MultiOtter: Multiprocess Symbolic Execution
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Symbolic execution can be an effective technique for exploring large numbers of program paths, but it has generally been applied to programs running in isolation, whose inputs are files or command-line arguments. Programs that take inputs from other programs---servers, for example---have been beyond the reach of symbolic execution. To address this, we developed a multiprocess symbolic executor called MultiOtter, along with an implementation of many of the POSIX functions, such as socket and select, that interactive programs usually rely on. However, that is just a first step. Next, we must determine what symbolic inputs to feed to an interactive program to make multiprocess symbolic execution effective. Providing completely unconstrained symbolic values causes symbolic execution to spend too much time exploring uninteresting paths, such as paths to handle invalid inputs. MultiOtter allows us to generate inputs that conform to a context-free grammar, similar to previous work, but it also enables new input generation capabilities because we can now run arbitrary programs concurrently with the program being studied. As examples, we symbolically executed a key-value store server, redis, and an FTP server, vsftpd, each with a variety of inputs, including symbolic versions of tests from redis's test suite and wget as a client for vsftpd. We report the coverage provided by symbolic execution with various forms of symbolic input, showing that different testing goals require different degrees of symbolic inputs.