New operating systems, such as the Capsicum capability system, allow a programmer to write an application that satisfies strong security properties by invoking security-specific system calls at a few key points in the program. However, rewriting an application to invoke such system calls correctly is an error-prone process: even the Capsicum developers have reported difficulties in rewriting programs to correctly invoke system calls.

This paper describes capweave, a tool that takes as input (i) an LLVM program, and (ii) a declarative policy of the possibly-changing capabilities that a program must hold during its execution, and rewrites the program to use Capsicum system calls to enforce the policy. Our experiments demonstrate that capweave can be applied to rewrite security-critical Unix utilities to satisfy practical security policies. capweave itself works quickly, and the runtime overhead incurred in the programs that capweave produces is generally low for practical workloads.

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