Linux: Simplify __opensock and fix race condition [BZ #28353]

AF_NETLINK support is not quite optional on modern Linux systems
anymore, so it is likely that the first attempt will always succeed.
Consequently, there is no need to cache the result.  Keep AF_UNIX
and the Internet address families as a fallback, for the rare case
that AF_NETLINK is missing.  The other address families previously
probed are totally obsolete be now, so remove them.

Use this simplified version as the generic implementation, disabling
Netlink support as needed.

1 files changed, 22 insertions, 43 deletions

diff --git a/socket/opensock.c b/socket/opensock.c
index 37148d4..ff94d27 100644
--- a/socket/opensock.c
+++ b/socket/opensock.c
@@ -1,4 +1,5 @@
-/* Copyright (C) 1999-2021 Free Software Foundation, Inc.
+/* Create socket with an unspecified address family for use with ioctl.
+   Copyright (C) 1999-2021 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -15,56 +16,34 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <stdio.h>
+#include <errno.h>
 #include <sys/socket.h>
-#include <libc-lock.h>
 
 /* Return a socket of any type.  The socket can be used in subsequent
    ioctl calls to talk to the kernel.  */
 int
 __opensock (void)
 {
-  /* Cache the last AF that worked, to avoid many redundant calls to
-     socket().  */
-  static int sock_af = -1;
-  int fd = -1;
-  __libc_lock_define_initialized (static, lock);
-
-  if (sock_af != -1)
-    {
-      fd = __socket (sock_af, SOCK_DGRAM, 0);
-      if (fd != -1)
-        return fd;
-    }
-
-  __libc_lock_lock (lock);
-
-  if (sock_af != -1)
-    fd = __socket (sock_af, SOCK_DGRAM, 0);
-
-  if (fd == -1)
-    {
-#ifdef AF_INET
-      fd = __socket (sock_af = AF_INET, SOCK_DGRAM, 0);
-#endif
-#ifdef AF_INET6
-      if (fd < 0)
-	fd = __socket (sock_af = AF_INET6, SOCK_DGRAM, 0);
-#endif
-#ifdef AF_IPX
-      if (fd < 0)
-	fd = __socket (sock_af = AF_IPX, SOCK_DGRAM, 0);
-#endif
-#ifdef AF_AX25
-      if (fd < 0)
-	fd = __socket (sock_af = AF_AX25, SOCK_DGRAM, 0);
-#endif
-#ifdef AF_APPLETALK
-      if (fd < 0)
-	fd = __socket (sock_af = AF_APPLETALK, SOCK_DGRAM, 0);
+  /* SOCK_DGRAM is supported by all address families.  (Netlink does
+     not support SOCK_STREAM.)  */
+  int type = SOCK_DGRAM | SOCK_CLOEXEC;
+  int fd;
+
+#ifdef AF_NETLINK
+  fd = __socket (AF_NETLINK, type, 0);
+  if (fd >= 0)
+    return fd;
 #endif
-    }
 
-  __libc_lock_unlock (lock);
+  fd = __socket (AF_UNIX, type, 0);
+  if (fd >= 0)
+    return fd;
+  fd = __socket (AF_INET, type, 0);
+  if (fd >= 0)
+    return fd;
+  fd = __socket (AF_INET6, type, 0);
+  if (fd >= 0)
+    return fd;
+  __set_errno (ENOENT);
   return fd;
 }