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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\title[\sectionline] {Reliable and Fast DWARF-based Stack Unwinding}
\author[\slidecountline]{\textbf{Théophile Bastian}\\
    \textbf{Stephen Kell} \\
\textbf{Francesco Zappa Nardelli}}
\date{}
%\subject{}
%\logo{}
\institute{ENS Paris, University of Kent, Inria}

\begin{document}

\begin{frame}
    \addtocounter{framenumber}{-1}
	\titlepage{}

    \vspace{-2em}
    \begin{center}
        {\large \url{https://huit.re/frdwarf}}\\
        {\todo{font size} Slides, paper, code}

        \todo{FUNDING: ONR Vertica + Google Research Fellowship (logos)}
    \end{center}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{DWARF and stack unwinding data}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Introduction}

\begin{frame}[fragile]{}
    \begin{columns}[c]
        \begin{column}{0.70\textwidth}
            \begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
$ ./a.out
Segmentation fault.

|\pause|(gdb) backtrace
#0   0x54625 in fct_b
#1   0x54663 in fct_a
#2   0x54674 in main
            \end{lstlisting}
            \pause{}
            \begin{center}
                \textbf{\Large How does it work?!}
            \end{center}
        \end{column}
        \begin{column}{0.35\textwidth}
            \pause{}
            \includegraphics[width=0.95\linewidth]{img/call_stack}
        \end{column}
    \end{columns}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Stack frames and unwinding}

\begin{frame}{Call stack and registers}
    \begin{columns}[c]
        \begin{column}{0.55\textwidth}
            \begin{center}
                \large\bf
                How do we get the RA\@?\\Easy, \reg{rbp}!

                \vspace{2em}

                \onslide<2>{What if we only have \reg{rsp}?}

            \end{center}
        \end{column}
        \begin{column}{0.45\textwidth}
            \includegraphics[width=0.95\linewidth]{img/call_stack}
        \end{column}
    \end{columns}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{DWARF tables}

\newcolumntype{a}{>{\columncolor{RedOrange}}l}

\begin{frame}{DWARF unwinding data}
    \vspace{2em}
    \tt \footnotesize
    \begin{tabular}{
            >{\columncolor{YellowGreen}}l
            >{\columncolor{Thistle}}l
            l l l l l l
            >{\columncolor{Apricot}}l}
~LOC & CFA & rbx & rbp & r12 & r13 & r14 & r15 & ra \\
0084950 & rsp+8 & u & u & u & u & u & u & c-8 \\
0084952 & rsp+16 & u & u & u & u & u & c-16 & c-8 \\
0084954 & rsp+24 & u & u & u & u & c-24 & c-16 & c-8 \\
0084956 & rsp+32 & u & u & u & c-32 & c-24 & c-16 & c-8 \\
0084958 & rsp+40 & u & u & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084959 & rsp+48 & u & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
\rowcolor{Aquamarine} 008495a & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084962 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a19 & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a1d & rsp+48 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a1e & rsp+40 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a20 & rsp+32 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a22 & rsp+24 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a24 & rsp+16 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a26 & rsp+8 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
0084a30 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
    \end{tabular}

    \pause{}

    \vspace{-3cm}
    \hfill\includegraphics[height=3cm, angle=45, origin=c]{img/dwarf_logo}
    \hspace{-1cm}
\end{frame}

\begin{frame}[t, fragile]{The real DWARF}
    \begin{lstlisting}[numbers=none, language=]
00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
  DW_CFA_advance_loc: 2 to 0000000000084952
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r15 (r15) at cfa-16
  DW_CFA_advance_loc: 2 to 0000000000084954
  DW_CFA_def_cfa_offset: 24
  DW_CFA_offset: r14 (r14) at cfa-24
  DW_CFA_advance_loc: 2 to 0000000000084956
  DW_CFA_def_cfa_offset: 32
  DW_CFA_offset: r13 (r13) at cfa-32
  DW_CFA_advance_loc: 2 to 0000000000084958
  DW_CFA_def_cfa_offset: 40
  DW_CFA_offset: r12 (r12) at cfa-40
  DW_CFA_advance_loc: 1 to 0000000000084959
  [...]
    \end{lstlisting}

    \begin{itemize}
        \item[\textbf{$\longrightarrow$}] \textbf{\alert{constructed} on-demand
            by a \alert{Turing-complete bytecode}!}
    \end{itemize}

    \pause{}

    \vspace{-6.5cm}
    \begin{center}
    \bf \fontsize{8cm}{1cm}
        \colorbox{white}{\alert{Complex}} \\
        \colorbox{white}{\alert{\& slow!}}
    \end{center}
\end{frame}

\begin{frame}{Why does slow matter?}
    \begin{itemize}

        \item{} After all, we're talking about \alert{debugging procedures} ran
            by a \alert{human being} (slower than the machine).

            \ldots{}or are we?
    \end{itemize}

    \pause{}
    \begin{center}
        \textbf{\Large{}No!}
    \end{center}

    \begin{itemize}
        \pause{}\item{} Pretty much any \alert{program analysis tool}
        \pause{}\item{} \alert{Profiling} with polling profilers

        \pause{}\item{} \alert{Exception handling} in C++

    \end{itemize}

    \vspace{2em}

    \begin{center}
        \textbf{\Large{}Debug data is not only for debugging}
    \end{center}

    \vspace{1em}
    $\leadsto$ we might want \alert{an alternative time/space trade-off}
\end{frame}

\newcommand{\LinusMailOne}{
    ``Sorry, but last time was too f\dots painful. The whole (and
    only) point of unwinders is to make debugging easy
    when a bug occurs. But \alert{the dwarf unwinder had bugs}
    itself, or \alert{our dwarf information had bugs}, and in either
    case it actually turned several trivial bugs into a \alert{total
    undebuggable hell}.''
}
\newcommand{\LinusMailTwo}{
    ``If you can \alert{mathematically prove that the unwinder is
    correct} — even in the presence of bogus and actively
    incorrect unwinding information — and never ever
    follows a bad pointer, \alert{I’ll reconsider}.''
}
\newcommand{\LinusSource}{
    \hfill ---~Linus Torvalds, 2012
}
\begin{frame}{A debugging hell: Linux kernel}
    \LinusMailOne{}

    \only<1-2>{
        \vspace{1em}
        \LinusSource{}
    }

    \vspace{1em}
    \only<2>{
        \begin{center}
            \Large\bf
            \alert{This is where we still are!}
        \end{center}
    }

    \only<3>{
        \LinusMailTwo{}

        \vspace{1em}
        \LinusSource{}
    }
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data as an abstract execution of the assembly}
\sectiontitleframe{}

\newcommand{\tblrowval}[4]{#1 & #2 & \only<2->{#3} & \only<2->{#4} \\}
\newcommand{\blknote}[1]
    {\begin{block}{}
            \centering\large
            #1
    \end{block}}
\newcommand{\blklnote}[1]
    {\begin{block}{}
            \large
            #1
    \end{block}}
\newcommand{\tblhl}{\rowcolor{Tan}}

\begin{frame}{Working on an example}
    \newcommand{\firsttblrows}{
\tblrowval{\hspace{-2ex}<{\bf foo}>:}{}{\textbf{CFA}}{\textbf{ra}}
\rowonly<3>{\tblhl{}}  \tblrowval{push}{\%r15}{rsp+8}{c-8}
\rowonly<4>{\tblhl{}} \tblrowval{push}{\%r14}{rsp+16}{c-8}
\rowonly<5>{\tblhl{}} \tblrowval{mov}{\$0x3,\%eax}{rsp+24}{c-8}
\rowonly<6>{\tblhl{}} \tblrowval{push}{\%r13}{rsp+24}{c-8}
\tblrowval{push}{\%r12}{rsp+32}{c-8}
\tblrowval{push}{\%rbp}{rsp+40}{c-8}
\tblrowval{push}{\%rbx}{rsp+48}{c-8}
\tblrowval{sub}{\$0x68,\%rsp}{rsp+56}{c-8}
}

    \only<-8>{
        \begin{table}
            \ttfamily\large
            \begin{tabularx}{0.9\linewidth}{
                    l
                    b
                    >{\columncolor{SkyBlue}}s
                    >{\columncolor{SkyBlue}}s
                }
\firsttblrows{}%
\tblrowval{add}{\$0x68,\%rsp}{rsp+160}{c-8}
\tblrowval{pop}{\%rbx}{rsp+56}{c-8}
\tblrowval{pop}{\%rbp}{rsp+48}{c-8}
            \end{tabularx}
        \end{table}
        \blknote{
            \centering
        \begin{overlayarea}{0.9\textwidth}{4.8ex}
            \only<3>{Upon function call, \alert{ra = *(\reg{rsp})}}
            \only<4>{\texttt{push} decreases \reg{rsp} by 8: %
                \alert{ra = *(\reg{rsp} + 8)}}
            \only<5>{and again: %
                \alert{ra = *(\reg{rsp} + 16)}}
            \only<6>{This \texttt{mov} leaves \reg{rsp} untouched: %
                \alert{ra = *(\reg{rsp} + 16)}}
            \only<7>{The unwinding table captures an \alert{abstract execution}
            of the code\ldots}
            \only<8>{\ldots and thus can be \alert{synthesized from the binary}.}
        \end{overlayarea}
    }
    }
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data synthesis from binaries}

%\begin{frame}{Why would synthesis be useful?}
%    \begin{itemize}
%        \item As said earlier, \alert{DWARF is complex}
%        \item Some compilers \alert{do not generate it}: hard to \alert{debug}
%            \& \alert{profile}.
%        \item Think of \alert{JIT-compiled assembly} (eg. JVM)
%        \item \ldots{}or even \alert{hand-written inlined assembly}!
%            \begin{itemize}
%                \item Painful enough to write for not bothering with DWARF
%                \item May not even be known by the programmer, breaks gdb
%                \item May be wrong (remember Linus!)
%            \end{itemize}
%    \end{itemize}
%\end{frame}

\begin{frame}{How do we actually synthesize?}
    \begin{itemize}
        \item Upon entering a function, we know (ABI)
            \[ \cfa = \reg{rsp} - 8
            \qquad \ra = \cfa + 8 \]
        \item The semantics of each instruction specifies \alert{how it changes \cfa}.
            \begin{itemize}
                \item Heuristic to decide whether we index with \reg{rbp} or
                    \reg{rsp}
            \end{itemize}
        \item By performing a symbolic execution, we can \alert{synthesize the
            unwinding table} line by line.
        \item Control flow: forward data-flow analysis
        \item The fixpoints are immediate, cf article
    \end{itemize}
\end{frame}

\begin{frame}{}
    \vfill
    \centering
    \begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
        \Large\bf
        Demo time!
    \end{beamercolorbox}
    \vfill
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data compilation}

\sectiontitleframe{}

\subsection{Compilation ahead-of-time}

\begin{frame}{Compilation overview}
    \begin{itemize}
        \item Compiled to \alert{C code}
        \item C code then \alert{compiled to native binary} (gcc)
            \begin{itemize}
                \item[$\leadsto$] gcc optimisations for free
            \end{itemize}
        \item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
            \bigskip{}
        \item Morally a \alert{monolithic switch} on IPs
        \item Each case contains assembly that computes a \alert{row of the
            table}
    \end{itemize}
\end{frame}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[shrink]{Compilation example: generated C}
    \lstinputlisting[language=C]{src/fib7/fib7.eh_elf_basic.c}

    \pause{}
    \vspace{1em}
    \begin{center}
        The real code is optimised, but boils down to this.
    \end{center}
\end{frame}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{frame}{Mostly plug-and-play: libunwind interface}
    \begin{itemize}
        \item \alert{libunwind}: \textit{de facto} standard library for
            unwinding

            \bigskip{}

        \item \texttt{libunwind-eh\_elf}: alternative implementation using
            \ehelfs{}

        \item[$\leadsto$] almost \alert{``relink-and-play''} for existing projects!
    \end{itemize}
\end{frame}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Results}

\begin{frame}{Performances}
    \begin{itemize}
        \item \alert{Speedup}: x15 (\prog{gzip}) to x25 (\prog{hackbench}) vs.
            libunwind

            \begin{itemize}
                \item libunwind: state of the art, aggressive caching.
            \end{itemize}

        \item \alert{Space overhead}: x2.6 to x3 vs. DWARF

            \vspace{2em}

        \item[$\leadsto$] Alternative time/space trade-off, favorable eg. for
            profiling.
    \end{itemize}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section*{Conclusion}
\setcounter{section}{0}

\begin{frame}{A fragment of our article}
    The original article \textbf{Reliable and Fast DWARF-based Stack Unwinding}
    contains

    \vspace{1em}

    \begin{itemize}
        \item{} DWARF unwinding tables validation;
        \item{} DWARF unwinding tables synthesis;
        \item{} DWARF-based unwinding speedup.
    \end{itemize}

    \vspace{1em}

    \begin{center}
        Come and chat if interested! \texttt{:)}
    \end{center}
\end{frame}

\end{document}