Re: passfd test failure on FreeBSD 12.0

[Bruno Haible]

I wrote in <https://lists.gnu.org/archive/html/bug-gnulib/2020-12/msg00077.html>
and <https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00053.html>:
> > On FreeBSD 12.0 — but not on FreeBSD 11.0 and 12.2 —, I'm seeing a test
> > failure of the Gnulib 'passfd' test:
> > 
> >   recvfd: Permission denied
> >   FAIL test-passfd (exit status: 16)
> 
> Likewise on FreeBSD 12.1.1 and NetBSD 9.0.

I looked on which platforms the test fails:

OS             version   bitness   compiler     result

freebsd          11      32        clang 3.8     OK
freebsd          11      64        clang 3.8     OK
freebsd/arm64    11      64        clang 4.0.0   OK
freebsd          12      32        clang 6.0.1   OK
freebsd          12      64        clang 6.0.1   FAIL
freebsd/arm64    12      64        clang 10.0.1  FAIL
freebsd/sparc64  12      64        gcc 4.2.1     FAIL

midnightbsd      2.0     32        gcc 7.4       OK
midnightbsd      2.0     32        clang 8.0.1   OK
midnightbsd      2.0     64        gcc 7.4       FAIL
midnightbsd      2.0     64        clang 8.0.1   FAIL

netbsd           8       32        gcc 5.5       OK
netbsd           8       64        gcc 5.5       FAIL
netbsd/sparc64   8       64        gcc 5.5       FAIL
netbsd           9       64        gcc 7.4       FAIL

openbsd          6.0     32        gcc 4.2.1     OK
openbsd          6.0     64        gcc 4.2.1     OK
openbsd          6.5     64        gcc 4.2.1     OK
openbsd          6.5     64        clang 7.0.1   OK

Result:
* FAILs only in 64-bit mode, never in 32-bit mode
* Only certain OSes (not OpenBSD)
* Only certain OS versions (FreeBSD >= 11)
* Independent of clang vs. gcc

It smells like a wrongly computed alignment or buffer size. And indeed,
this patch fixes it:


2021-02-16  Bruno Haible  <bruno@clisp.org>

        passfd: Fix test failure on FreeBSD >= 12 and NetBSD in 64-bit mode.
        * lib/passfd.c (recvfd): Use the CMSG_SPACE macro to compute the value
        for msg_controllen.

diff --git a/lib/passfd.c b/lib/passfd.c
index ca10ba5..a993f39 100644
--- a/lib/passfd.c
+++ b/lib/passfd.c
@@ -142,19 +142,23 @@ recvfd (int sock, int flags)
   cmsg->cmsg_len = CMSG_LEN (sizeof fd);
   /* Initialize the payload: */
   memcpy (CMSG_DATA (cmsg), &fd, sizeof fd);
-  msg.msg_controllen = cmsg->cmsg_len;
+  msg.msg_controllen = CMSG_SPACE (sizeof fd);
 
   len = recvmsg (sock, &msg, flags_recvmsg);
   if (len < 0)
     return -1;
-
+  if (len == 0)
+    {
+      /* fake errno: at end the file is not available */
+      errno = ENOTCONN;
+      return -1;
+    }
   cmsg = CMSG_FIRSTHDR (&msg);
   /* be paranoiac */
-  if (len == 0 || cmsg == NULL || cmsg->cmsg_len != CMSG_LEN (sizeof fd)
+  if (cmsg == NULL || cmsg->cmsg_len != CMSG_LEN (sizeof fd)
       || cmsg->cmsg_level != SOL_SOCKET || cmsg->cmsg_type != SCM_RIGHTS)
     {
-      /* fake errno: at end the file is not available */
-      errno = len ? EACCES : ENOTCONN;
+      errno = EACCES;
       return -1;
     }