Another new ABI for fanotify

"Fanotify" is a much-revised system for providing filesystem event notifications to user space, and, possibly, allowing user space to block open() operations on specific files. The intended use case is malware-scanning utilities, but there are others: hierarchical storage has been cited as one possibility. This code has had a long, hard path into the kernel for a couple of reasons: kernel developers are not big fans of malware scanning, and nailing down the user-space ABI has been challenging.

The first obstacle has been more-or-less overcome. Even developers who think that malware scanning is the worst sort of security snake oil can agree that having these utilities use a well-defined kernel interface is better than having them employ nasty tricks like hooking into the system call table. ABI difficulties can be harder to overcome, though. With the latest fanotify posting, developer Eric Paris may have resolved this issue for at least a portion of the fanotify functionality.

The new version does away with the novel interface using setsockopt() in favor of a couple of new system calls. The first of these is fanotify_init():

    int fanotify_init(unsigned int flags, unsigned int event_f_flags,
		      int priority);

This system call initializes the fanotify subsystem, returning a file descriptor which is used for further operations. There are two flags values implemented: FAN_NONBLOCK creates a nonblocking file descriptor, and FAN_CLOEXEC sets the close-on-exec flag. Currently, event_f_flags and priority are unused; they should be set to zero.

Management of notification events is then done with fanotify_mark():

    int fanotify_mark(int fanotify_fd, unsigned int flags,
		      int dfd, const char *pathname, u64 mask,
		      u64 ignored_mask);

This call is used to "mark" specific parts of the filesystem hierarchy, indicating an interest in events involving those files. fanotify_fd is the file descriptor returned by fanotify_init(). The flags parameter must be one of FAN_MARK_ADD or FAN_MARK_REMOVE, indicating whether this call adds new marks or removes existing ones; there are also a couple of flags to control following of symbolic links and the marking of directories (without their contents).

The file(s) to be marked are determined by dfd and pathname; these parameters work much like in any of the *at() system calls. If dfd is AT_FDCWD, the pathname is resolved using the current working directory. If, instead, dfd points to a directory, the pathname lookup starts at that directory. If pathname is null, though, then dfd is interpreted as the actual object to mark.

Finally, mask and ignored_mask control which events are reported. To generate a specific event, a file must have the appropriate flag set in mask and clear in ignored_mask. The flags are FAN_ACCESS (file access), FAN_MODIFY (a file is modified), FAN_CLOSE_WRITE (a writable file has been closed), FAN_CLOSE_NOWRITE (a read-only file has been closed), FAN_OPEN (a file has been opened), and FAN_EVENT_ON_CHILD (events on children of a directory). There is also a FAN_Q_OVERFLOW event for event queue overflows, but that is not currently implemented.

Once files have been marked, the application can simply read from the fanotify file descriptor to get events. The events look like:

    struct fanotify_event_metadata {
	__u32 event_len;
	__u32 vers;
	__s32 fd;
	__u64 mask;
    };

Here, event_len is the length of the structure, vers indicates which version of fanotify generated the structure, fd is an open file descriptor for the object being accessed, and mask describes what is actually happening.

There is one crucial component missing in these patches: there is no way for the fanotify user to react to these events. In particular, the ability to block an open() call, a core part of the malware-scanning process, is missing. That, presumably, is to be added in a future revision. Meanwhile, Eric has requested permission to put the notification code into linux-next, presumably with a 2.6.33 merge in mind. As of this writing, objections have not been forthcoming.

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