[newlib-cygwin] cygwin: try unprivileged symlink creation on W10 1703 and later

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=ffcfbf4b66904529635d0b85687a1be48392ca15

commit ffcfbf4b66904529635d0b85687a1be48392ca15
Author: Corinna Vinschen <cori...@vinschen.de>
Date:   Mon Apr 24 17:17:29 2017 +0200

    cygwin: try unprivileged symlink creation on W10 1703 and later
    
    Add new SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag to
    CreateSymbolicLinkW call when running on W10 1703 or later.
    Don't do that on older versions to avoid ERROR_INVALID_PARAMETER.
    
    Preliminary, needs testing.  There's an off-chance that the
    flag results in the same ERROR_INVALID_PARAMETER on 1703 if the
    developer settings are not enabled.
    
    Signed-off-by: Corinna Vinschen <cori...@vinschen.de>

Diff:
---
 winsup/cygwin/path.cc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 16de025..7d1d23d 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -1621,6 +1621,10 @@ cnt_bs (PWCHAR s, PWCHAR e)
   return num;
 }
 
+#ifndef SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+#define SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE 2
+#endif
+
 static int
 symlink_native (const char *oldpath, path_conv &win32_newpath)
 {
@@ -1628,6 +1632,7 @@ symlink_native (const char *oldpath, path_conv 
&win32_newpath)
   path_conv win32_oldpath;
   PUNICODE_STRING final_oldpath, final_newpath;
   UNICODE_STRING final_oldpath_buf;
+  DWORD flags;
 
   if (isabspath (oldpath))
     {
@@ -1724,9 +1729,11 @@ symlink_native (const char *oldpath, path_conv 
&win32_newpath)
        final_oldpath->Buffer[1] = L'\\';
     }
   /* Try to create native symlink. */
+  flags = win32_oldpath.isdir () ? SYMBOLIC_LINK_FLAG_DIRECTORY : 0;
+  if (wincap.has_unprivileged_createsymlink ())
+    flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
   if (!CreateSymbolicLinkW (final_newpath->Buffer, final_oldpath->Buffer,
-                           win32_oldpath.isdir ()
-                           ? SYMBOLIC_LINK_FLAG_DIRECTORY : 0))
+                           flags))
     {
       /* Repair native newpath, we still need it. */
       final_newpath->Buffer[1] = L'?';