The TALPA malware scanning API was covered in LWN in December, 2007. Several months later, TALPA is back - in the form of a patch set posted by a Red Hat employee. The resulting discussion has certainly not been what the TALPA developers would have hoped for; it is, instead, a good example of how a potentially useful idea can be set back by poor execution and presentation to the kernel community.
The idea behind TALPA is simple: various companies in the virus-scanning business would like a hook into the kernel which allows them to check for malware and prevent its spread. So the patch adds a hook into the VFS code which intercepts every file open operation. A series of filters can be attached to this intercept, with the most important one being a mechanism which makes the file being opened available to a user-space process as a read-only file descriptor. That process can scan the file and tell the kernel whether the open operation should be allowed to proceed or not. In this way, the scanning process can prevent any sort of access to files which are deemed to contain bits with evil intentions.
There are a few other details, of course. A caching mechanism prevents rescanning of unchanged files, increasing performance considerably. There is also a hook on close() calls which can trigger the rescanning of a file. Processes can exempt themselves from scanning if it might get in their way; scanning can also be turned off for specific files, such as those used for relational database storage. But the patch set is relatively small, as it really does not have that much to do.
This capability could well prove to be useful. Even if one is not concerned about malware infections on Linux systems, a lot of files destined for more vulnerable platforms can pass through Linux servers. There is also the potential for the detection of attempted exploits of the Linux host. Normally, in the Linux world, the way we respond to knowledge of a specific vulnerability is to patch the problem rather than scan for exploits, but there may be systems which cannot be restarted on short notice, and which could benefit from an updated scanning database while running code with known vulnerabilities. Also, as Alan Cox pointed out, this feature could be useful for entirely different objectives, such as efficient indexing of files as they change.
What might be best of all, though, is that this hook could replace a number of rather less pleasant things being done by anti-malware vendors now. Some of these products use binary-only modules, plant hooks into the system call table, and generally behave in unwelcome ways. Moving all of that to a user-space process behind a well-defined API could be beneficial for everybody involved.