+  memset(&link_settings_request, 0, sizeof(link_settings_request));
+  link_settings->cmd = ETHTOOL_GLINKSETTINGS;
 
-  ifr.ifr_data = reinterpret_cast<char *>(&link_settings);
+  ifr.ifr_data = reinterpret_cast<char *>(link_settings);
   int error = ioctl(sockFd, SIOCETHTOOL, static_cast<void *>(&ifr));
   if (error < 0) {
     LOG(ERROR) << __FUNCTION__ << " handshake failed error=" << error;
     close(sockFd);
     return error;
   }
 
-  if (link_settings.req.link_mode_masks_nwords >= 0 ||
-      link_settings.req.cmd != ETHTOOL_GLINKSETTINGS) {
+  if (link_settings->link_mode_masks_nwords >= 0 ||
+      link_settings->cmd != ETHTOOL_GLINKSETTINGS) {
     close(sockFd);
     return -EOPNOTSUPP;
   }
 
-  link_settings.req.link_mode_masks_nwords =
-      -link_settings.req.link_mode_masks_nwords;
+  link_settings->link_mode_masks_nwords =
+      -link_settings->link_mode_masks_nwords;
 
-  ifr.ifr_data = reinterpret_cast<char *>(&link_settings);
+  ifr.ifr_data = reinterpret_cast<char *>(link_settings);
   error = ioctl(sockFd, SIOCETHTOOL, static_cast<void *>(&ifr));
   if (error < 0) {
     LOG(ERROR) << __FUNCTION__ << " failed to get port= " << name
@@ -423,11 +425,11 @@ int tap_manager::set_port_speed(const std::string name, uint32_t speed,
     return error;
   }
 
-  link_settings.req.duplex = duplex;
-  link_settings.req.speed = ETHTOOL_SPEED(speed);
-  link_settings.req.cmd = ETHTOOL_SLINKSETTINGS;
+  link_settings->duplex = duplex;
+  link_settings->speed = ETHTOOL_SPEED(speed);
+  link_settings->cmd = ETHTOOL_SLINKSETTINGS;
 
-  ifr.ifr_data = reinterpret_cast<char *>(&link_settings);
+  ifr.ifr_data = reinterpret_cast<char *>(link_settings);
   error = ioctl(sockFd, SIOCETHTOOL, static_cast<void *>(&ifr));
   if (error < 0) {
     LOG(ERROR) << __FUNCTION__ << " failed to set port= " << name