blog/data/2010-08-26-fs-sematics.txt

   1 Title: Broken umask handling with sshfs
   2 Tags: english, nuug, debian edu
   3 Date: 2010-08-26 13:30
   4
   5 <p>My file system sematics program
   6 <a href="http://people.skolelinux.org/pere/blog/Testing_if_a_file_system_can_be_used_for_home_directories___.html">presented
   7 a few days ago</a> is very useful to verify that a file system can
   8 work as a unix home directory,and today I had to extend it a bit.  I'm
   9 looking into alternatives for home directory access here at the
  10 University of Oslo, and one of the options is sshfs.  My friend
  11 Finn-Arne mentioned a while back that they had used sshfs with Debian
  12 Edu, but stopped because of problems.  I asked today what the problems
  13 where, and he mentioned that sshfs failed to handle umask properly.
  14 Trying to detect the problem I wrote this addition to my fs testing
  15 script:</p>
  16
  17 <pre>
  18 mode_t touch_get_mode(const char *name, mode_t mode) {
  19   mode_t retval = 0;
  20   int fd = open(name, O_RDWR|O_CREAT|O_LARGEFILE, mode);
  21   if (-1 != fd) {
  22     unlink(name);
  23     struct stat statbuf;
  24     if (-1 != fstat(fd, &statbuf)) {
  25       retval = statbuf.st_mode & 0x1ff;
  26     }
  27     close(fd);
  28   }
  29   return retval;
  30 }
  31
  32 /* Try to detect problem discovered using sshfs */
  33 int test_umask(void) {
  34   printf("info: testing umask effect on file creation\n");
  35
  36   mode_t orig_umask = umask(000);
  37   mode_t newmode;
  38   if (0666 != (newmode = touch_get_mode("foobar", 0666))) {
  39     printf("  error: Wrong file mode %o when creating using mode 666 and umask 000\n",
  40            newmode);
  41   }
  42   umask(007);
  43   if (0660 != (newmode = touch_get_mode("foobar", 0666))) {
  44     printf("  error: Wrong file mode %o when creating using mode 666 and umask 007\n",
  45            newmode);
  46   }
  47
  48   umask (orig_umask);
  49   return 0;
  50 }
  51
  52 int main(int argc, char **argv) {
  53   [...]
  54   test_umask();
  55   return 0;
  56 }
  57 </pre>
  58
  59 <p>Sure enough.  On NFS to a netapp, I get this result:</p>
  60
  61 <pre>
  62 Testing POSIX/Unix sematics on file system
  63 info: testing symlink creation
  64 info: testing subdirectory creation
  65 info: testing fcntl locking
  66   Read-locking 1 byte from 1073741824
  67   Read-locking 510 byte from 1073741826
  68   Unlocking 1 byte from 1073741824
  69   Write-locking 1 byte from 1073741824
  70   Write-locking 510 byte from 1073741826
  71   Unlocking 2 byte from 1073741824
  72 info: testing umask effect on file creation
  73 </pre>
  74
  75 <p>When mounting the same directory using sshfs, I get this
  76 result:</p>
  77
  78 <pre>
  79 Testing POSIX/Unix sematics on file system
  80 info: testing symlink creation
  81 info: testing subdirectory creation
  82 info: testing fcntl locking
  83   Read-locking 1 byte from 1073741824
  84   Read-locking 510 byte from 1073741826
  85   Unlocking 1 byte from 1073741824
  86   Write-locking 1 byte from 1073741824
  87   Write-locking 510 byte from 1073741826
  88   Unlocking 2 byte from 1073741824
  89 info: testing umask effect on file creation
  90   error: Wrong file mode 644 when creating using mode 666 and umask 000
  91   error: Wrong file mode 640 when creating using mode 666 and umask 007
  92 </pre>
  93
  94 <p>So, I can conclude that sshfs is better than smb to a Netapp or a
  95 Windows server, but not good enough to be used as a home
  96 directory.</p>
  97
  98 <p>Update 2010-08-26: Reported the issue in
  99 <a href="http://bugs.debian.org/594498">BTS report #594498</a></p>
 100
 101 <p>Update 2010-08-27: Michael Gebetsroither report that he found the
 102 script so useful that he created a GIT repository and stored it in
 103 <a href="http://github.com/gebi/fs-test">http://github.com/gebi/fs-test</a>.</p>