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\section*{Internship synthesis}

\subsection*{The general context}

The standard debugging data format for ELF binary files, DWARF, contains a lot
of information, which is generated mostly when passing \eg{} the switch
\lstbash{-g} to \prog{gcc}. This information, essentially provided for
debuggers, contains all that is needed to connect the generated assembly with
the original code, information that can be used by sanitizers (\eg{} the type
of each variable in the source language), etc.

Even in stripped (non-debug) binaries, a small portion of DWARF data remains.
Among this essential data that is never stripped is the stack unwinding data,
which allows to unwind stack frames, restoring machine registers to the value
they had in the previous frame, for instance within the context of a debugger
or a profiler.

This data is structured into tables, each row corresponding to an program
counter (PC) range for which it describes valid unwinding data, and each column
describing how to unwind a particular machine register (or virtual register
used for various purposes). These rules are mostly basic, consisting in offsets
from memory addresses stored in registers (such as \reg{rbp} or \reg{rsp}), but
in some cases, they can take the form of a stack-machine expression that can
access virtually all the process's memory and perform Turing-complete
computation~\cite{oakley2011exploiting}.

\subsection*{The research problem}

As debugging data can easily get heavy beyond reasonable if stored carelessly,
the DWARF standard pays a great attention to data compactness and compression,
and succeeds particularly well at it. But this, as always, is at the expense
of efficiency: accessing stack unwinding data for a particular program point
can be quite costly.

This is often not a huge problem, as stack unwinding is mostly thought of as a
debugging procedure: when something behaves unexpectedly, the programmer might
be interested in exploring the stack.  Yet, stack unwinding might, in some
cases, be performance-critical: for instance, profiler programs needs to
perform a whole lot of stack unwindings. Even worse, exception handling relies
on stack unwinding in order to find a suitable catch-block! For such
applications, it might be desirable to find a different time/space trade-off,
allowing a slightly space-heavier, but far more time-efficient unwinding
procedure.

This different trade-off is the question that I explored during this
internship: what good alternative trade-off is reachable when storing the stack
unwinding data completely differently?

It seems that the subject has not really been explored yet, and as of now, the
most widely used library for stack unwinding,
\prog{libunwind}~\cite{libunwind}, essentially makes use of aggressive but
fine-tuned caching and optimized code to mitigate this problem.

% What is the question that you studied?
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\subsection*{Your contribution}

This internship explored the possibility to compile DWARF's stack unwinding
data directly into native assembly on the x86\_64 architecture. Instead of
parsing and interpreting at runtime the debug data, the stack unwinding data is
accessed as a function of a dynamically-loaded shared library.

Multiple approaches have been tried, in order to determine which compilation
process leads to the best time/space trade-off.

Quite unexpectedly, the part that proved hardest of the project was finding a
benchmarking protocol that was both relevant and reliable. Unwinding one single
frame is way too fast to be benched on a few samples (around $10\,\mu s$ per
frame), and having a lot of samples is quite complex, since one must avoid
unwinding the same frame over and over again, which would only benchmark the
caching mechanism. The other problem is to distribute evenly the unwinding
measures across the various program positions, including directly into the
loaded libraries (\eg{} the \prog{libc}).

The solution eventually chosen was to modify \prog{perf}, the standard
profiling program for Linux, in order to gather statistics and benchmarks of
its unwindings, and produce an alternative version of \prog{libunwind} using
the compiled debugging data, in order to interface it with \prog{perf},
allowing to benchmark \prog{perf} with both the standard stack unwinding data
and the alternative experimental compiled format. As a free and enjoyable
side-effect, the experimental unwinding data is perfectly interfaced with
\prog{libunwind}, and thus interfaceable at practically no cost with any
existing project using the \textit{de facto} standard library \prog{libunwind}.

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\subsection*{Arguments supporting its validity}

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The goal was to obtain a compiled version of unwinding data that was faster
than DWARF, reasonably heavier and reliable. The benchmarks mentioned have
yielded convincing results: on the experimental setup created (detailed later
in this report), the compiled version is up to 25 times faster than the DWARF
version, while it remains only around 2.5 times bigger than the original data.

Even though the implementation is more a research prototype than a release
version, is still reasonably robust, compared to \prog{libunwind}, which is
built for robustness. Corner cases are frequent while analyzing stack data, and
even more when analyzing them through a profiler; yet the prototype fails only
on around 200 cases more than \prog{libunwind} on a 27000 samples test (1099
failures, against 885 for \prog{libunwind}).

The prototype, unlike \prog{libunwind}, does not support $100\,\%$ of the DWARF
instructions present in the DWARF5 standard~\cite{dwarf5std}. It is also
limited to the x86\_64 architecture, and relies to some extent on the Linux
operating system. But none of those limitations are real problems in practice.
As argued later on, the vast majority of the DWARF instruction set actually
used in the wild is implemented; other processor architectures and ABIs are
only a matter of time spent and engineering work; and the operating system
dependency is only present in the libraries developed in order to interact with
the compiled unwinding data, which can be developed for virtually any operating
system.

\subsection*{Summary and future work}

In most cases of everyday's life, a slow stack unwinding is not a problem, or
even an annoyance. Yet, having a 25 times speed-up on stack unwinding-heavy
tasks, such as profiling, can be really useful to profile heavy programs,
particularly if one wants to profile many times in order to analyze the impact
of multiple changes. It can also be useful for exception-heavy programs. Thus,
it might be interesting to implement a more stable version, and try to
interface it cleanly with mainstream tools, such as \prog{perf}.

Another question worth exploring might be whether it is possible to shrink even
more the original DWARF unwinding data, which would be stored in a format not
too far from the original standard, by applying techniques close to those
used to shrink the compiled unwinding data.

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