gpg: Fix export porting of zero length user ID packets.

* g10/build-packet.c (do_user_id): Avoid indeterminate length header.
--

We are able to import such user ids but when exporting them the
exported data could not be imported again because the parser bails out
on invalid keyrings.  This is now fixed and should be backported.

Signed-off-by: Werner Koch <wk@gnupg.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/g10/build-packet.c b/g10/build-packet.c
index 60e7d45e8..1ee57e009 100644
--- a/g10/build-packet.c
+++ b/g10/build-packet.c
@@ -415,18 +415,26 @@ static int
 do_user_id( IOBUF out, int ctb, PKT_user_id *uid )
 {
   int rc;
+  int hdrlen;
 
   log_assert (ctb_pkttype (ctb) == PKT_USER_ID
               || ctb_pkttype (ctb) == PKT_ATTRIBUTE);
 
+  /* We need to take special care that doe user ID with a length of 0:
+   * Without forcing HDRLEN to 2 in this case an indeterminate length
+   * packet would be written which is not allowed.  Note that we are
+   * always called with a CTB indicating an old packet header format,
+   * so that forcing a 2 octet header works.  */
   if (uid->attrib_data)
     {
-      write_header(out, ctb, uid->attrib_len);
+      hdrlen = uid->attrib_len? 0 : 2;
+      write_header2 (out, ctb, uid->attrib_len, hdrlen);
       rc = iobuf_write( out, uid->attrib_data, uid->attrib_len );
     }
   else
     {
-      write_header2( out, ctb, uid->len, 0 );
+      hdrlen = uid->len? 0 : 2;
+      write_header2 (out, ctb, uid->len, hdrlen);
       rc = iobuf_write( out, uid->name, uid->len );
     }
   return rc;