[newlib-cygwin] Cygwin: fix fchmod on sockets that are not socket files

[Ken Brown]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=1a9ceab5dd64bc1689b5c48e9a74938b5ee24dcb

commit 1a9ceab5dd64bc1689b5c48e9a74938b5ee24dcb
Author: Ken Brown <kbrown@cornell.edu>
Date:   Mon Feb 22 09:34:36 2021 -0500

    Cygwin: fix fchmod on sockets that are not socket files
    
    If fchmod(2) is called on an AF_LOCAL or AF_UNIX socket that is not a
    socket file, the current code calls fhandler_disk_file::fchmod 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::fchmod only if the
    fhandler_socket object is a file (determined by testing dev().isfs()).

Diff:
---
 winsup/cygwin/fhandler_socket_local.cc | 6 +++++-
 winsup/cygwin/fhandler_socket_unix.cc  | 8 +++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/winsup/cygwin/fhandler_socket_local.cc b/winsup/cygwin/fhandler_socket_local.cc
index 5ca6d8550..d1faa079a 100644
--- a/winsup/cygwin/fhandler_socket_local.cc
+++ b/winsup/cygwin/fhandler_socket_local.cc
@@ -710,8 +710,12 @@ fhandler_socket_local::fstatvfs (struct statvfs *sfs)
 int
 fhandler_socket_local::fchmod (mode_t newmode)
 {
-  if (get_sun_path () && get_sun_path ()[0] == '\0')
+  if (!dev ().isfs ())
+    /* fchmod called on a socket. */
     return fhandler_socket_wsock::fchmod (newmode);
+
+  /* chmod on a socket file.  [We won't get here if fchmod is called
+     on a socket opened w/ O_PATH.] */
   fhandler_disk_file fh (pc);
   fh.get_device () = FH_FS;
   return fh.fchmod (S_IFSOCK | adjust_socket_file_mode (newmode));
diff --git a/winsup/cygwin/fhandler_socket_unix.cc b/winsup/cygwin/fhandler_socket_unix.cc
index 06db929ed..e08e9bdd9 100644
--- a/winsup/cygwin/fhandler_socket_unix.cc
+++ b/winsup/cygwin/fhandler_socket_unix.cc
@@ -2374,10 +2374,12 @@ fhandler_socket_unix::fstatvfs (struct statvfs *sfs)
 int
 fhandler_socket_unix::fchmod (mode_t newmode)
 {
-  if (sun_path ()
-      && (sun_path ()->un_len <= (socklen_t) sizeof (sa_family_t)
-	  || sun_path ()->un.sun_path[0] == '\0'))
+  if (!dev ().isfs ())
+    /* fchmod called on a socket. */
     return fhandler_socket::fchmod (newmode);
+
+  /* chmod on a socket file.  [We won't get here if fchmod is called
+     on a socket opened w/ O_PATH.] */
   fhandler_disk_file fh (pc);
   fh.get_device () = FH_FS;
   /* Kludge: Don't allow to remove read bit on socket files for