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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\title[] {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{columns}
        \begin{column}{0.55\textwidth}
            \begin{tcolorbox}[halign=center, colframe=blue]
                \textbf{Webpage} (incl. slides)

                \smallskip
                \vspace{0.5em}
                {\url{https://huit.re/frdwarf}}\\
                \vspace{0.5em}
            \end{tcolorbox}
        \end{column}

        \begin{column}{0.55\textwidth}
            \begin{tcolorbox}[colframe=blue]
                \begin{center}\textbf{Funding}\end{center}
                \vspace{-1em}

                \smallskip
                ONR Vertica \\
                Google Research Fellowship
            \end{tcolorbox}
        \end{column}
    \end{columns}
\end{frame}

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

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

\begin{frame}[fragile]{}
    \begin{columns}[c]
        \begin{column}{0.65\textwidth}
            \begin{lstlisting}[basicstyle=\tt,language=gdb, numbers=none, escapechar=|]
$ ./a.out
Segmentation fault.

|\pause|(gdb) backtrace
#0   |0x54625| in fct_b
#1   |\color{blue}0x54663| in fct_a
#2   |\color{red}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}
    \begin{columns}[c]
        \begin{column}{0.65\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}{0.35\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{center}
        \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 \\
        \end{tabular}
    \end{center}
    \vspace{1em}

    \only<1>{\vspace{19mm}}
    \begin{columns}
        \begin{column}{0.50\textwidth}
            \only<2->{
                \begin{tcolorbox}[enhanced, halign=center, frame hidden, colback=YellowGreen]
                    \textbf{For each instruction\ldots}\\
                    (identified by its program counter)
                \end{tcolorbox}
            }
        \end{column}
        \begin{column}{0.50\textwidth}
            \only<3->{
                \begin{tcolorbox}[enhanced, halign=center, frame hidden,
                        interior style={right color=Apricot, left color=Thistle}]
                        \textbf{\ldots{}an expression to compute its return address
                        location on the stack}
                \end{tcolorbox}
            }
        \end{column}
    \end{columns}
\end{frame}

\begin{frame}[t, fragile]{The real DWARF}
    \begin{lstlisting}[numbers=none, language=]
30 24 34 FDE pc=004020..004040
  DW_CFA_def_cfa_offset: 16
  DW_CFA_advance_loc: 6 to 0000000000004026
  DW_CFA_def_cfa_offset: 24
  DW_CFA_advance_loc: 10 to 0000000000004030
  DW_CFA_def_cfa_expression (DW_OP_breg7 (rsp): 8; DW_OP_breg16 (rip): 0; DW_OP_lit15; DW_OP_and; DW_OP_lit11; DW_OP_ge; DW_OP_lit3; DW_OP_shl; DW_OP_plus)
  [...]
    \end{lstlisting}

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

    \only<2->{
        \begin{textblock*}{0.90\textwidth}[0.5,0](0.5\paperwidth,0.17\paperheight)%
            \begin{tcolorbox}[halign=center, colframe=red, colback=Lavender]
                \bf \huge
                Complex \,\& \,slow
            \end{tcolorbox}
        \end{textblock*}
    }

    \only<3->{
        \begin{textblock*}{0.90\textwidth}[0.5,0](0.5\paperwidth,0.35\paperheight)%
            \begin{tcolorbox}[halign=center, colframe=red, colback=Lavender]
                \huge
                \textbf{Pervasive:}\\ relied upon by debuggers, profilers, C++
                exceptions \\
                \medskip{}
                \textbf{$\leadsto$ not only for debuggers!}
            \end{tcolorbox}
        \end{textblock*}
    }
\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}
    \begin{columns}
        \begin{column}{0.75\textwidth}
            \LinusMailOne{}
        \end{column}
        \begin{column}{0.25\textwidth}
            \includegraphics[width=\textwidth]{img/roundtorvalds.png}
        \end{column}
    \end{columns}

    \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}
    \newcommand{\firsttblrows}{
\tblrowval{\hspace{-2ex}<{\bf foo}>:}{}{\textbf{CFA}}{\textbf{ra}}
\rowonly<4>{\tblhl{}}  \tblrowval{push}{\%r15}{rsp+8}{c-8}
\rowonly<5>{\tblhl{}} \tblrowval{push}{\%r14}{rsp+16}{c-8}
\rowonly<6>{\tblhl{}} \tblrowval{mov}{\$0x3,\%eax}{rsp+24}{c-8}
\rowonly<7>{\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<3>{
		\begin{textblock*}{0.90\textwidth}[0.5,0.5](0.5\paperwidth,0.5\paperheight)%
			\begin{tcolorbox}[halign=center, colframe=red, colback=Lavender]
				\large
				\alert{\bf Assumptions:}
                \begin{itemize}
                    \item the assembly is was generated by a compiler
                    \item which also generated unwinding data
                    \item and I have a reliable DWARF parser
                \end{itemize}
			\end{tcolorbox}
		\end{textblock*}
	}}
    \only<-9>{
        \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<4>{Upon function call, \alert{ra = *(\reg{rsp})}}
            \only<5>{\texttt{push} decreases \reg{rsp} by 8: %
                \alert{ra = *(\reg{rsp} + 8)}}
            \only<6>{and again: %
                \alert{ra = *(\reg{rsp} + 16)}}
            \only<7>{This \texttt{mov} leaves \reg{rsp} untouched: %
                \alert{ra = *(\reg{rsp} + 16)}}
            \only<8>{The unwinding table captures an \alert{abstract execution}
            of the code\ldots}
            \only<9>{\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
            \[ \cfa = \reg{rsp} - 8
            \qquad \ra = \cfa + 8 \]
        \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
        Demo time!
    \end{beamercolorbox}
    \vfill
\end{frame}

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

\sectiontitleframe{}

\subsection{Compilation ahead-of-time}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[shrink]
    \vspace{0.5cm}
    \begin{tikzpicture}
        \begin{scope}[every node/.style={rectangle,thick,draw,scale=0.95}]
            \node (dwarf) at (0, 0) {
                \lstinputlisting[basicstyle=\tiny\tt,numbers=none,language=]{src/dw_plt_abbr}
            };

            \only<2->{
                \node (table) at (0.5\textwidth, -0.23\textheight) {
                    \tiny\tt
                    \begin{tabular}{
                            >{\columncolor{YellowGreen}}l
                            >{\columncolor{Thistle}}l
                            l l
                        >{\columncolor{Apricot}}l}
                        LOC & CFA & rbx & rbp & ra \\
                        0084950 & rsp+8 & u & u & c-8 \\
                        0084952 & rsp+16 & u & u & c-8 \\
                        0084954 & rsp+24 & u & u & c-8 \\
                        0084956 & rsp+32 & u & u & c-8 \\
                    \end{tabular}
                };
            }
            \only<3->{
                \node (csrc) at (0, -0.6\textheight) {
                    \lstinputlisting[basicstyle=\tiny,numbers=none,language=C]{src/fib7/fib7.eh_elf_basic.c}
                };
                \node (ehelf) at (0.55\textwidth, -0.75\textheight) {
                    ELF file:
                    ``\ehelf{}''
                };
            }
        \end{scope}

        \begin{scope}[>={Stealth[black]},
                every node/.style={fill=white,rectangle},
                every path/.style={draw=black,very thick}]
            \only<2->{\path [->] (dwarf) -| node {runtime} (table);}
            \only<3->{
                \path [->] (dwarf) edge node {ahead of time} (csrc);
                \path [->] (csrc) -| node {gcc, AoT} (ehelf);
            }
        \end{scope}
    \end{tikzpicture}
\end{frame}


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

\begin{frame}
    \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 lot of things remain to be done!}
    \begin{itemize}
        \item{} Synthesis + compare = verification of unwinding data!
        \item{} Synthesis could be integrated in compilers: support for inline
            assembly, fallback, \ldots
        \item{} Speedup could be implemented in profilers, debuggers,
            \ldots{}
        \item{} Probably many more cool things to do!
    \end{itemize}

    \vspace{1em}

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

\end{document}