Petter Reinholdtsen

Broken umask handling with sshfs
26th August 2010

My file system sematics program presented a few days ago is very useful to verify that a file system can work as a unix home directory,and today I had to extend it a bit. I'm looking into alternatives for home directory access here at the University of Oslo, and one of the options is sshfs. My friend Finn-Arne mentioned a while back that they had used sshfs with Debian Edu, but stopped because of problems. I asked today what the problems where, and he mentioned that sshfs failed to handle umask properly. Trying to detect the problem I wrote this addition to my fs testing script:

mode_t touch_get_mode(const char *name, mode_t mode) {
  mode_t retval = 0;
  int fd = open(name, O_RDWR|O_CREAT|O_LARGEFILE, mode);
  if (-1 != fd) {
    unlink(name);
    struct stat statbuf;
    if (-1 != fstat(fd, &statbuf)) {
      retval = statbuf.st_mode & 0x1ff;
    }
    close(fd);
  }
  return retval;
}

/* Try to detect problem discovered using sshfs */
int test_umask(void) {
  printf("info: testing umask effect on file creation\n");

  mode_t orig_umask = umask(000);
  mode_t newmode;
  if (0666 != (newmode = touch_get_mode("foobar", 0666))) {
    printf("  error: Wrong file mode %o when creating using mode 666 and umask 000\n",
           newmode);
  }
  umask(007);
  if (0660 != (newmode = touch_get_mode("foobar", 0666))) {
    printf("  error: Wrong file mode %o when creating using mode 666 and umask 007\n",
           newmode);
  }

  umask (orig_umask);
  return 0;
}

int main(int argc, char **argv) {
  [...]
  test_umask();
  return 0;
}

Sure enough. On NFS to a netapp, I get this result:

Testing POSIX/Unix sematics on file system
info: testing symlink creation
info: testing subdirectory creation
info: testing fcntl locking
  Read-locking 1 byte from 1073741824
  Read-locking 510 byte from 1073741826
  Unlocking 1 byte from 1073741824
  Write-locking 1 byte from 1073741824
  Write-locking 510 byte from 1073741826
  Unlocking 2 byte from 1073741824
info: testing umask effect on file creation

When mounting the same directory using sshfs, I get this result:

Testing POSIX/Unix sematics on file system
info: testing symlink creation
info: testing subdirectory creation
info: testing fcntl locking
  Read-locking 1 byte from 1073741824
  Read-locking 510 byte from 1073741826
  Unlocking 1 byte from 1073741824
  Write-locking 1 byte from 1073741824
  Write-locking 510 byte from 1073741826
  Unlocking 2 byte from 1073741824
info: testing umask effect on file creation
  error: Wrong file mode 644 when creating using mode 666 and umask 000
  error: Wrong file mode 640 when creating using mode 666 and umask 007

So, I can conclude that sshfs is better than smb to a Netapp or a Windows server, but not good enough to be used as a home directory.

Update 2010-08-26: Reported the issue in BTS report #594498

Update 2010-08-27: Michael Gebetsroither report that he found the script so useful that he created a GIT repository and stored it in http://github.com/gebi/fs-test.

Tags: debian edu, english, nuug.

Created by Chronicle v4.6