When long jumps fall short: control-flow tracking and misuse detection for nonlocal jumps in C: Extended version
The C programming language offers setjmp/longjmp as a mechanism for nonlocal control flow. This mechanism has complicated semantics. As most developers do not encounter it day-to-day, they may be unfamiliar with all its intricacies – leading to subtle programming errors. At the same time, most static analyzers lack proper support, implying that otherwise sound tools miss whole classes of program deficiencies. We propose a concrete semantics of a subset of C with setjmp/longjmp, where interprocedural longjmps are performed directly, as well as an equivalent formulation where such jumps are implemented via stack-unwinding at the call-sites. Reflecting this semantic equivalence, we propose an approach for lifting existing interprocedural analyses to support setjmp/longjmp and to flag their misuse. To deal with the nonlocal semantics, our approach leverages side-effecting transfer functions, which, when executed, may additionally trigger contributions for program points that are not static control-flow successors. We showcase our analysis on a real-world example and propose a set of litmus tests for other analyzers.