FIXED: Windows: UNC support for relative file names and shared object loading.

When the process working directory is a UNC path (e.g. set via a
Windows shortcut's "Start in" field), resolving a relative file name
via absolute_file_name/3, access_file/2 and friends failed, and
open_shared_object/2, use_foreign_library/1 could not load DLLs
identified by a UNC path.

_xos_os_filenameW() used to unconditionally prepend "\\?\" and let
GetFullPathNameW() append its result.  When the CWD is UNC, that
produced a malformed "\\?\\\host\share\..." path that NT rejects.
Now run GetFullPathNameW() first, inspect the result, and use the
"\\?\UNC\" prefix when it resolves to a UNC path.

LoadLibraryExW() and AddDllDirectoryW() do not accept extended-length
"\\?\" or "\\?\UNC\" prefixes and return ERROR_INVALID_PARAMETER,
especially in combination with LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR.
Strip the prefix (converting "\\?\UNC\host\..." back to "\\host\...")
before calling them.  This also fixes a latent bug in
win_add_dll_directory/2 where _xos_win_prefix_length() returned 4 for
"\\?\UNC\..." (WIN_PATH_PREFIX matches first), leaving "UNC\host\..."
as the directory passed to AddDllDirectoryW().

Discussion: https://swi-prolog.discourse.group/t/issues-with-unc-path/9010

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JanWielemaker

src/os/windows/uxnt.c

@@ -387,13 +387,34 @@ _xos_os_filenameW(const char *cname, wchar_t *osname, size_t len)
       return osname;
     }
   } else
-  { wcscpy(s, WIN_PATH_PREFIX);
-    s += wcslen(s);
-    len -= (s-osname);
-    rc = GetFullPathNameW(buf, len, s, NULL);
-    if ( rc <= len )
-    { remove_win_prefix(s);
-      return osname;
+  { TCHAR full[PATH_MAX];
+
+    rc = GetFullPathNameW(buf, PATH_MAX, full, NULL);
+    if ( rc > 0 && rc < PATH_MAX )
+    { remove_win_prefix(full);
+      /* GetFullPathNameW may resolve a relative name to a UNC path when
+	 the process CWD is a UNC path.  In that case we must use the
+	 \\?\UNC\ prefix rather than \\?\, or NT rejects the path.
+      */
+      if ( full[0] == '\\' && full[1] == '\\' )
+      { size_t plen = wcslen(WIN_UNC_PREFIX);
+	size_t tlen = wcslen(full+1);		/* skip one leading '\' */
+
+	if ( plen + tlen + 1 <= len )
+	{ wcscpy(s, WIN_UNC_PREFIX);
+	  wcscpy(s+plen, full+1);
+	  return osname;
+	}
+      } else
+      { size_t plen = wcslen(WIN_PATH_PREFIX);
+	size_t tlen = wcslen(full);
+
+	if ( plen + tlen + 1 <= len )
+	{ wcscpy(s, WIN_PATH_PREFIX);
+	  wcscpy(s+plen, full);
+	  return osname;
+	}
+      }
     }
   }
 

src/pl-nt.c

@@ -718,6 +718,24 @@ load_library_search_flags(void)
 }
 
 
+/* LoadLibraryExW() and AddDllDirectoryW() do not accept extended-length
+ * "\\?\" prefixes.  Strip a "\\?\" prefix in place; convert "\\?\UNC\"
+ * into its plain UNC form "\\".  Returns a pointer into buf.
+ */
+
+static wchar_t *
+strip_win_prefix(wchar_t *buf)
+{ if ( wcsncmp(buf, L"\\\\?\\UNC\\", 8) == 0 )
+  { buf[6] = L'\\';			/* overwrite 'C' of "UNC" */
+    return buf + 6;			/* now begins with "\\host\..." */
+  }
+  if ( wcsncmp(buf, L"\\\\?\\", 4) == 0 )
+    return buf + 4;
+
+  return buf;
+}
+
+
 static
 PRED_IMPL("win_add_dll_directory", 2, win_add_dll_directory, 0)
 { PRED_LD
@@ -733,7 +751,7 @@ PRED_IMPL("win_add_dll_directory", 2, win_add_dll_directory, 0)
     { int eno;
 
       /* AddDllDirectoryW() cannot handle "\\?\" */
-      if ( (cookie = (*f_AddDllDirectoryW)(dirw + _xos_win_prefix_length(dirw))) )
+      if ( (cookie = (*f_AddDllDirectoryW)(strip_win_prefix(dirw))) )
       { DEBUG(MSG_WIN_API,
 	      SdprintfX("AddDllDirectory(%Ws) ok\n", dirw));
 
@@ -786,12 +804,17 @@ PL_dlopen(const char *file, int flags)	/* file is in UTF-8, POSIX path */
 { HINSTANCE h;
   DWORD llflags = 0;
   wchar_t wfile[PATH_MAX];
+  wchar_t *load_path = wfile;
 
   if ( strchr(file, '/') || strchr(file, '\\' ) )
   { if ( _xos_os_filenameW(file, wfile, PATH_MAX) == NULL )
     { dlmsg = "Name too long";
       return NULL;
     }
+    /* LoadLibraryExW() rejects "\\?\"-prefixed paths, especially in
+     * combination with LOAD_LIBRARY_SEARCH_* flags.
+     */
+    load_path = strip_win_prefix(wfile);
   } else
   { wchar_t *w = wfile;
     wchar_t *e = &w[PATH_MAX-1];
@@ -809,12 +832,12 @@ PL_dlopen(const char *file, int flags)	/* file is in UTF-8, POSIX path */
     *w = 0;
   }
 
-  DEBUG(MSG_WIN_API, SdprintfX("dlopen(%Ws)\n", wfile));
+  DEBUG(MSG_WIN_API, SdprintfX("dlopen(%Ws)\n", load_path));
 
-  if ( is_windows_abs_path(wfile) )
+  if ( is_windows_abs_path(load_path) )
     llflags |= load_library_search_flags();
 
-  if ( (h = LoadLibraryExW(wfile, NULL, llflags)) )
+  if ( (h = LoadLibraryExW(load_path, NULL, llflags)) )
   { dlmsg = "No Error";
     return (void *)h;
   }