[Android] Port getifaddrs implementation from Xamarin.Android (#71943)

In recent Android versions the data returned by getifaddrs is invalid and doesn't allow us to implement System.Net.NetworkInformation.GetAllNetworkInterfaces(). It's possible to reimplement getifaddrs using Netlink and there's already an existing implementation in Xamarin.Android.

Fixes xamarin/xamarin-android#6973
Ref #62780
Ref #51303

src/mono/cmake/config.h.in

@@ -517,9 +517,6 @@
 /* Have getifaddrs */
 #cmakedefine HAVE_GETIFADDRS 1
 
-/* Have struct ifaddrs */
-#cmakedefine HAVE_IFADDRS 1
-
 /* Have access */
 #cmakedefine HAVE_ACCESS 1
 

src/native/libs/Common/pal_config.h.in

@@ -11,7 +11,6 @@
 #cmakedefine01 HAVE_F_FULLFSYNC
 #cmakedefine01 HAVE_O_CLOEXEC
 #cmakedefine01 HAVE_GETIFADDRS
-#cmakedefine01 HAVE_IFADDRS
 #cmakedefine01 HAVE_UTSNAME_DOMAINNAME
 #cmakedefine01 HAVE_STAT64
 #cmakedefine01 HAVE_FORK

src/native/libs/System.Native/CMakeLists.txt

@@ -83,6 +83,13 @@ include(${CMAKE_CURRENT_LIST_DIR}/extra_libs.cmake)
 set(NATIVE_LIBS_EXTRA)
 append_extra_system_libs(NATIVE_LIBS_EXTRA)
 
+if (CLR_CMAKE_TARGET_ANDROID)
+    add_compile_options(-Wno-gnu-zero-variadic-macro-arguments)
+
+    list (APPEND NATIVE_LIBS_EXTRA -llog)
+    list (APPEND NATIVE_SOURCES pal_ifaddrs.c)
+endif ()
+
 if (GEN_SHARED_LIB)
     add_library(System.Native
         SHARED

src/native/libs/System.Native/pal_ifaddrs.h

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#pragma once
+
+#ifndef TARGET_ANDROID
+#error The pal_ifaddrs.h shim is intended only for Android
+#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.
+
+#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
+
+int getifaddrs (struct ifaddrs **ifap);
+void freeifaddrs (struct ifaddrs *ifap);

src/native/libs/System.Native/pal_interfaceaddresses.c

@@ -11,12 +11,11 @@
 #include <stdlib.h>
 #include <sys/types.h>
 #include <assert.h>
-#if HAVE_IFADDRS || HAVE_GETIFADDRS
+#if HAVE_GETIFADDRS && !defined(TARGET_ANDROID)
 #include <ifaddrs.h>
 #endif
-#if !HAVE_GETIFADDRS && TARGET_ANDROID
-#include <dlfcn.h>
-#include <pthread.h>
+#ifdef TARGET_ANDROID
+#include "pal_ifaddrs.h"
 #endif
 #include <net/if.h>
 #include <netinet/in.h>
@@ -101,66 +100,12 @@ static inline uint8_t mask2prefix(uint8_t* mask, int length)
     return len;
 }
 
-#if !HAVE_IFADDRS && TARGET_ANDROID
-// This structure is exactly the same as struct ifaddrs defined in ifaddrs.h but since the header
-// might not be available (e.g., in bionics used in Android before API 24) we need to mirror it here
-// so that we can dynamically load the getifaddrs function and use it.
-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;
-};
-#endif
-
-#if !HAVE_GETIFADDRS && TARGET_ANDROID
-// 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.
-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)
-    {
-        getifaddrs = (int (*)(struct ifaddrs**)) dlsym(libc, "getifaddrs");
-        freeifaddrs = (void (*)(struct ifaddrs*)) dlsym(libc, "freeifaddrs");
-    }
-}
-
-static bool ensure_getifaddrs_is_loaded()
-{
-    static pthread_once_t getifaddrs_is_loaded = PTHREAD_ONCE_INIT;
-    pthread_once(&getifaddrs_is_loaded, try_loading_getifaddrs);
-    return getifaddrs != NULL && freeifaddrs != NULL;
-}
-#endif
-
 int32_t SystemNative_EnumerateInterfaceAddresses(void* context,
                                                IPv4AddressFound onIpv4Found,
                                                IPv6AddressFound onIpv6Found,
                                                LinkLayerAddressFound onLinkLayerFound)
 {
-#if !HAVE_GETIFADDRS && TARGET_ANDROID
-    // Workaround for Android API < 24
-    if (!ensure_getifaddrs_is_loaded())
-    {
-        errno = ENOTSUP;
-        return -1;
-    }
-#endif
-
-#if HAVE_GETIFADDRS || TARGET_ANDROID
+#if HAVE_GETIFADDRS || defined(TARGET_ANDROID)
     struct ifaddrs* headAddr;
     if (getifaddrs(&headAddr) == -1)
     {
@@ -305,16 +250,7 @@ int32_t SystemNative_EnumerateInterfaceAddresses(void* context,
 
 int32_t SystemNative_GetNetworkInterfaces(int32_t * interfaceCount, NetworkInterfaceInfo **interfaceList, int32_t * addressCount, IpAddressInfo **addressList )
 {
-#if !HAVE_GETIFADDRS && TARGET_ANDROID
-    // Workaround for Android API < 24
-    if (!ensure_getifaddrs_is_loaded())
-    {
-        errno = ENOTSUP;
-        return -1;
-    }
-#endif
-
-#if HAVE_GETIFADDRS || TARGET_ANDROID
+#if HAVE_GETIFADDRS || defined(TARGET_ANDROID)
     struct ifaddrs* head;   // Pointer to block allocated by getifaddrs().
     struct ifaddrs* ifaddrsEntry;
     IpAddressInfo *ai;

src/native/libs/configure.cmake

@@ -121,18 +121,6 @@ check_c_source_compiles(
     "
     HAVE_FLOCK64)
 
-check_c_source_compiles(
-    "
-    #include <sys/types.h>
-    #include <ifaddrs.h>
-    int main(void)
-    {
-        struct ifaddrs ia;
-        return 0;
-    }
-    "
-    HAVE_IFADDRS)
-
 check_symbol_exists(
     O_CLOEXEC
     fcntl.h