September 23, 2019Francesco Zappa Nardelli (INRIA - ENS)
Debug information, usually encoded in the DWARF format, is a hidden and obscure component of our computing infrastructure. Debug information is obviously used by debuggers, but it also plays a key role in program analysis tools, and, most surprisingly, it can be relied upon by the runtime of high-level programming languages. For instance the C++ runtime leverages DWARF stack unwind tables to implement exceptions! Alas, generating debug information adds significant burden to compiler implementations, and the debug information itself can be pervaded by subtle bugs, making the whole infrastructure unreliable. Additionally, interpreting the debug tables is time consuming and for applications as sampling profilers it is a performance bottle-neck.
In this talk I will focus on the DWARF unwind table, that enables stack unwinding in absence of frame-pointer information. I will describe two techniques to perform validation and synthesis of the DWARF stack unwinding tables, and their implementation for the x86_64 architecture. The validation tool has proven effective for compiler and inline assembly testing, while the synthesis tool can generate DWARF unwind tables for arbitrary binaries lacking debug information. Additionally, I will report on a technique to precompile unwind tables into native x86_64 code, which we have implemented and integrated into libunwind, resulting in a 25x DWARF-based unwind speedup.