Re: [PATCH 3/4] Add a CLOEXEC recvfd

[Bruno Haible]

Hi Bastien,

> In order to avoid a race add a recvfd(int fd, int flags). flags could be 
> O_CLOEXEC.
> ---
>  lib/passfd.c   |   58 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  lib/passfd.h   |    1 +
>  m4/afunix.m4   |   22 +++++++++++++++++++++
>  modules/passfd |    1 +

This is pretty good as well. But there's no need for two functions recvfd and
recvfd2. Better merge them into a single function. It's easy for the callers
to pass a 0 flags argument. In the POSIX API, a separate function was
introduced only when the previous function already existed for a long time,
like pipe()/pipe2() and accept()/accept4().

Also, it is better to test 'flags & O_CLOEXEC' instead of 'flags == O_CLOEXEC'
because
  - The flags argument is conceptually a bit mask.
  - O_CLOEXEC is 0 on some platforms (that don't support this flag), and
    passing a 0 flags argument ought to _not_ set the close-on-exec flag.

Other than this, I applied the usual untabification and .m4 file indentation.

A possible simplification would be to assume that MSG_CMSG_CLOEXEC is a macro
when it exists, and thus simplify "#if HAVE_MSG_CMSG_CLOEXEC" to
"#if defined MSG_CMSG_CLOEXEC".


2011-03-13  Bastien Roucariès  <address@hidden>
            Bruno Haible  <address@hidden>

        passfd module, part 3.
        * lib/passfd.h (recvfd): Add a flags argument.
        * lib/passfd.c: Include <fcntl.h>, cloexec.h.
        (recvfd): Add a flags argument.
        * m4/afunix.m4 (gl_SOCKET_AFUNIX): Test whether MSG_CMSG_CLOEXEC
        exists.
        * modules/passfd (Depends-on): Add cloexec.
        Suggested by Eric Blake.

--- lib/passfd.c.orig   Sun Mar 13 16:26:20 2011
+++ lib/passfd.c        Sun Mar 13 15:53:28 2011
@@ -19,6 +19,7 @@
 #include "passfd.h"
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
@@ -35,6 +36,8 @@
 # include <winsock2.h>
 #endif
 
+#include "cloexec.h"
+
 /* sendfd sends the file descriptor fd along the socket
    to a process calling recvfd on the other end.
 
@@ -84,11 +87,12 @@
 }
 
 /* recvfd receives a file descriptor through the socket.
+   The flags are a bitmask, possibly including O_CLOEXEC (defined in 
<fcntl.h>).
 
    Return 0 on success, or -1 with errno set in case of error.
 */
 int
-recvfd (int sock)
+recvfd (int sock, int flags)
 {
   char recv = 0;
   const int mone = -1;
@@ -96,6 +100,12 @@
   struct iovec iov[1];
   struct msghdr msg;
 
+  if ((flags & ~O_CLOEXEC) != 0)
+    {
+      errno = EINVAL;
+      return -1;
+    }
+
   /* send at least one char */
   iov[0].iov_base = &recv;
   iov[0].iov_len = 1;
@@ -108,6 +118,11 @@
 #if HAVE_UNIXSOCKET_SCM_RIGHTS_BSD44_WAY
     struct cmsghdr *cmsg;
     char buf[CMSG_SPACE (sizeof (fd))];
+# if HAVE_MSG_CMSG_CLOEXEC
+    int flags_recvmsg = (flags & O_CLOEXEC ? MSG_CMSG_CLOEXEC : 0);
+# else
+    int flags_recvmsg = 0;
+# endif
 
     msg.msg_control = buf;
     msg.msg_controllen = sizeof (buf);
@@ -119,7 +134,7 @@
     memcpy (CMSG_DATA (cmsg), &mone, sizeof (mone));
     msg.msg_controllen = cmsg->cmsg_len;
 
-    if (recvmsg (sock, &msg, 0) < 0)
+    if (recvmsg (sock, &msg, flags_recvmsg) < 0)
       return -1;
 
     cmsg = CMSG_FIRSTHDR (&msg);
@@ -133,13 +148,43 @@
       }
 
     memcpy (&fd, CMSG_DATA (cmsg), sizeof (fd));
+
+# if !HAVE_MSG_CMSG_CLOEXEC
+    /* set close-on-exec flag */
+    if (flags & O_CLOEXEC)
+      {
+        if (set_cloexec_flag (fd, true) < 0)
+          {
+            int saved_errno = errno;
+            (void) close (fd);
+            errno = saved_errno;
+            return -1;
+          }
+      }
+# endif
+
     return fd;
+
 #elif HAVE_UNIXSOCKET_SCM_RIGHTS_BSD43_WAY
     msg.msg_accrights = &fd;
     msg.msg_accrightslen = sizeof (fd);
     if (recvmsg (sock, &msg, 0) < 0)
       return -1;
+
+    /* set close-on-exec flag */
+    if (flags & O_CLOEXEC)
+      {
+        if (set_cloexec_flag (fd, true) < 0)
+          {
+            int saved_errno = errno;
+            (void) close (fd);
+            errno = saved_errno;
+            return -1;
+          }
+      }
+
     return fd;
+
 #else
     errno = ENOSYS;
     return -1;
--- lib/passfd.h.orig   Sun Mar 13 16:26:20 2011
+++ lib/passfd.h        Sun Mar 13 15:43:03 2011
@@ -23,7 +23,7 @@
 #endif
 
 extern int sendfd (int sock, int fd);
-extern int recvfd (int sock);
+extern int recvfd (int sock, int flags);
 
 #ifdef __cplusplus
 }
--- m4/afunix.m4.orig   Sun Mar 13 16:26:20 2011
+++ m4/afunix.m4        Sun Mar 13 15:58:06 2011
@@ -1,4 +1,4 @@
-# afunix.m4 serial 2
+# afunix.m4 serial 3
 dnl Copyright (C) 2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -109,4 +109,31 @@
     AC_DEFINE([HAVE_UNIXSOCKET_SCM_RIGHTS_BSD43_WAY], [1],
       [Define to 1 if fd can be sent/received in the BSD4.3 way.])
   fi
+
+  AC_MSG_CHECKING([for UNIX domain sockets recvmsg() MSG_CMSG_CLOEXEC flag])
+  AC_CACHE_VAL([gl_cv_socket_unix_msg_cmsg_cloexec],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM(
+          [[#include <sys/types.h>
+            #ifdef HAVE_SYS_SOCKET_H
+            #include <sys/socket.h>
+            #endif
+            #ifdef HAVE_SYS_UN_H
+            #include <sys/un.h>
+            #endif
+            #ifdef HAVE_WINSOCK2_H
+            #include <winsock2.h>
+            #endif
+            ]],
+            [[int flags = MSG_CMSG_CLOEXEC;
+              if (&flags) return 0;
+            ]])],
+       [gl_cv_socket_unix_msg_cmsg_cloexec=yes],
+       [gl_cv_socket_unix_msg_cmsg_cloexec=no])
+    ])
+  AC_MSG_RESULT([$gl_cv_socket_unix_msg_cmsg_cloexec])
+  if test $gl_cv_socket_unix_msg_cmsg_cloexec = yes; then
+    AC_DEFINE([HAVE_MSG_CMSG_CLOEXEC], [1],
+      [Define to 1 if recvmsg could be specified with MSG_CMSG_CLOEXEC.])
+  fi
 ])
--- modules/passfd.orig Sun Mar 13 16:26:20 2011
+++ modules/passfd      Sun Mar 13 16:03:40 2011
@@ -8,6 +8,7 @@
 m4/sockpfaf.m4
 
 Depends-on:
+cloexec
 sys_socket
 extensions
 
-- 
In memoriam Odette Sansom <http://en.wikipedia.org/wiki/Odette_Hallowes>