[newlib-cygwin] Cygwin: normalize_win32_path: improve error checking

[Ken Brown <kbrown@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=283cb372e4e25d1d11123d98e3d14e85b525e48d

commit 283cb372e4e25d1d11123d98e3d14e85b525e48d
Author: Ken Brown <kbrown@cornell.edu>
Date:   Wed Sep 25 14:18:18 2019 -0400

    Cygwin: normalize_win32_path: improve error checking
    
    If the source path starts with the Win32 long path prefix '\\?\' or
    the NT object directory prefix '\??\', require the prefix to be
    followed by 'UNC\' or '<drive letter>:\'.  Otherwise return EINVAL.
    
    This fixes the assertion failure in symlink_info::check that was
    reported here:
    
      https://cygwin.com/ml/cygwin/2019-09/msg00228.html
    
    That assertion failure was caused by normalize_win32_path returning a
    path with no backslashes when the source path was '\\?\DRIVE'.

Diff:
---
 winsup/cygwin/path.cc | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 2fbacd8..f610035 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -1406,15 +1406,18 @@ normalize_win32_path (const char *src, char *dst, char *&tail)
   bool beg_src_slash = isdirsep (src[0]);
 
   tail = dst;
-  /* Skip long path name prefixes in Win32 or NT syntax. */
+  /* Skip Win32 long path name prefix and NT object directory prefix. */
   if (beg_src_slash && (src[1] == '?' || isdirsep (src[1]))
       && src[2] == '?' && isdirsep (src[3]))
     {
       src += 4;
-      if (src[1] != ':') /* native UNC path */
+      if (isdrive (src) && isdirsep (src[2]))
+	beg_src_slash = false;
+      else if (!strncmp (src, "UNC", 3) && isdirsep (src[3]))
+	/* native UNC path */
 	src += 2; /* Fortunately the first char is not copied... */
       else
-	beg_src_slash = false;
+	return EINVAL;
     }
   if (beg_src_slash && isdirsep (src[1]))
     {