  Despite the advancements in software testing, bugs still plague deployed
software and result in crashes in production. When debugging issues --
sometimes caused by "heisenbugs" -- there is the need to interpret core dumps
and reproduce the issue offline on the same binary deployed. This requires the
entire toolchain (compiler, linker, debugger) to correctly generate and use
debug information. Little attention has been devoted to checking that such
information is correctly preserved by modern toolchains' optimization stages.
This is particularly important as managing debug information in optimized
production binaries is non-trivial, often leading to toolchain bugs that may
hinder post-deployment debugging efforts. In this paper, we present
Debug$^{2}$, a framework to find debug information bugs in modern toolchains.
Our framework feeds random source programs to the target toolchain and
surgically compares the debugging behavior of their optimized/unoptimized
binary variants. Such differential analysis allows Debug$^{2}$ to check
invariants at each debugging step and detect bugs from invariant violations.
Our invariants are based on the (in)consistency of common debug entities, such
as source lines, stack frames, and function arguments. We show that, while
simple, this strategy yields powerful cross-toolchain and cross-language
invariants, which can pinpoint several bugs in modern toolchains. We have used
Debug$^{2}$ to find 23 bugs in the LLVM toolchain (clang/lldb), 8 bugs in the
GNU toolchain (GCC/gdb), and 3 in the Rust toolchain (rustc/lldb) -- with 14
bugs already fixed by the developers.
