[newlib-cygwin] Cygwin: Allow to set SO_PEERCRED zero (v2)

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=9e573ba50f129bff7b6e1be10cf4bbcb70f1efdd

commit 9e573ba50f129bff7b6e1be10cf4bbcb70f1efdd
Author: Mark Geisert <mark@maxrnd.com>
Date:   Mon Dec 7 02:29:36 2020 -0800

    Cygwin: Allow to set SO_PEERCRED zero (v2)
    
    The existing code errors as EINVAL any attempt to set a value for
    SO_PEERCRED via setsockopt() on an AF_UNIX/AF_LOCAL socket.  But to
    enable the workaround set_no_getpeereid behavior for Python one has
    to be able to set SO_PEERCRED to zero.  Ergo, this patch.  Python has
    no way to specify a NULL pointer for 'optval'.
    
    This v2 of patch allows the original working (i.e., allow NULL,0 for
    optval,optlen to mean turn off SO_PEERCRED) in addition to the new
    working described above.  The sense of the 'if' stmt is reversed for
    readability.

Diff:
---
 winsup/cygwin/fhandler_socket_local.cc | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/fhandler_socket_local.cc b/winsup/cygwin/fhandler_socket_local.cc
index c94bf828f..964f3e819 100644
--- a/winsup/cygwin/fhandler_socket_local.cc
+++ b/winsup/cygwin/fhandler_socket_local.cc
@@ -1430,10 +1430,14 @@ fhandler_socket_local::setsockopt (int level, int optname, const void *optval,
 	     FIXME: In the long run we should find a more generic solution
 	     which doesn't require a blocking handshake in accept/connect
 	     to exchange SO_PEERCRED credentials. */
-	  if (optval || optlen)
-	    set_errno (EINVAL);
-	  else
+	  /* Temporary: Allow SO_PEERCRED to only be zeroed. Two ways to
+	     accomplish this: pass NULL,0 for optval,optlen; or pass the
+	     address,length of an '(int) 0' set up by the caller. */
+	  if ((!optval && !optlen) ||
+		(optlen == (socklen_t) sizeof (int) && !*(int *) optval))
 	    ret = af_local_set_no_getpeereid ();
+	  else
+	    set_errno (EINVAL);
 	  return ret;
 
 	case SO_REUSEADDR: