Hook functions for Linux filesystem

Question

I want to do something before writing data into hard disk. I don't known any solutions. To avoid editing kernel source code, is there any locations that I can use hook function as a loadable module?

UPDATE: Thanks all, LSM is good for API hooks. But I want to find other solution that provides mechanism hook read/write block of data. It can avoid re-encrypting all file after updating file.

I think there is something that I can modify between filesystem (ext2, ext3,...) and buffer cache.

Answer 1

Use Linux Security Modules. These are loadable kernel modules, that provide hooks to mediate accesses to various internal objects in the kernel. You can use the hook to filesystem, or inodes as per your requirement. A god place to start is to read Greg Kroah Hartman's paper on LSM. Then you can visit this link, where it is shown along with an example how to make use of LSM hooks. The example is to mediate access to the system only when a particular USB is inserted, and is a good reference point on how to start with LSM hooks.