[Android] Fix NetworkInterface.GetAllNetworkInterfaces on API 21-23 (dotnet#76541)

* Bring back pal_ifaddrs

* Update the header file

Author: simonrozsival

src/libraries/Native/Unix/System.Native/CMakeLists.txt

@@ -81,7 +81,6 @@ append_extra_system_libs(NATIVE_LIBS_EXTRA)
 
 if (CLR_CMAKE_TARGET_ANDROID AND NOT HAVE_GETIFADDRS)
     add_definitions(-DANDROID_GETIFADDRS_WORKAROUND)
-
     add_compile_options(-Wno-gnu-zero-variadic-macro-arguments)
 
     list (APPEND NATIVE_LIBS_EXTRA -llog)

src/libraries/Native/Unix/System.Native/pal_ifaddrs.c

@@ -312,15 +312,15 @@ static struct ifaddrs *get_link_info(struct nlmsghdr *message)
                 break;
 
             case IFLA_BROADCAST:
-                LOG_DEBUG("   interface broadcast (%lu bytes)\n", RTA_PAYLOAD(attribute));
+                LOG_DEBUG("   interface broadcast (%zu bytes)\n", RTA_PAYLOAD(attribute));
                 if (fill_ll_address(&sa, net_interface, RTA_DATA(attribute), RTA_PAYLOAD(attribute)) < 0) {
                     goto error;
                 }
                 ifa->ifa_broadaddr = (struct sockaddr*)sa;
                 break;
 
             case IFLA_ADDRESS:
-                LOG_DEBUG("   interface address (%lu bytes)\n", RTA_PAYLOAD(attribute));
+                LOG_DEBUG("   interface address (%zu bytes)\n", RTA_PAYLOAD(attribute));
                 if (fill_ll_address(&sa, net_interface, RTA_DATA(attribute), RTA_PAYLOAD(attribute)) < 0) {
                     goto error;
                 }
@@ -352,7 +352,7 @@ static struct ifaddrs *find_interface_by_index(int index, struct ifaddrs **ifadd
         return NULL;
 
     /* Normally expensive, but with the small amount of links in the chain we'll deal with it's not
-     * worth the extra houskeeping and memory overhead
+     * worth the extra housekeeping and memory overhead
      */
     cur = *ifaddrs_head;
     while (cur) {
@@ -509,7 +509,7 @@ static struct ifaddrs *get_link_address(struct nlmsghdr *message, struct ifaddrs
                 LOG_DEBUG("     attribute type: LOCAL");
                 if (ifa->ifa_addr) {
                     /* P2P protocol, set the dst/broadcast address union from the original address.
-                     * Since ifa_addr is set it means IFA_ADDRESS occured earlier and that address
+                     * Since ifa_addr is set it means IFA_ADDRESS occurred earlier and that address
                      * is indeed the P2P destination one.
                      */
                     ifa->ifa_dstaddr = ifa->ifa_addr;
@@ -531,7 +531,7 @@ static struct ifaddrs *get_link_address(struct nlmsghdr *message, struct ifaddrs
             case IFA_ADDRESS:
                 LOG_DEBUG("     attribute type: ADDRESS");
                 if (ifa->ifa_addr) {
-                    /* Apparently IFA_LOCAL occured earlier and we have a P2P connection
+                    /* Apparently IFA_LOCAL occurred earlier and we have a P2P connection
                      * here. IFA_LOCAL carries the destination address, move it there
                      */
                     ifa->ifa_dstaddr = ifa->ifa_addr;
@@ -570,7 +570,7 @@ static struct ifaddrs *get_link_address(struct nlmsghdr *message, struct ifaddrs
         attribute = RTA_NEXT(attribute, length);
     }
 
-    /* glibc stores the associated interface name in the address if IFA_LABEL never occured */
+    /* glibc stores the associated interface name in the address if IFA_LABEL never occurred */
     if (!ifa->ifa_name) {
         char *name = get_interface_name_by_index((int)(net_address->ifa_index), ifaddrs_head);
         LOG_DEBUG("   address has no name/label, getting one from interface\n");
@@ -708,7 +708,7 @@ static int parse_netlink_reply(struct netlink_session *session, struct ifaddrs *
     return ret;
 }
 
-int getifaddrs(struct ifaddrs **ifap)
+int _netlink_getifaddrs(struct ifaddrs **ifap)
 {
     int ret = -1;
 
@@ -728,7 +728,7 @@ int getifaddrs(struct ifaddrs **ifap)
             (parse_netlink_reply(&session, &ifaddrs_head, &last_ifaddr) < 0) ||
             (send_netlink_dump_request(&session, RTM_GETADDR) < 0) ||
             (parse_netlink_reply(&session, &ifaddrs_head, &last_ifaddr) < 0)) {
-        freeifaddrs (ifaddrs_head);
+        _netlink_freeifaddrs (ifaddrs_head);
         goto cleanup;
     }
 
@@ -744,7 +744,7 @@ int getifaddrs(struct ifaddrs **ifap)
     return ret;
 }
 
-void freeifaddrs(struct ifaddrs *ifa)
+void _netlink_freeifaddrs(struct ifaddrs *ifa)
 {
     struct ifaddrs *cur, *next;
 

src/libraries/Native/Unix/System.Native/pal_ifaddrs.h

@@ -7,37 +7,15 @@
 #error The pal_ifaddrs.h shim is intended only for Android
 #endif
 
+#if __ANDROID_API__ >= 24
+#error The pal_ifaddrs.h shim is only necessary for Android API 21-23 and it should be removed now that the minimum supported API level is 24 or higher
+#endif
+
 // Android doesn't include the getifaddrs and freeifaddrs functions in older Bionic libc (pre API 24).
-// In recent Android versions (Android 11+) the data returned by the getifaddrs function is not valid.
 // This shim is a port of Xamarin Android's implementation of getifaddrs using Netlink.
+// https://github.com/xamarin/xamarin-android/blob/681887ebdbd192ce7ce1cd02221d4939599ba762/src/monodroid/jni/xamarin_getifaddrs.h
 
-#include "pal_compiler.h"
-#include "pal_config.h"
-#include "pal_types.h"
-
-#include <sys/cdefs.h>
-#include <netinet/in.h>
-#include <sys/socket.h>
-
-struct ifaddrs
-{
-    struct ifaddrs *ifa_next;
-    char *ifa_name;
-    unsigned int ifa_flags;
-    struct sockaddr *ifa_addr;
-    struct sockaddr *ifa_netmask;
-    union
-    {
-        struct sockaddr *ifu_broadaddr;
-        struct sockaddr *ifu_dstaddr;
-    } ifa_ifu;
-    void *ifa_data;
-};
-
-// Synonym for `ifa_ifu.ifu_broadaddr` in `struct ifaddrs`.
-#define ifa_broadaddr ifa_ifu.ifu_broadaddr
-// Synonym for `ifa_ifu.ifu_dstaddr` in `struct ifaddrs`.
-#define ifa_dstaddr ifa_ifu.ifu_dstaddr
+#include <ifaddrs.h>
 
-int getifaddrs (struct ifaddrs **ifap);
-void freeifaddrs (struct ifaddrs *ifap);
+int _netlink_getifaddrs (struct ifaddrs **ifap);
+void _netlink_freeifaddrs (struct ifaddrs *ifap);

src/libraries/Native/Unix/System.Native/pal_interfaceaddresses.c

@@ -17,6 +17,7 @@
 #ifdef ANDROID_GETIFADDRS_WORKAROUND
 #include <dlfcn.h>
 #include <pthread.h>
+#include "pal_ifaddrs.h" // fallback for Android API 21-23
 #endif
 #include <net/if.h>
 #include <netinet/in.h>
@@ -102,19 +103,30 @@ static inline uint8_t mask2prefix(uint8_t* mask, int length)
 }
 
 #ifdef ANDROID_GETIFADDRS_WORKAROUND
-// Try to load the getifaddrs and freeifaddrs functions manually.
-// This workaround is necessary on Android prior to API 24 and it can be removed once
-// we drop support for earlier Android versions.
+// This workaround is necessary as long as we support Android API 21-23 and it can be removed once
+// we drop support for these old Android versions.
 static int (*getifaddrs)(struct ifaddrs**) = NULL;
 static void (*freeifaddrs)(struct ifaddrs*) = NULL;
 
 static void try_loading_getifaddrs()
 {
-    void *libc = dlopen("libc.so", RTLD_NOW);
-    if (libc)
+    if (android_get_device_api_level() >= 24)
     {
-        getifaddrs = (int (*)(struct ifaddrs**)) dlsym(libc, "getifaddrs");
-        freeifaddrs = (void (*)(struct ifaddrs*)) dlsym(libc, "freeifaddrs");
+        // Bionic on API 24+ contains the getifaddrs/freeifaddrs functions but the NDK doesn't expose those functions
+        // in ifaddrs.h when the minimum supported SDK is lower than 24 and therefore we need to load them manually
+        void *libc = dlopen("libc.so", RTLD_NOW);
+        if (libc)
+        {
+            getifaddrs = (int (*)(struct ifaddrs**)) dlsym(libc, "getifaddrs");
+            freeifaddrs = (void (*)(struct ifaddrs*)) dlsym(libc, "freeifaddrs");
+        }
+    }
+    else
+    {
+        // Bionic on API 21-23 doesn't contain the implementation of getifaddrs/freeifaddrs at all
+        // and we need to reimplement it using netlink (see pal_ifaddrs)
+        getifaddrs = _netlink_getifaddrs;
+        freeifaddrs = _netlink_freeifaddrs;
     }
 }