Writing Stackable Filesystems
In the wrapfs_unlink example, we suggested that instead of deleting a file, you could rename it, thus saving a single backup of deleted files. Suppose we call this filesystem unrmfs, in which deleted files are instead renamed from their original name F to F.unrm. It might be annoying if all of these .unrm files started appearing in your directory, especially if you're expecting nothing there. Moreover, this kind of functionality also could be used to fool attackers who try to delete log files that may be used to track their actions. To achieve this, however, the .unrm files must not be visible or accessible to users by default.
To hide certain files in a filesystem, you have to do two things. First, prevent the file from showing up in ->readdir(). This is done by writing code in wrapfs_filldir that checks each filename passed to ->filldir() and returning NULL for those files you do not want listed. Second, prevent users from directly looking up the file by its name; this is done by checking for .unrm files in the beginning of wrapfs_lookup.
Of course, hiding those files from all users isn't very useful. Legitimate users must be able to access those files under certain conditions. A simple approach might be to check the UID of the calling process and to hide the .unrm files only from certain users. A better approach would be to use the mother of all system calls, ioctl. In Wrapfs, you can define as many new ioctls as you like, and then write small user-level programs to use those ioctls. This is, for example, the mechanism we use in encryption filesystems for a user-level tool to pass a user's cipher key to the kernel.
For our unrmfs, you could write a restore ioctl that takes a file's name, F, checks whether the file F.unrm exists and then renames F.unrm back to F, effectively unhiding it from unrmfs. The following example shows a sketch of this code:
/* len: length of source file */ newname = kmalloc(len+6, GFP_KERNEL); strncpy_from_user(newname, ioctl_arg, len); strcat(newname, ".unrm"); lower_dir = get_lower_inode(dir); src = lookup_one_len(lower_dir, newname); if (IS_ERR(src)) return PTR_ERR(src); dst = lookup_one_len(lower_dir, name); vfs_rename(lower_dir, src, lower_dir, dst);
Filesystem development need not be difficult. Using stackable filesystems, you can create new, useful and efficient filesystems quickly—all without changing kernels or existing filesystems. The examples in this article hopefully demonstrate the power of stacking, from which you gradually can build more complex filesystems. You can get the FiST software, documentation and many more examples from www.cs.sunysb.edu/~ezk/research/fist. Happy stacking.
Erez Zadok (firstname.lastname@example.org) is on the Computer Science faculty at Stony Brook University, the author of Linux NFS and Automounter Administration (Sybex, 2001), the creator and maintainer of the FiST stackable templates system and the primary maintainer of the Am-utils (aka, Amd) automounter. Erez conducts operating systems research with a focus on filesystems, security and networking.
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One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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