lok: provide global random symbol to find random device.

This is vital for lok when used in a jail with no random device,
but with an inherited file-handle to /dev/urandom.

Use 'dup' to avoid changing code that wants to 'close' the handle
after use.

Change-Id: I15f40fb251f180a2394af030f56e47f2bf9651d4
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166113
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

bin/find-can-be-private-symbols.functions.results

@@ -13449,6 +13449,7 @@ linguistic::SpellAlternatives::SetFailureType(short)
 linguistic::SpellAlternatives::SetWordLanguage(rtl::OUString const&, o3tl::strong_int<unsigned short, LanguageTypeTag>)
 linguistic::SpellAlternatives::SpellAlternatives()
 load_BLAS
+lok_open_urandom
 longdual_testset
 lower_utf(w_char, int)
 lp_solve_version

desktop/source/lib/init.cxx

@@ -31,8 +31,10 @@
 
 #undef HAVE_MALLOC_TRIM
 
+#ifdef UNX
+#  include <fcntl.h>
+#endif
 #ifdef LINUX
-#include <fcntl.h>
 #if defined __GLIBC__
 #  include <malloc.h>
 #  define HAVE_MALLOC_TRIM
@@ -234,6 +236,20 @@ using namespace bridge;
 using namespace uno;
 using namespace lang;
 
+#ifdef UNX
+
+static int urandom = -1;
+
+extern "C" {
+    int SAL_JNI_EXPORT lok_open_urandom()
+    {
+        return dup(urandom);
+    }
+};
+
+#endif
+
+
 using LanguageToolCfg = officecfg::Office::Linguistic::GrammarChecking::LanguageTool;
 
 static LibLibreOffice_Impl *gImpl = nullptr;
@@ -7903,6 +7919,10 @@ static int lo_initialize(LibreOfficeKit* pThis, const char* pAppPath, const char
         const char* tz = ::getenv("TZ");
         SfxLokHelper::setDefaultTimezone(!!tz, tz ? OStringToOUString(tz, RTL_TEXTENCODING_UTF8)
                                                   : OUString());
+#ifdef UNX
+        if (urandom < 0)
+            urandom = open("/dev/urandom", O_RDONLY);
+#endif
     }
 
     if (eStage != SECOND_INIT)

sal/osl/unx/random.cxx

@@ -13,13 +13,19 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <dlfcn.h>
 
 bool osl_get_system_random_data(char* buffer, size_t desired_len)
 {
     int fd;
 
     assert(buffer);
-    fd = open("/dev/urandom", O_RDONLY);
+
+    static int (*lok_open_urandom)()
+        = reinterpret_cast<int (*)()>(dlsym(RTLD_DEFAULT, "lok_open_urandom"));
+    if (!lok_open_urandom || (fd = lok_open_urandom()) < 0)
+        fd = open("/dev/urandom", O_RDONLY);
+
     if (fd != -1)
     {
         while (desired_len)