[newlib-cygwin] Cygwin: fix linkat(2) on sockets that are not socket files

[Ken Brown <kbrown@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=425203384a6c3e19af782c86570740464423eb4d

commit 425203384a6c3e19af782c86570740464423eb4d
Author: Ken Brown <kbrown@cornell.edu>
Date:   Wed Feb 24 10:43:15 2021 -0500

    Cygwin: fix linkat(2) on sockets that are not socket files
    
    If linkat(2) is called with AT_EMPTY_PATH on an AF_LOCAL or
    AF_UNIX socket that is not a socket file, the current code calls
    fhandler_disk_file::link in most cases.  The latter expects to be
    operating on a disk file and uses the socket's io_handle, which
    is not a file handle.
    
    Fix this by calling fhandler_disk_file::link only if the
    fhandler_socket object is a file (determined by testing
    dev().isfs()).
    
    Also fix the case of a socket file opened with O_PATH by setting
    the fhandler_disk_file's io_handle.

Diff:
---
 winsup/cygwin/fhandler_socket_local.cc | 7 ++++++-
 winsup/cygwin/fhandler_socket_unix.cc  | 9 ++++++---
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/winsup/cygwin/fhandler_socket_local.cc b/winsup/cygwin/fhandler_socket_local.cc
index 22586c0dd..ad7dd0a98 100644
--- a/winsup/cygwin/fhandler_socket_local.cc
+++ b/winsup/cygwin/fhandler_socket_local.cc
@@ -750,9 +750,14 @@ fhandler_socket_local::facl (int cmd, int nentries, aclent_t *aclbufp)
 int
 fhandler_socket_local::link (const char *newpath)
 {
-  if (get_sun_path () && get_sun_path ()[0] == '\0')
+  if (!dev ().isfs ())
+    /* linkat w/ AT_EMPTY_PATH called on a socket not opened w/ O_PATH. */
     return fhandler_socket_wsock::link (newpath);
+  /* link on a socket file or linkat w/ AT_EMPTY_PATH called on a
+     socket opened w/ O_PATH. */
   fhandler_disk_file fh (pc);
+  if (get_flags () & O_PATH)
+    fh.set_handle (get_handle ());
   return fh.link (newpath);
 }
 
diff --git a/winsup/cygwin/fhandler_socket_unix.cc b/winsup/cygwin/fhandler_socket_unix.cc
index fae07367d..252bcd9a9 100644
--- a/winsup/cygwin/fhandler_socket_unix.cc
+++ b/winsup/cygwin/fhandler_socket_unix.cc
@@ -2421,11 +2421,14 @@ fhandler_socket_unix::facl (int cmd, int nentries, aclent_t *aclbufp)
 int
 fhandler_socket_unix::link (const char *newpath)
 {
-  if (sun_path ()
-      && (sun_path ()->un_len <= (socklen_t) sizeof (sa_family_t)
-	  || sun_path ()->un.sun_path[0] == '\0'))
+  if (!dev ().isfs ())
+    /* linkat w/ AT_EMPTY_PATH called on a socket not opened w/ O_PATH. */
     return fhandler_socket::link (newpath);
+  /* link on a socket file or linkat w/ AT_EMPTY_PATH called on a
+     socket opened w/ O_PATH. */
   fhandler_disk_file fh (pc);
+  if (get_flags () & O_PATH)
+    fh.set_handle (get_handle ());
   return fh.link (newpath);
 }