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

   1 Title: Testing if a file system can be used for home directories...
   2 Tags: english, nuug, debian edu
   3 Date: 2010-08-08 21:20
   4
   5 <p>A few years ago, I was involved in a project planning to use
   6 Windows file servers as home directory servers for Debian
   7 Edu/Skolelinux machines.  This was thought to be no problem, as the
   8 access would be through the SMB network file system protocol, and we
   9 knew other sites used SMB with unix and samba as the file server to
  10 mount home directories without any problems.  But, after months of
  11 struggling, we had to conclude that our goal was impossible.</p>
  12
  13 <p>The reason is simply that while SMB can be used for home
  14 directories when the file server is Samba running on Unix, this only
  15 work because of Samba have some extensions and the fact that the
  16 underlying file system is a unix file system.  When using a Windows
  17 file server, the underlying file system do not have POSIX semantics,
  18 and several programs will fail if the users home directory where they
  19 want to store their configuration lack POSIX semantics.</p>
  20
  21 <p>As part of this work, I wrote a small C program I want to share
  22 with you all, to replicate a few of the problematic applications (like
  23 OpenOffice.org and GCompris) and see if the file system was working as
  24 it should.  If you find yourself in spooky file system land, it might
  25 help you find your way out again.  This is the fs-test.c source:</p>
  26
  27 <pre>
  28 /*
  29  * Some tests to check the file system sematics.  Used to verify that
  30  * CIFS from a windows server do not work properly as a linux home
  31  * directory.
  32  * License: GPL v2 or later
  33  *
  34  * needs libsqlite3-dev and build-essential installed
  35  * compile with: gcc -Wall -lsqlite3 -DTEST_SQLITE fs-test.c -o fs-test
  36 */
  37
  38 #define _FILE_OFFSET_BITS 64
  39 #define _LARGEFILE_SOURCE 1
  40 #define _LARGEFILE64_SOURCE 1
  41
  42 #define _GNU_SOURCE /* for asprintf() */
  43
  44 #include &lt;errno.h>
  45 #include &lt;fcntl.h>
  46 #include &lt;stdio.h>
  47 #include &lt;string.h>
  48 #include &lt;stdlib.h>
  49 #include &lt;sys/file.h>
  50 #include &lt;sys/stat.h>
  51 #include &lt;sys/types.h>
  52 #include &lt;unistd.h>
  53
  54 #ifdef TEST_SQLITE
  55 /*
  56  * Test sqlite open, as done by gcompris require the libsqlite3-dev
  57  * package and linking with -lsqlite3.  A more low level test is
  58  * below.
  59  * See also &lt;URL: http://www.sqlite.org./faq.html#q5 >.
  60  */
  61 #include &lt;sqlite3.h>
  62 #define CREATE_TABLE_USERS                                              \
  63   "CREATE TABLE users (user_id INT UNIQUE, login TEXT, lastname TEXT, firstname TEXT, birthdate TEXT, class_id INT ); "
  64 int test_sqlite_open(void) {
  65   char *zErrMsg;
  66   char *name = "testsqlite.db";
  67   sqlite3 *db=NULL;
  68   unlink(name);
  69   int rc = sqlite3_open(name, &db);
  70   if( rc ){
  71     printf("error: sqlite open of %s failed: %s\n", name, sqlite3_errmsg(db));
  72     sqlite3_close(db);
  73     return -1;
  74   }
  75
  76   /* create tables */
  77   rc = sqlite3_exec(db,CREATE_TABLE_USERS, NULL,  0, &zErrMsg);
  78   if( rc != SQLITE_OK ){
  79     printf("error: sqlite table create failed: %s\n", zErrMsg);
  80     sqlite3_close(db);
  81     return -1;
  82   }
  83   printf("info: sqlite worked\n");
  84   sqlite3_close(db);
  85   return 0;
  86 }
  87 #endif /* TEST_SQLITE */
  88
  89 /*
  90  * Demonstrate locking issue found in gcompris using sqlite3.  This
  91  * work with ext3, but not with cifs server on Windows 2003.  This is
  92  * done in the sqlite3 library.
  93  * See also
  94  * &lt;URL:http://www.cygwin.com/ml/cygwin/2001-08/msg00854.html> and the
  95  * POSIX specification
  96  * &lt;URL:http://www.opengroup.org/onlinepubs/009695399/functions/fcntl.html>.
  97  */
  98 int test_gcompris_locking(void) {
  99   struct flock fl;
 100   char *name = "testsqlite.db";
 101   unlink(name);
 102   int fd = open(name, O_RDWR|O_CREAT|O_LARGEFILE, 0644);
 103   printf("info: testing fcntl locking\n");
 104
 105   fl.l_whence = SEEK_SET;
 106   fl.l_pid    = getpid();
 107   printf("  Read-locking 1 byte from 1073741824");
 108   fl.l_start  = 1073741824;
 109   fl.l_len    = 1;
 110   fl.l_type   = F_RDLCK;
 111   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 112
 113   printf("  Read-locking 510 byte from 1073741826");
 114   fl.l_start  = 1073741826;
 115   fl.l_len    = 510;
 116   fl.l_type   = F_RDLCK;
 117   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 118
 119   printf("  Unlocking 1 byte from 1073741824");
 120   fl.l_start  = 1073741824;
 121   fl.l_len    = 1;
 122   fl.l_type   = F_UNLCK;
 123   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 124
 125   printf("  Write-locking 1 byte from 1073741824");
 126   fl.l_start  = 1073741824;
 127   fl.l_len    = 1;
 128   fl.l_type   = F_WRLCK;
 129   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 130
 131   printf("  Write-locking 510 byte from 1073741826");
 132   fl.l_start  = 1073741826;
 133   fl.l_len    = 510;
 134   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 135
 136   printf("  Unlocking 2 byte from 1073741824");
 137   fl.l_start  = 1073741824;
 138   fl.l_len    = 2;
 139   fl.l_type   = F_UNLCK;
 140   if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
 141
 142   close(fd);
 143   return 0;
 144 }
 145
 146 /*
 147  * Test if permissions of freshly created directories allow entries
 148  * below them.  This was a problem with OpenOffice.org and gcompris.
 149  * Mounting with option 'sync' seem to solve this problem while
 150  * slowing down file operations.
 151  */
 152 int test_subdirectory_creation(void) {
 153 #define LEVELS 5
 154   char *path = strdup("test");
 155   char *dirs[LEVELS];
 156   int level;
 157   printf("info: testing subdirectory creation\n");
 158   for (level = 0; level &lt; LEVELS; level++) {
 159     char *newpath = NULL;
 160     if (-1 == mkdir(path, 0777)) {
 161       printf("  error: Unable to create directory '%s': %s\n",
 162              path, strerror(errno));
 163       break;
 164     }
 165     asprintf(&newpath, "%s/%s", path, "test");
 166     free(path);
 167     path = newpath;
 168   }
 169   return 0;
 170 }
 171
 172 /*
 173  * Test if symlinks can be created.  This was a problem detected with
 174  * KDE.
 175  */
 176 int test_symlinks(void) {
 177   printf("info: testing symlink creation\n");
 178   unlink("symlink");
 179   if (-1 == symlink("file", "symlink"))
 180     printf("  error: Unable to create symlink\n");
 181   return 0;
 182 }
 183
 184 int main(int argc, char **argv) {
 185   printf("Testing POSIX/Unix sematics on file system\n");
 186   test_symlinks();
 187   test_subdirectory_creation();
 188 #ifdef TEST_SQLITE
 189   test_sqlite_open();
 190 #endif /* TEST_SQLITE */
 191   test_gcompris_locking();
 192   return 0;
 193 }
 194 </pre>
 195
 196 <p>When everything is working, it should print something like
 197 this:</p>
 198
 199 <pre>
 200 Testing POSIX/Unix sematics on file system
 201 info: testing symlink creation
 202 info: testing subdirectory creation
 203 info: sqlite worked
 204 info: testing fcntl locking
 205   Read-locking 1 byte from 1073741824
 206   Read-locking 510 byte from 1073741826
 207   Unlocking 1 byte from 1073741824
 208   Write-locking 1 byte from 1073741824
 209   Write-locking 510 byte from 1073741826
 210   Unlocking 2 byte from 1073741824
 211 </pre>
 212
 213 <p>I do not remember the exact details of the problems we saw, but one
 214 of them was with locking, where if I remember correctly, POSIX allow a
 215 read-only lock to be upgraded to a read-write lock without unlocking
 216 the read-only lock (while Windows do not).  Another was a bug in the
 217 CIFS/SMB client implementation in the Linux kernel where directory
 218 meta information would be wrong for a fraction of a second, making
 219 OpenOffice.org fail to create its deep directory tree because it was
 220 not allowed to create files in its freshly created directory.</p>
 221
 222 <p>Anyway, here is a nice tool for your tool box, might you never need
 223 it. :)</p>
 224
 225 <p>Update 2010-08-27: Michael Gebetsroither report that he found the
 226 script so useful that he created a GIT repository and stored it in
 227 <a href="http://github.com/gebi/fs-test">http://github.com/gebi/fs-test</a>.</p>