compat/w32dlfcn.h: Add safe win32 dlopen/dlclose/dlsym functions.

Signed-off-by: Matt Oliver <protogonoi@gmail.com>

compat/w32dlfcn.h

+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef COMPAT_W32DLFCN_H
+#define COMPAT_W32DLFCN_H
+
+#ifdef _WIN32
+#include <windows.h>
+#if _WIN32_WINNT < 0x0602
+#include "libavutil/wchar_filename.h"
+#endif
+/**
+ * Safe function used to open dynamic libs. This attempts to improve program security
+ * by removing the current directory from the dll search path. Only dll's found in the
+ * executable or system directory are allowed to be loaded.
+ * @param name  The dynamic lib name.
+ * @return A handle to the opened lib.
+ */
+static inline HMODULE win32_dlopen(const char *name)
+{
+#if _WIN32_WINNT < 0x0602
+    // Need to check if KB2533623 is available
+    if (!GetProcAddress(GetModuleHandleW(L"kernel32.dll"), "SetDefaultDllDirectories")) {
+        HMODULE module = NULL;
+        wchar_t *path = NULL, *name_w = NULL;
+        DWORD pathlen;
+        if (utf8towchar(name, &name_w))
+            goto exit;
+        path = (wchar_t *)av_mallocz_array(MAX_PATH, sizeof(wchar_t));
+        // Try local directory first
+        pathlen = GetModuleFileNameW(NULL, path, MAX_PATH);
+        pathlen = wcsrchr(path, '\\') - path;
+        if (pathlen == 0 || pathlen + wcslen(name_w) + 2 > MAX_PATH)
+            goto exit;
+        path[pathlen] = '\\';
+        wcscpy(path + pathlen + 1, name_w);
+        module = LoadLibraryExW(path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+        if (module == NULL) {
+            // Next try System32 directory
+            pathlen = GetSystemDirectoryW(path, MAX_PATH);
+            if (pathlen == 0 || pathlen + wcslen(name_w) + 2 > MAX_PATH)
+                goto exit;
+            path[pathlen] = '\\';
+            wcscpy(path + pathlen + 1, name_w);
+            module = LoadLibraryExW(path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+        }
+exit:
+        av_free(path);
+        av_free(name_w);
+        return module;
+    }
+#endif
+#ifndef LOAD_LIBRARY_SEARCH_APPLICATION_DIR
+#   define LOAD_LIBRARY_SEARCH_APPLICATION_DIR 0x00000200
+#endif
+#ifndef LOAD_LIBRARY_SEARCH_SYSTEM32
+#   define LOAD_LIBRARY_SEARCH_SYSTEM32        0x00000800
+#endif
+    return LoadLibraryExA(name, NULL, LOAD_LIBRARY_SEARCH_APPLICATION_DIR | LOAD_LIBRARY_SEARCH_SYSTEM32);
+}
+#define dlopen(name, flags) win32_dlopen(name)
+#define dlclose FreeLibrary
+#define dlsym GetProcAddress
+#else
+#include <dlfcn.h>
+#endif
+
+#endif /* COMPAT_W32DLFCN_H */