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

[Florian Weimer <fweimer@redhat.com>]

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 fixing.  The other address families previously
probed are totally obsolete be now, so remove them.

---
Tested on i686-linux-gnu and x86_64-linux-gnu (albeit with all four
address families present).  Initial build-many-glibcs.py results look
okay as well.

 sysdeps/unix/sysv/linux/opensock.c | 103 ++++++-------------------------------
 1 file changed, 16 insertions(+), 87 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/opensock.c b/sysdeps/unix/sysv/linux/opensock.c
index e87d6e58b0..47464d3a30 100644
--- a/sysdeps/unix/sysv/linux/opensock.c
+++ b/sysdeps/unix/sysv/linux/opensock.c
@@ -15,11 +15,7 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <assert.h>
 #include <errno.h>
-#include <stdio.h>
-#include <string.h>
-#include <unistd.h>
 #include <sys/socket.h>
 
 /* Return a socket of any type.  The socket can be used in subsequent
@@ -27,88 +23,21 @@
 int
 __opensock (void)
 {
-  static int last_family;	/* Available socket family we will use.  */
-  static int last_type;
-  static const struct
-  {
-    int family;
-    const char procname[15];
-  } afs[] =
-    {
-      { AF_UNIX, "net/unix" },
-      { AF_INET, "" },
-      { AF_INET6, "net/if_inet6" },
-      { AF_AX25, "net/ax25" },
-      { AF_NETROM, "net/nr" },
-      { AF_ROSE, "net/rose" },
-      { AF_IPX, "net/ipx" },
-      { AF_APPLETALK, "net/appletalk" },
-      { AF_ECONET, "sys/net/econet" },
-      { AF_ASH, "sys/net/ash" },
-      { AF_X25, "net/x25" },
-#ifdef NEED_AF_IUCV
-      { AF_IUCV, "net/iucv" }
-#endif
-    };
-#define nafs (sizeof (afs) / sizeof (afs[0]))
-  char fname[sizeof "/proc/" + 14];
-  int result;
-  int has_proc;
-  size_t cnt;
-
-  /* We already know which family to use from the last call.  Use it
-     again.  */
-  if (last_family != 0)
-    {
-      assert (last_type != 0);
-
-      result = __socket (last_family, last_type | SOCK_CLOEXEC, 0);
-      if (result != -1 || errno != EAFNOSUPPORT)
-	/* Maybe the socket type isn't supported anymore (module is
-	   unloaded).  In this case again try to find the type.  */
-	return result;
-
-      /* Reset the values.  They seem not valid anymore.  */
-      last_family = 0;
-      last_type = 0;
-    }
-
-  /* Check whether the /proc filesystem is available.  */
-  has_proc = __access ("/proc/net", R_OK) != -1;
-  strcpy (fname, "/proc/");
-
-  /* Iterate over the interface families and find one which is
-     available.  */
-  for (cnt = 0; cnt < nafs; ++cnt)
-    {
-      int type = SOCK_DGRAM;
-
-      if (has_proc && afs[cnt].procname[0] != '\0')
-	{
-	  strcpy (fname + 6, afs[cnt].procname);
-	  if (__access (fname, R_OK) == -1)
-	    /* The /proc entry is not available.  I.e., we cannot
-	       create a socket of this type (without loading the
-	       module).  Don't look for it since this might trigger
-	       loading the module.  */
-	    continue;
-	}
-
-      if (afs[cnt].family == AF_NETROM || afs[cnt].family == AF_X25)
-	type = SOCK_SEQPACKET;
-
-      result = __socket (afs[cnt].family, type | SOCK_CLOEXEC, 0);
-      if (result != -1)
-	{
-	  /* Found an available family.  */
-	  last_type = type;
-	  last_family = afs[cnt].family;
-	  return result;
-	}
-    }
-
-  /* None of the protocol families is available.  It is unclear what kind
-     of error is returned.  ENOENT seems like a reasonable choice.  */
+  /* SOCK_DGRAM is supported by all address families.  (Netlink does
+     not support SOCK_STREAM.)  */
+  int type = SOCK_DGRAM | SOCK_CLOEXEC;
+  int fd = __socket (AF_NETLINK, type, 0);
+  if (fd >= 0)
+    return fd;
+  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 -1;
+  return fd;
 }