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Third-party external functions

The final two languages supported by FFPF are C and OKE-Cyclone. They are not intended to be used for packet processing by normal users on a day-to-day basis (although this is not precluded), but rather for implementing fast filters or external functions that can be called from FPL-1 or FPL-2. Writing kernel modules in C is too complex for most users, and writing code in the OKE is even more complex than that. We expect only power users to exploit the `native code extensibility' features. Even so, once written and declared safe, the code and credentials needed to install such code can be given to third-parties.

External functions written in C and compiled as kernel modules can only be loaded by the system administrator. However, in the Open Kernel Environment [7]2 it was shown how third-party users can load fully optimised native code in the Linux kernel, without compromising safety in any way. OKE support was added to FFPF, so that even non-root users are allowed to load fast native functions in the kernel and register them with FFPF. Subsequently, these functions can be called by or connected to filter expressions just like ordinary external functions.

The FPL-2 way of injecting code was directly modelled after the OKE, so compilation and instantiation are as sketched in Figures 8 and 9, except that the language used in the OKE is OKE-Cyclone , a `crash-free' version of C [20]. Unlike FPL-2, this is a language that supports pointer memory allocation and full interaction with the kernel. Accordingly, to be able to generate `resource safe' code, the compiler must check and instrument the user code much more strictly. Depending on the credentials provided by the user the OKE compiler restricts the user code in terms of access to resources, e.g. CPU, heap, and stack usage, access to APIs, access to sensitive fields in packets, accesses to kernel heap, etc.

Using the OKE, users no longer depend on root users to load the desired functionality as native code. The cost of full resource control in the OKE is roughly 10% compared to plain C. Like authorisation control, the OKE is optional. We refer to [7] for a discussion of related work in safe kernel programming.


next up previous
Next: FFPF packet sources Up: Implementation Previous: Authorisation
Herbert Bos 2004-10-06