QDnsLookup: fix "Resolver functions not found" error on FreeBSD

The current code only tries to load the required functions from
LIBRESOLV_SO (if defined) and resolv, but on FreeBSD they are in libc:
https://www.freebsd.org/cgi/man.cgi?query=res_query&sektion=3&apropos=0&manpath=freebsd

This commit changes the code so that, after failing to load the
non-existent libraries, it attempts to load the functions with dlsym()
using the special handle RTLD_DEFAULT, which searches for the specified
symbol in the loaded libraries.

Task-number: QTBUG-74844
Change-Id: If97aaae233cabbfa01c30d26d9a7fb01ec3ff5c2
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 33 insertions, 14 deletions

diff --git a/src/network/kernel/kernel.pri b/src/network/kernel/kernel.pri
index 11b80d59d5..d7a92a12eb 100644
--- a/src/network/kernel/kernel.pri
+++ b/src/network/kernel/kernel.pri
@@ -38,7 +38,11 @@ qtConfig(dnslookup) {
 }
 
 unix {
-    !integrity:qtConfig(dnslookup): SOURCES += kernel/qdnslookup_unix.cpp
+    !integrity:qtConfig(dnslookup) {
+        SOURCES += kernel/qdnslookup_unix.cpp
+        qtConfig(dlopen): QMAKE_USE_PRIVATE += libdl
+    }
+
     SOURCES += kernel/qhostinfo_unix.cpp
 
     qtConfig(linux-netlink): SOURCES += kernel/qnetworkinterface_linux.cpp
diff --git a/src/network/kernel/qdnslookup_unix.cpp b/src/network/kernel/qdnslookup_unix.cpp
index ce1ec6442a..ee7484ab35 100644
--- a/src/network/kernel/qdnslookup_unix.cpp
+++ b/src/network/kernel/qdnslookup_unix.cpp
@@ -59,6 +59,10 @@
 #  include <gnu/lib-names.h>
 #endif
 
+#if defined(Q_OS_FREEBSD) || QT_CONFIG(dlopen)
+#  include <dlfcn.h>
+#endif
+
 #include <cstring>
 
 QT_BEGIN_NAMESPACE
@@ -87,6 +91,18 @@ struct QDnsLookupStateDeleter
     }
 };
 
+static QFunctionPointer resolveSymbol(QLibrary &lib, const char *sym)
+{
+    if (lib.isLoaded())
+        return lib.resolve(sym);
+
+#if defined(RTLD_DEFAULT) && (defined(Q_OS_FREEBSD) || QT_CONFIG(dlopen))
+    return reinterpret_cast<QFunctionPointer>(dlsym(RTLD_DEFAULT, sym));
+#else
+    return nullptr;
+#endif
+}
+
 static bool resolveLibraryInternal()
 {
     QLibrary lib;
@@ -96,31 +112,30 @@ static bool resolveLibraryInternal()
 #endif
     {
         lib.setFileName(QLatin1String("resolv"));
-        if (!lib.load())
-            return false;
+        lib.load();
     }
 
-    local_dn_expand = dn_expand_proto(lib.resolve("__dn_expand"));
+    local_dn_expand = dn_expand_proto(resolveSymbol(lib, "__dn_expand"));
     if (!local_dn_expand)
-        local_dn_expand = dn_expand_proto(lib.resolve("dn_expand"));
+        local_dn_expand = dn_expand_proto(resolveSymbol(lib, "dn_expand"));
 
-    local_res_nclose = res_nclose_proto(lib.resolve("__res_nclose"));
+    local_res_nclose = res_nclose_proto(resolveSymbol(lib, "__res_nclose"));
     if (!local_res_nclose)
-        local_res_nclose = res_nclose_proto(lib.resolve("res_9_nclose"));
+        local_res_nclose = res_nclose_proto(resolveSymbol(lib, "res_9_nclose"));
     if (!local_res_nclose)
-        local_res_nclose = res_nclose_proto(lib.resolve("res_nclose"));
+        local_res_nclose = res_nclose_proto(resolveSymbol(lib, "res_nclose"));
 
-    local_res_ninit = res_ninit_proto(lib.resolve("__res_ninit"));
+    local_res_ninit = res_ninit_proto(resolveSymbol(lib, "__res_ninit"));
     if (!local_res_ninit)
-        local_res_ninit = res_ninit_proto(lib.resolve("res_9_ninit"));
+        local_res_ninit = res_ninit_proto(resolveSymbol(lib, "res_9_ninit"));
     if (!local_res_ninit)
-        local_res_ninit = res_ninit_proto(lib.resolve("res_ninit"));
+        local_res_ninit = res_ninit_proto(resolveSymbol(lib, "res_ninit"));
 
-    local_res_nquery = res_nquery_proto(lib.resolve("__res_nquery"));
+    local_res_nquery = res_nquery_proto(resolveSymbol(lib, "__res_nquery"));
     if (!local_res_nquery)
-        local_res_nquery = res_nquery_proto(lib.resolve("res_9_nquery"));
+        local_res_nquery = res_nquery_proto(resolveSymbol(lib, "res_9_nquery"));
     if (!local_res_nquery)
-        local_res_nquery = res_nquery_proto(lib.resolve("res_nquery"));
+        local_res_nquery = res_nquery_proto(resolveSymbol(lib, "res_nquery"));
 
     return true;
 }