POSIX.1-2008 requires that the third argument to open(2) on
O_CREAT does not affect whether the file is open for reading,
writing, or for both. A subsequent ftruncate should not
depend on access permission bits of the file, but solely
on the read/write flags specified on the open(2).

The bug affects a mailer Postfix, which reports a
permission problem on ftruncate, affecting the smtpd
service when option speed_adjust is requested.
As the problem is in the file system violating a POSIX
specification and not in the application, it is unlikely
that the program will be modified.

Reproducible on:
  FreeBSD 8.1-RELEASE-p2, ZFS: pool version 14, ZFS version 3
as well as on:
  FreeBSD 8.2-RC3, ZFS: pool version 15, ZFS version 4

Fix: 

Fix unknown.
Work around by not using ZFS, or by allowing less strict
access permission bits on open(2) - which may be hard to achieve
if code is buried inside some application.
How-To-Repeat: Run the following test program. It will report:
  Error truncating: Permission denied
when (cwd) on a ZFS file system, but will pass clean
when on UFS or on some other file system.


#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char *argv[]) {
  const char fname[] = "truncate-posix-test.tmp";

  /* POSIX.1-2008: ( http://www.opengroup.org/onlinepubs/9699919799/ )
   * open() [...] O_CREAT [...] The file status flags and file
   * access modes of the open file description shall be set according
   * to the value of oflag. [...] The argument following the oflag
   * argument does not affect whether the file is open for reading,
   * writing, or for both.
   *
   * In other words, read/write access is controlled with the
   * O_RDWR flags, not the read/write permissions argument.
   *
   * Create a file with mode 0, i.e. all access permission bits off:
  */
  int fd = open(fname, O_CREAT|O_RDWR|O_EXCL, 0);
  if (fd < 0) { perror("Error creating file"); return 1; }

  if (unlink(fname) < 0) perror("Error unlinking");

  /* ftruncate should succeed,
   * it must not depend on access permission bits, its rights
   * should solely be governed by an O_RDWR file access flag.
   * 
   * This FAILS on a ZFS file system, reporting "Permission denied"!
   */
  if (ftruncate(fd,0) < 0) perror("Error truncating");

  if (close(fd) < 0) perror("Error closing");
  return 0;
}