[newlib-cygwin] Cygwin: format_process_fd: add directory check

[Ken Brown]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=58cc67d6536c73c463114a30b4f14f331137baeb

commit 58cc67d6536c73c463114a30b4f14f331137baeb
Author: Ken Brown <kbrown@cornell.edu>
Date:   Tue Sep 8 11:45:09 2020 -0400

    Cygwin: format_process_fd: add directory check
    
    The incoming path is allowed to have the form "$PID/fd/[0-9]*/.*"
    provided the descriptor symlink points to a directory.  Check that
    this is indeed the case.

Diff:
---
 winsup/cygwin/fhandler_process.cc | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/winsup/cygwin/fhandler_process.cc b/winsup/cygwin/fhandler_process.cc
index a6c358217..888604e3d 100644
--- a/winsup/cygwin/fhandler_process.cc
+++ b/winsup/cygwin/fhandler_process.cc
@@ -398,6 +398,21 @@ format_process_fd (void *data, char *&destbuf)
 	*((process_fd_t *) data)->fd_type = virt_fdsymlink;
       else /* trailing path */
 	{
+	  /* Does the descriptor link point to a directory? */
+	  bool dir;
+	  if (*destbuf != '/')	/* e.g., "pipe:[" or "socket:[" */
+	    dir = false;
+	  else
+	    {
+	      path_conv pc (destbuf);
+	      dir = pc.isdir ();
+	    }
+	  if (!dir)
+	    {
+	      set_errno (ENOTDIR);
+	      cfree (destbuf);
+	      return -1;
+	    }
 	  char *newbuf = (char *) cmalloc_abort (HEAP_STR, strlen (destbuf)
 							   + strlen (e) + 1);
 	  stpcpy (stpcpy (newbuf, destbuf), e);