From: Petter Reinholdtsen Date: Sun, 8 Aug 2010 19:17:03 +0000 (+0000) Subject: New post. X-Git-Url: https://pere.pagekite.me/gitweb/homepage.git/commitdiff_plain/02bfc320cc764ab2b8fdb63b48b6dc53013ea3e7?ds=sidebyside New post. --- diff --git a/blog/data/2010-08-08-fs-sematics.txt b/blog/data/2010-08-08-fs-sematics.txt new file mode 100644 index 0000000000..0c1d4045f8 --- /dev/null +++ b/blog/data/2010-08-08-fs-sematics.txt @@ -0,0 +1,223 @@ +Title: Testing if a file system can be used for home directories... +Tags: english, nuug, debian edu +Date: 2010-08-08 21:20 + +

A few years ago, I was involved in a project planning to use +Windows file servers as home directory servers for Skolelinux +machines. This was thought to be no problem, as the access would be +through the SMB network file system protocol, and we knew other sites +used SMB with unix and samba as the file server to mount home +directories without any problems. But, after months of struggling, we +had to conclude that our goal was impossible.

+ +

The reason is simply that while SMB can be used for home +directories when the file server is Samba running on Unix, this only +work because of Samba have some extensions and the fact that the +underlying file system is a unix file system. When using a Windows +file server, the underlying file system do not have POSIX semantics, +and several programs will fail if the users home directory where they +want to store their configuration lack POSIX semantics.

+ +

As part of this work, I wrote a small C program I want to share +with you all, to replicate a few of the problematic applications (like +OpenOffice.org and GCompris) and see if the file system was working as +it should. If you find yourself in spooky file system land, it might +help you find your way out again. This is the fs-test.c source:

+ +
+/*
+ * Some tests to check the file system sematics.  Used to verify that
+ * CIFS from a windows server do not work properly as a linux home
+ * directory.
+ * License: GPL v2 or later
+ * 
+ * needs libsqlite3-dev and build-essential installed
+ * compile with: gcc -Wall -lsqlite3 -DTEST_SQLITE fs-test.c -o fs-test
+*/
+
+#define _FILE_OFFSET_BITS 64
+#define _LARGEFILE_SOURCE 1
+#define _LARGEFILE64_SOURCE 1
+
+#define _GNU_SOURCE /* for asprintf() */
+
+#include 
+#include 
+#include 
+#include 
+#include 
+#include 
+#include 
+#include 
+#include 
+
+#ifdef TEST_SQLITE
+/*
+ * Test sqlite open, as done by gcompris require the libsqlite3-dev
+ * package and linking with -lsqlite3.  A more low level test is
+ * below.
+ * See also .
+ */
+#include 
+#define CREATE_TABLE_USERS                                              \
+  "CREATE TABLE users (user_id INT UNIQUE, login TEXT, lastname TEXT, firstname TEXT, birthdate TEXT, class_id INT ); "
+int test_sqlite_open(void) {
+  char *zErrMsg;
+  char *name = "testsqlite.db";
+  sqlite3 *db=NULL;
+  unlink(name);
+  int rc = sqlite3_open(name, &db);
+  if( rc ){
+    printf("error: sqlite open of %s failed: %s\n", name, sqlite3_errmsg(db));
+    sqlite3_close(db);
+    return -1;
+  }
+
+  /* create tables */
+  rc = sqlite3_exec(db,CREATE_TABLE_USERS, NULL,  0, &zErrMsg);
+  if( rc != SQLITE_OK ){
+    printf("error: sqlite table create failed: %s\n", zErrMsg);
+    sqlite3_close(db);
+    return -1;
+  }
+  printf("info: sqlite worked\n");
+  sqlite3_close(db);
+  return 0;
+}
+#endif /* TEST_SQLITE */
+
+/*
+ * Demonstrate locking issue found in gcompris using sqlite3.  This
+ * work with ext3, but not with cifs server on Windows 2003.  This is
+ * done in the sqlite3 library.
+ * See also
+ *  and the
+ * POSIX specification
+ * .
+ */
+int test_gcompris_locking(void) {
+  struct flock fl;
+  char *name = "testsqlite.db";
+  unlink(name);
+  int fd = open(name, O_RDWR|O_CREAT|O_LARGEFILE, 0644);
+  printf("info: testing fcntl locking\n");
+
+  fl.l_whence = SEEK_SET;
+  fl.l_pid    = getpid();
+  printf("  Read-locking 1 byte from 1073741824");
+  fl.l_start  = 1073741824;
+  fl.l_len    = 1;
+  fl.l_type   = F_RDLCK;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  printf("  Read-locking 510 byte from 1073741826");
+  fl.l_start  = 1073741826;
+  fl.l_len    = 510;
+  fl.l_type   = F_RDLCK;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  printf("  Unlocking 1 byte from 1073741824");
+  fl.l_start  = 1073741824;
+  fl.l_len    = 1;
+  fl.l_type   = F_UNLCK;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  printf("  Write-locking 1 byte from 1073741824");
+  fl.l_start  = 1073741824;
+  fl.l_len    = 1;
+  fl.l_type   = F_WRLCK;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  printf("  Write-locking 510 byte from 1073741826");
+  fl.l_start  = 1073741826;
+  fl.l_len    = 510;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  printf("  Unlocking 2 byte from 1073741824");
+  fl.l_start  = 1073741824;
+  fl.l_len    = 2;
+  fl.l_type   = F_UNLCK;
+  if (0 != fcntl(fd, F_SETLK, &fl) ) printf(" - error!\n"); else printf("\n");
+
+  close(fd);
+  return 0;
+}
+
+/*
+ * Test if permissions of freshly created directories allow entries
+ * below them.  This was a problem with OpenOffice.org and gcompris.
+ * Mounting with option 'sync' seem to solve this problem while
+ * slowing down file operations.
+ */
+int test_subdirectory_creation(void) {
+#define LEVELS 5
+  char *path = strdup("test");
+  char *dirs[LEVELS];
+  int level;
+  printf("info: testing subdirectory creation\n");
+  for (level = 0; level < LEVELS; level++) {
+    char *newpath = NULL;
+    if (-1 == mkdir(path, 0777)) {
+      printf("  error: Unable to create directory '%s': %s\n",
+	     path, strerror(errno));
+      break;
+    }
+    asprintf(&newpath, "%s/%s", path, "test");
+    free(path);
+    path = newpath;
+  }
+  return 0;
+}
+
+/*
+ * Test if symlinks can be created.  This was a problem detected with
+ * KDE.
+ */
+int test_symlinks(void) {
+  printf("info: testing symlink creation\n");
+  unlink("symlink");
+  if (-1 == symlink("file", "symlink"))
+    printf("  error: Unable to create symlink\n");
+  return 0;
+}
+
+int main(int argc, char **argv) {
+  printf("Testing POSIX/Unix sematics on file system\n");
+  test_symlinks();
+  test_subdirectory_creation();
+#ifdef TEST_SQLITE
+  test_sqlite_open();
+#endif /* TEST_SQLITE */
+  test_gcompris_locking();
+  return 0;
+}
+
+ +

When everything is working, it should print something like +this:

+ +
+Testing POSIX/Unix sematics on file system
+info: testing symlink creation
+info: testing subdirectory creation
+info: sqlite worked
+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
+
+ +

I do not remember the exact details of the problems we saw, but one +of them was with locking, where if I remember correctly, POSIX allow a +read-only lock to be upgraded to a read-write lock without unlocking +the read-only lock (while Windows do not). Another was a bug in the +CIFS/SMB client implementation in the Linux kernel where directory +meta information would be wrong for a fraction of a second, making +OpenOffice.org fail to create its deep directory tree because it was +not allowed to create files in its freshly created directory.

+ +

Anyway, here is a nice tool for your tool box, might you never need +it. :)