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report/fiche_synthese.tex
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@ -9,37 +9,54 @@
\subsection*{The general context} \subsection*{The general context}
The standard debugging data format for ELF binary files, DWARF, contains a lot The standard debugging data format for ELF binary files, DWARF, contains a lot
of information. Among those are the stack unwinding data, which allows to of information, which is generated mostly when passing \eg{} the switch
unwind stack frames, restoring machine registers to their proper values, for \lstbash{-g} to \prog{gcc}. This information, essentially provided for
instance within the context of a debugger. 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, As debugging data can easily get heavy beyond reasonable if stored carelessly,
the DWARF standard pays a great attention to data compactness and compression, 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 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 of efficiency: accessing stack unwinding data for a particular program point
can be quite costly. can be quite costly.
This is often not a huge problem, as stack unwinding is mostly thought of as a 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 debugging procedure: when something behaves unexpectedly, the programmer might
be interested in exploring the stack, moving around between stack frames, be interested in exploring the stack. Yet, stack unwinding might, in some
tracing the program path leading to some bug, \ldots{} Yet, stack unwinding cases, be performance-critical: for instance, profiler programs needs to
might, in some cases, be performance-critical: for instance, profiler programs perform a whole lot of stack unwindings. Even worse, exception handling relies
needs to perform a whole lot of stack unwindings. Even worse, exception on stack unwinding in order to find a suitable catch-block! For such
handling relies on stack unwinding in order to find a suitable catch-block! 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.
The most widely used library used for stack unwinding, 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 \prog{libunwind}~\cite{libunwind}, essentially makes use of aggressive but
fine-tuned caching and optimized code to mitigate this problem. fine-tuned caching and optimized code to mitigate this problem.
\subsection*{The research problem}
\todo{Split the previous paragraph into two paragraphs, fitting this section as
well}
\note{I have trouble figuring out what is expected here, and what is expected
in the previous section…}
% What is the question that you studied? % What is the question that you studied?
% Why is it important, what are the applications/consequences? % Why is it important, what are the applications/consequences?
% Is it a new problem? % Is it a new problem?
@ -48,11 +65,10 @@ in the previous section…}
\subsection*{Your contribution} \subsection*{Your contribution}
This internship explored the possibility to compile the standard ELF debugging This internship explored the possibility to compile DWARF's stack unwinding
information format, DWARF, directly into native assembly on the x86\_64 data directly into native assembly on the x86\_64 architecture. Instead of
architecture. Instead of parsing and interpreting at runtime the debug data, parsing and interpreting at runtime the debug data, the stack unwinding data is
the stack unwinding data is accessed as a function of a dynamically-loaded accessed as a function of a dynamically-loaded shared library.
shared library.
Multiple approaches have been tried, in order to determine which compilation Multiple approaches have been tried, in order to determine which compilation
process leads to the best time/space trade-off. process leads to the best time/space trade-off.
@ -74,7 +90,7 @@ allowing to benchmark \prog{perf} with both the standard stack unwinding data
and the alternative experimental compiled format. As a free and enjoyable and the alternative experimental compiled format. As a free and enjoyable
side-effect, the experimental unwinding data is perfectly interfaced with side-effect, the experimental unwinding data is perfectly interfaced with
\prog{libunwind}, and thus interfaceable at practically no cost with any \prog{libunwind}, and thus interfaceable at practically no cost with any
existing project using the common library \prog{libunwind}. existing project using the \textit{de facto} standard library \prog{libunwind}.
% What is your solution to the question described in the last paragraph? % What is your solution to the question described in the last paragraph?
% %
@ -103,30 +119,27 @@ on around 200 cases more than \prog{libunwind} on a 27000 samples test (1099
failures, against 885 for \prog{libunwind}). failures, against 885 for \prog{libunwind}).
The prototype, unlike \prog{libunwind}, does not support $100\,\%$ of the DWARF The prototype, unlike \prog{libunwind}, does not support $100\,\%$ of the DWARF
instruction present in the DWARF5 standard~\cite{dwarf5std}. It is also limited instructions present in the DWARF5 standard~\cite{dwarf5std}. It is also
to the x86\_64 architecture, and relies to some extent on the Linux operating limited to the x86\_64 architecture, and relies to some extent on the Linux
system. But none of those limitations are real problems in practice. As argued operating system. But none of those limitations are real problems in practice.
later on, the vast majority of the DWARF instructions actually used in the wild As argued later on, the vast majority of the DWARF instruction set actually
are implemented; other processor architectures and ABIs are only a matter of used in the wild is implemented; other processor architectures and ABIs are
time spent and engineering work; and the operating system dependency is only only a matter of time spent and engineering work; and the operating system
present in the libraries developed in order to interact with the compiled dependency is only present in the libraries developed in order to interact with
unwinding data, which can be developed for virtually any operating system. the compiled unwinding data, which can be developed for virtually any operating
system.
\subsection*{Summary and future work} \subsection*{Summary and future work}
In most cases of everyday's life, the slowness of stack unwinding is not a In most cases of everyday's life, a slow stack unwinding is not a problem, or
problem, or even an annoyance. Yet, having a 25 times speed-up on stack even an annoyance. Yet, having a 25 times speed-up on stack unwinding-heavy
unwinding-heavy tasks, such as profiling, can be really useful to analyse heavy tasks, such as profiling, can be really useful to profile heavy programs,
programs, particularly if one wants to profile many times in order to analyze particularly if one wants to profile many times in order to analyze the impact
the impact of multiple changes. It can also be useful for exception-heavy of multiple changes. It can also be useful for exception-heavy
programs~\qtodo{cite Stephen's software?}. Thus, it might be interesting to programs~\qtodo{cite Stephen's software?}. Thus, it might be interesting to
implement a more stable version, and try to interface it cleanly with implement a more stable version, and try to interface it cleanly with
mainstream tools, such as \prog{perf}. mainstream tools, such as \prog{perf}.
It might also be interesting to investigate whether it is possible to reach
even greater speeds by using some more complex compilation process that would
have yet to be determined.
Another question worth exploring might be whether it is possible to shrink even 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 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 too far from the original standard, by applying techniques close to those

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shared/report.bib
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@ -16,3 +16,12 @@
title = {Libunwind webpage}, title = {Libunwind webpage},
url = {http://www.nongnu.org/libunwind/}, url = {http://www.nongnu.org/libunwind/},
} }
@inproceedings{oakley2011exploiting,
title={Exploiting the Hard-Working DWARF: Trojan and Exploit Techniques with No Native Executable Code.},
author={Oakley, James and Bratus, Sergey},
booktitle={WOOT},
pages={91--102},
year={2011}
}