Rework fiche de synthèse

report/fiche_synthese.tex

@@ -9,37 +9,54 @@
 \subsection*{The general context}
 
 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
-unwind stack frames, restoring machine registers to their proper values, for
-instance within the context of a debugger.
+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
+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, moving around between stack frames,
-tracing the program path leading to some bug, \ldots{} 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!
+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.
 
-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
 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?
 % Why is it important, what are the applications/consequences?
 % Is it a new problem?
@@ -48,11 +65,10 @@ in the previous section…}
 
 \subsection*{Your contribution}
 
-This internship explored the possibility to compile the standard ELF debugging
-information format, DWARF, 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.
+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.
@@ -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
 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 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?
 %
@@ -103,30 +119,27 @@ 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
-instruction 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 instructions actually used in the wild
-are 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.
+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, the slowness of 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 analyse 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
+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~\qtodo{cite Stephen's software?}. Thus, it might be interesting to
 implement a more stable version, and try to interface it cleanly with
 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
 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

shared/report.bib

@@ -16,3 +16,12 @@
     title           = {Libunwind webpage},
     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}
+}
+