forked from m2-internship/talk-2018-11-uppsala
825 lines
25 KiB
TeX
825 lines
25 KiB
TeX
% vim: spell spelllang=en
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\documentclass[11pt,xcolor={usenames,dvipsnames}]{beamer}
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\usetheme{Warsaw}
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\usepackage[utf8]{inputenc}
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\usepackage[english]{babel}
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\usepackage[T1]{fontenc}
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\usepackage{amsmath}
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\usepackage{amsfonts}
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\usepackage{amssymb}
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\usepackage{booktabs}
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\usepackage{makecell}
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\usepackage{ifthen}
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\usepackage{colortbl}
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\usepackage{tabularx}
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\usepackage{pifont}
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\usepackage{texlib/my_listings}
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\usepackage{texlib/specific}
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\usepackage{texlib/common}
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\usepackage{texlib/todo}
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\usepackage{inconsolata}
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\lstset{basicstyle=\footnotesize\ttfamily}
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\renewcommand\theadalign{c}
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\newcommand{\thenalert}[1]{\only<1>{#1}\only<2>{\alert{#1}}}
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\newcommand{\slidecountline}{
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\ifthenelse{\theframenumber = 0}
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{}
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{\insertframenumber/\inserttotalframenumber}}
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\newcommand{\sectionline}{
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\ifthenelse{\thesection = 0}
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{}
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{\Roman{section}~-- \insertsection}}
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\newcommand{\cmark}{\color{OliveGreen}\ding{52}}
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\newcommand{\xmark}{\color{BrickRed}\ding{56}}
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\AtBeginSection[]{
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\begin{frame}
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\vfill
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\centering
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\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
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\usebeamerfont{title}\insertsectionhead\par%
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\end{beamercolorbox}
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\vfill
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\end{frame}
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}
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\lstdefinelanguage{gdb}{
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morekeywords={gdb},
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sensitive=false,
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}
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\newcolumntype{b}{X}
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\newcolumntype{s}{>{\hsize=.43\hsize}X}
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\newcommand{\lstinl}
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{\lstinline[language=C, keepspaces=true, basicstyle=\ttfamily]}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\title[\sectionline] {Growing the DWARF tougher:\\
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synthesis, validation and compilation}
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\author[\slidecountline]{\textbf{Théophile Bastian} \\ \vspace{0.5em}
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{{\footnotesize{}Based on work done with}\\
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\textbf{Francesco Zappa Nardelli},
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\textbf{Stephen Kell},
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\textbf{Simon Ser}}}
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\date{}
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%\subject{}
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%\logo{}
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\institute{ENS Paris, INRIA}
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\begin{document}
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\begin{frame}
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\addtocounter{framenumber}{-1}
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\titlepage{}
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\vspace{-2em}
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\begin{center}
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\todo{}
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\begin{align*}
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\text{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
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\text{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
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\end{align*}
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\end{center}
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\end{frame}
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\begin{frame}{~}
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\addtocounter{framenumber}{-1}
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\tableofcontents[hideallsubsections]
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{DWARF and stack unwinding data}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Introduction}
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\begin{frame}[fragile]{We often use stack unwinding!}
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\begin{columns}[c]
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\begin{column}{0.70\textwidth}
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\begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
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Program received signal SIGSEGV.
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0x54625 in fct_b at segfault.c:5
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5 printf("%l\n", *b);
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|\pause| (gdb) backtrace
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#0 0x54625 in fct_b at segfault.c:5
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#1 0x54663 in fct_a at segfault.c:10
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#2 0x54674 in main at segfault.c:14
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|\pause| (gdb) frame 1
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#1 0x54663 in fct_a at segfault.c:10
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10 fct_b((int*) a);
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|\pause| (gdb) print a
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$1 = 84
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\end{lstlisting}
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\vspace{-1em}
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\pause{}
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\begin{center}
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\textbf{\Large How does it work?!}
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\end{center}
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\end{column}
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\begin{column}{0.35\textwidth}
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\pause{}
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\includegraphics[width=0.95\linewidth]{img/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Stack frames and unwinding}
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\begin{frame}{Call stack and registers}
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\begin{columns}[c]
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\begin{column}{0.55\textwidth}
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\begin{center}
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\large\bf
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How do we get the grandparent RA\@?
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\medskip
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Isn't it as trivial as \texttt{pop()}?
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\vspace{2em}
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\only<2>{We only have \reg{rsp} and \reg{rip}.}
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\end{center}
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\end{column}
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\begin{column}{0.45\textwidth}
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\includegraphics[width=0.95\linewidth]{img/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{DWARF tables}
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\newcolumntype{a}{>{\columncolor{RedOrange}}l}
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\begin{frame}{DWARF unwinding data}
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\vspace{2em}
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\tt \footnotesize
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\begin{tabular}{
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>{\columncolor{YellowGreen}}l
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>{\columncolor{Thistle}}l
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l l l l l l
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>{\columncolor{Apricot}}l}
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~LOC & CFA & rbx & rbp & r12 & r13 & r14 & r15 & ra \\
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0084950 & rsp+8 & u & u & u & u & u & u & c-8 \\
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0084952 & rsp+16 & u & u & u & u & u & c-16 & c-8 \\
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0084954 & rsp+24 & u & u & u & u & c-24 & c-16 & c-8 \\
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0084956 & rsp+32 & u & u & u & c-32 & c-24 & c-16 & c-8 \\
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0084958 & rsp+40 & u & u & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084959 & rsp+48 & u & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\rowcolor{Aquamarine} 008495a & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084962 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a19 & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1d & rsp+48 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1e & rsp+40 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a20 & rsp+32 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a22 & rsp+24 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a24 & rsp+16 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a26 & rsp+8 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a30 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\end{tabular}
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\pause{}
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\vspace{-3cm}
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\hfill\includegraphics[height=3cm, angle=45, origin=c]{img/dwarf_logo}
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\hspace{-1cm}
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\end{frame}
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\begin{frame}[t, fragile]{The real DWARF}
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\begin{lstlisting}[numbers=none, language=]
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00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
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DW_CFA_advance_loc: 2 to 0000000000084952
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DW_CFA_def_cfa_offset: 16
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DW_CFA_offset: r15 (r15) at cfa-16
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DW_CFA_advance_loc: 2 to 0000000000084954
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DW_CFA_def_cfa_offset: 24
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DW_CFA_offset: r14 (r14) at cfa-24
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DW_CFA_advance_loc: 2 to 0000000000084956
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DW_CFA_def_cfa_offset: 32
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DW_CFA_offset: r13 (r13) at cfa-32
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DW_CFA_advance_loc: 2 to 0000000000084958
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DW_CFA_def_cfa_offset: 40
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DW_CFA_offset: r12 (r12) at cfa-40
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DW_CFA_advance_loc: 1 to 0000000000084959
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[...]
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\end{lstlisting}
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\begin{itemize}
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\item[\textbf{$\longrightarrow$}] \textbf{\alert{constructed} on-demand
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by a \alert{Turing-complete bytecode}!}
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\end{itemize}
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\pause{}
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\vspace{-6.5cm}
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\begin{center}
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\bf \fontsize{8cm}{1cm}
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\colorbox{white}{\alert{Complex}} \\
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\colorbox{white}{\alert{\& slow!}}
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\end{center}
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\end{frame}
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\begin{frame}{Why does slow matter?}
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\begin{itemize}
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\item{} After all, we're talking about \alert{debugging procedures} ran
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by a \alert{human being} (slower than the machine).
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\ldots{}or are we?
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\end{itemize}
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\pause{}
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\begin{center}
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\textbf{\Large{}No!}
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\end{center}
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\begin{itemize}
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\pause{}\item{} Pretty much any \alert{program analysis tool}
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\pause{}\item{} \alert{Profiling} with polling profilers
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\pause{}\item{} \alert{Exception handling} in C++
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\end{itemize}
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\vspace{2em}
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\begin{center}
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\textbf{\Large{}Debug data is not only for debugging}
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\end{center}
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\end{frame}
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\begin{frame}{Difficult to generate}
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Major concern with DWARF\@: it is \alert{difficult to generate}
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(correctly).
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\begin{itemize}
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\item{} \alert{Hard to generate}: each compiler pass must keep it in
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sync
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\item{} Most of it is \alert{seldom used} (\eg{} unwinding data of
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dusty code), and thus \alert{seldom tested}
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\end{itemize}
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\vspace{1em}
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Yields to
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\begin{itemize}
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\item{} unreliable DWARF\@: can cause headaches when debugging
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\item{} or not generated at all (eg. OCaml until recently) \todo{Check
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this}
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\end{itemize}
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\vspace{1em}
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\begin{center}
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\Large\bf
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$\leadsto$ Complex, buggy, untested
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\end{center}
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\end{frame}
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\begin{frame}{A debugging hell: Linux kernel}
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``Sorry, but last time was too f\dots painful. The whole (and
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only) point of unwinders is to make debugging easy
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when a bug occurs. But \alert{the dwarf unwinder had bugs}
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itself, or \alert{our dwarf information had bugs}, and in either
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case it actually turned several trivial bugs into a \alert{total
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undebuggable hell}.''
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\vspace{1em}
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\only<1>{\hfill ---~Linus Torvalds, Kernel mailing list, 2012}
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\pause{}
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``If you can \alert{mathematically prove that the unwinder is
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correct} — even in the presence of bogus and actively
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incorrect unwinding information — and never ever
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follows a bad pointer, \alert{I’ll reconsider}.''
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\vspace{1em}
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\hfill ---~Linus Torvalds, Kernel mailing list, 2012
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\pause{}\vspace{1em}
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\begin{center}
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\Large\bf
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\alert{This is where we still are!}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Unwinding data validation}
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\begin{frame}{Main idea}
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\begin{itemize}
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\item If we follow \alert{one path of execution}, we can track the
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return address location
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\item If we read unwinding data \alert{at runtime}, we can \alert{check
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the RA consistency} at each step
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\end{itemize}
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\end{frame}
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\newcommand{\tblrowval}[4]{#1 & #2 & \only<2->{#3} & \only<2->{#4} \\}
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\newcommand{\blknote}[1]
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{\begin{block}{}
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\centering\large
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#1
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\end{block}}
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\newcommand{\blklnote}[1]
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{\begin{block}{}
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\large
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#1
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\end{block}}
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\newcommand{\tblhl}{\rowcolor{Tan}}
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\begin{frame}{Example}
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\newcommand{\firsttblrows}{
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\tblrowval{\hspace{-2ex}<{\bf foo}>:}{}{\textbf{CFA}}{\textbf{ra}}
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\rowonly<3>{\tblhl{}} \tblrowval{push}{\%r15}{rsp+8}{c-8}
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\rowonly<4>{\tblhl{}} \tblrowval{push}{\%r14}{rsp+16}{c-8}
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\rowonly<5>{\tblhl{}} \tblrowval{mov}{\$0x3,\%eax}{rsp+24}{c-8}
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\rowonly<6>{\tblhl{}} \tblrowval{push}{\%r13}{rsp+24}{c-8}
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\tblrowval{push}{\%r12}{rsp+32}{c-8}
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\tblrowval{push}{\%rbp}{rsp+40}{c-8}
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\tblrowval{push}{\%rbx}{rsp+48}{c-8}
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\tblrowval{sub}{\$0x68,\%rsp}{rsp+56}{c-8}
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\rowonly<9>{\tblhl{}} \tblrowval{cmp}{\$0x1,\%edi}{rsp+160}{c-8}%
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}
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\only<-8>{
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\begin{table}
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\ttfamily\large
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\begin{tabularx}{0.9\linewidth}{
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l
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b
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>{\columncolor{SkyBlue}}s
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>{\columncolor{SkyBlue}}s
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}
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\firsttblrows{}%
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\tblrowval{add}{\$0x68,\%rsp}{rsp+160}{c-8}
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\tblrowval{pop}{\%rbx}{rsp+56}{c-8}
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\tblrowval{pop}{\%rbp}{rsp+48}{c-8}
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\end{tabularx}
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\end{table}
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\blknote{
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\centering
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\begin{overlayarea}{0.9\textwidth}{4.8ex}
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\only<3>{Upon function call, \alert{ra = *(\reg{rsp})} (ABI)}
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\only<4>{\texttt{push} decreases \reg{rsp} by 8: %
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\alert{ra = *(\reg{rsp} + 8)}}
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\only<5>{and again: %
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\alert{ra = *(\reg{rsp} + 16)}}
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\only<6>{This \texttt{mov} leaves \reg{rsp} untouched: %
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\alert{ra = *(\reg{rsp} + 16)}}
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\only<7>{The unwinding table can actually be seen as\\
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an \alert{abstract interpretation} of the code\ldots}
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\only<8>{\ldots and thus, for a given run, be
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\alert{re-computed for verification}}
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\end{overlayarea}
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}
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}
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\only<9->{
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\begin{table}
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\ttfamily\large
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\begin{tabularx}{0.9\linewidth}{
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l
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b
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>{\columncolor{SkyBlue}}s
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>{\columncolor{SkyBlue}}s
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}
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\firsttblrows{}%
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\end{tabularx}
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\end{table}
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\vspace{-0.8em}
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\only<9>{\blklnote{If, within an execution,
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\begin{itemize}
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\item ra = \texttt{*(0xFFFF1098)}
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\item \reg{rsp} = \texttt{0xFFFF1000}
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\end{itemize}
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We can \alert{evaluate both expressions} and \alert{compare}
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}}
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}
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\end{frame}
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\begin{frame}{Dynamic validation}
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\textbf{Abstract state}
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\begin{itemize}
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\item \alert{Stack} of actual \alert{addresses} where
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\alert{return addresses} are stored
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\end{itemize}
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\vspace{2em}\pause{}
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\textbf{Abstract instruction semantics}
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\begin{itemize}
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\item[\alert{\texttt{call}}] push \alert{\reg{rsp}} on the stack
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\item[\alert{\texttt{ret}}] pop from the stack
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\end{itemize}
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\vspace{2em}\pause{}
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\textbf{Validation of each instruction}
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\begin{itemize}
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\item Evaluate the return address provided by DWARF
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\item Compare it with the value at the top of the stack
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\end{itemize}
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\end{frame}
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\begin{frame}{In practice: \texttt{eh\_frame\_check}}
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Strategy implemented and working: \alert{\texttt{eh\_frame\_check}}
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\begin{itemize}
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\item \texttt{gdb} allows for Python instrumentation
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\vspace{1em}\pause{}
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\item Parse ELF and DWARF data (\texttt{pyelftools})
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\item Run the binary inside gdb
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\item Pause at each (assembly) step
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\item Jointly evaluate DWARF data and the abstract stack
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\item Report upon error
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\end{itemize}
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\vspace{1em}
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Works, but… \alert{Python is slow}!
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A few thousand of ASM instructions/second (good enough)
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\end{frame}
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\begin{frame}{A real bug!}
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\begin{columns}[c]
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\begin{column}{0.65\textwidth}
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\lstinputlisting[language=C]{src/llvm_bug.c}
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\end{column} \begin{column}{0.35\textwidth}
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\textbf{\texttt{CSmith}\\+ \texttt{Creduce}\\+
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\texttt{eh\_frame\_check}}
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\vspace{2em}$\leadsto$ \alert{\bf LLVM (3.8) bug!}
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\end{column}
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\end{columns}
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\end{frame}
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\renewcommand{\tblrowval}[5]{#1 & #2 & #3 & #4 & #5 \\}
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\begin{frame}{A real bug!}
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\begin{columns}[c]
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\column{0.7\textwidth}
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\begin{align*}
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\onslide<2->{\textbf{Abstract state} \qquad &
|
||
\left[\texttt{0xFFFF1000}\right]} \\
|
||
\onslide<3->{\reg{rsp} \qquad & %
|
||
~\,\texttt{%
|
||
\only<3-4>{0xFFFF1000}%
|
||
\only<5-8>{0xFFFF0FF8}%
|
||
\only<9->{0xFFFF1000}%
|
||
}
|
||
}
|
||
\end{align*}
|
||
|
||
\column{0.3\textwidth}
|
||
{\vspace{-4mm}\bf \fontsize{2cm}{5.5cm}\selectfont %
|
||
\only<4>{\cmark}%
|
||
\only<6>{\cmark}%
|
||
\only<8>{\cmark}%
|
||
\only<10->{\xmark}%
|
||
}
|
||
\end{columns}
|
||
|
||
|
||
\vspace{1em}
|
||
|
||
\begin{table}
|
||
\ttfamily\large
|
||
\begin{tabularx}{0.95\linewidth}{
|
||
l
|
||
l
|
||
b
|
||
>{\columncolor{SkyBlue}}s
|
||
>{\columncolor{SkyBlue}}s
|
||
}
|
||
\tblrowval{\hspace{-2ex}<{\bf foo}>:}{}{}{\textbf{CFA}}{\textbf{ra}}
|
||
%\rowonly<3>{\tblhl{}} \tblrowval{4004e0}{push}{\%rbx}{rsp+8}{c-8}
|
||
\rowonly<2-4>{\tblhl{}} \tblrowval{4004e0}{push}{\%rbx}{rsp+8}{c-8}
|
||
\rowonly<5-6>{\tblhl{}} \tblrowval{}{}{}{rsp+16}{c-8}
|
||
\tblrowval{}{[\ldots]}{}{}{}
|
||
\tblrowval{}{}{}{}{}
|
||
\rowonly<7-8>{\tblhl{}} \tblrowval{40061d}{pop}{\%rbx}{rsp+16}{c-8}
|
||
\rowonly<9->{\tblhl{}} \tblrowval{40061e}{retq}{}{rsp+16}{c-8}
|
||
\end{tabularx}
|
||
\end{table}
|
||
|
||
\begin{center}
|
||
\bf\Large %
|
||
\onslide<11>{$\leadsto$ LLVM bug \#13161}
|
||
\end{center}
|
||
|
||
\end{frame}
|
||
|
||
\begin{frame}{What for, in the end?}
|
||
\begin{itemize}[<+->]
|
||
\item We can \alert{find bugs} in compilers
|
||
\item We can \alert{validate DWARF tables}!
|
||
\item \ldots{}well, only along \alert{one execution path}\ldots
|
||
\item but mostly we are close to a working \alert{algorithm} to
|
||
\alert{synthesize unwinding data from assembly}!
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\section{Unwinding data synthesis from assembly}
|
||
|
||
\begin{frame}{What have we got so far?}
|
||
We now want to \alert{synthesize unwinding data}. \pause{}That means
|
||
\alert{forgetting the blue part of the previous schemes}.
|
||
|
||
\begin{itemize}[<+->]
|
||
\item Upon entering a function, we know (ABI)
|
||
\[ \cfa = \reg{rsp} - 8
|
||
\qquad \ra = \cfa + 8 \]
|
||
\item For each instruction, we know \alert{how it changes \cfa}.
|
||
\item We assume \alert{\ra{} constant wrt. \cfa}.
|
||
\begin{itemize}
|
||
\item[$\leadsto$] only \cfa{} tracking matters (for unwinding)
|
||
\end{itemize}
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\section{Unwinding data compilation}
|
||
|
||
\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}{Compilation example: original C, DWARF}
|
||
\lstinputlisting[language=C]{src/fib7/fib7.cfde}
|
||
\end{frame}
|
||
|
||
\begin{frame}[shrink]{Compilation example: generated C}
|
||
\lstinputlisting[language=C]{src/fib7/fib7.eh_elf_basic.c}
|
||
\end{frame}
|
||
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
|
||
\begin{frame}{Compilation choices}
|
||
\textbf{In order to keep the compiler \alert{simple} and \alert{easily
|
||
testable}, the whole DWARF5 instruction set is not supported.}
|
||
|
||
\begin{itemize}
|
||
\item Focus on \alert{x86\_64}
|
||
\item Focus on unwinding return address \\
|
||
\vspace{0.3ex}
|
||
$\leadsto$ \textit{Allows building a backtrace}
|
||
\begin{itemize}
|
||
\item \alert{suitable for perf, not for gdb}
|
||
\item Only supports \alert{unwinding registers}: \reg{rip}, \reg{rsp},
|
||
\reg{rbp}, \reg{rbx}
|
||
\item Supports the \alert{wide majority} ($> 99.9\%$) of instructions
|
||
used
|
||
\item Among \alert{4000} randomly sampled filed, only \alert{24}
|
||
containing unsupported instructions
|
||
\end{itemize}
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
\begin{frame}{Interface: libunwind}
|
||
\begin{itemize}
|
||
\item \alert{libunwind}: \textit{de facto} standard library for
|
||
unwinding
|
||
\item Relies on DWARF
|
||
|
||
\bigskip{}
|
||
|
||
\item \texttt{libunwind-eh\_elf}: alternative implementation using
|
||
\ehelfs{}
|
||
|
||
\item[$\leadsto$] \alert{alternative implementation} of libunwind,
|
||
almost plug-and-play for existing projects!
|
||
\begin{itemize}
|
||
\item[$\leadsto$] It is \alert{easy} to use \ehelfs{}: just
|
||
link against the right library!
|
||
\end{itemize}
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
|
||
\begin{frame}{Size optimisation: outlining}
|
||
\begin{itemize}
|
||
\item This \alert{works}, but \alert{takes space}: about \alert{7 times
|
||
larger in size} than regular DWARF\@.
|
||
|
||
\item DWARF optimisation strategy: \alert{alter previous row}. \\
|
||
Causes slowness: we cannot do that.
|
||
|
||
\item Remark: a lot of lines appear often.
|
||
\begin{itemize}
|
||
\item[$\leadsto$] \textbf{\emph{outline} them!}
|
||
\end{itemize}
|
||
|
||
\pause{}
|
||
|
||
\item On libc, $20\,827$ rows $\rightarrow$ $302$ outlined ($1.5\,\%$)
|
||
\item Turn the big switch into a binary search \alert{if/else tree}
|
||
\end{itemize}
|
||
|
||
\pause{}
|
||
|
||
\bigskip{}
|
||
\begin{center}
|
||
$\leadsto$ only \textbf{2.5 times bigger than DWARF}
|
||
\end{center}
|
||
\end{frame}
|
||
|
||
\begin{frame}{Example with outlining}
|
||
\lstinputlisting[language=C]{src/fib7/fib7.eh_elf_outline.c}
|
||
\end{frame}
|
||
|
||
|
||
\begin{frame}[t]{A word on formalization}
|
||
\begin{itemize}
|
||
\item First task: \alert{writing semantics} for DWARF, written as
|
||
mapping to C code.
|
||
\item DWARF5 specification: \alert{plain English}, no proper semantics
|
||
\item Compiled code is in substance equivalent to semantics
|
||
\item What remains to prove is mostly \alert{simple or classic
|
||
optimisations}
|
||
\end{itemize}
|
||
|
||
\pause{}
|
||
\vspace{-3cm}
|
||
\begin{center}
|
||
\includegraphics[width=0.8\linewidth, angle=10]{img/dw_spec.png}
|
||
\end{center}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\subsection{Benchmarking}
|
||
|
||
\begin{frame}{Benchmarking requirements}
|
||
\begin{enumerate}
|
||
\item Thousands of samples (single unwind: $10\,\mu{}s$)
|
||
\item Interesting enough program to unwind: nested functions, complex
|
||
FDEs
|
||
\item Mitigate caching: don't always unwind from the \emph{same} point
|
||
\item Yet be fair: don't always unwind from totally different places
|
||
\item Distribute evenly: if possible, also from within libraries
|
||
\end{enumerate}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\begin{frame}{perf instrumentation}
|
||
\textbf{\alert{perf} is the state-of-the-art polling profiler for Linux.}
|
||
\begin{itemize}
|
||
\item{} used to get readings of the time spent in each function
|
||
\item{} works by regularly stopping the program, unwinding its stack,
|
||
then aggregating the gathered data
|
||
\end{itemize}
|
||
|
||
\pause{}\bigskip{}
|
||
\textbf{Instrumenting perf matches all the requirements!}
|
||
|
||
\begin{itemize}
|
||
\item{} \alert{Plug \ehelfs{} into perf}: use \ehelfs{} instead of
|
||
DWARF to unwind the stack
|
||
\item{} Implement \alert{unwinding performance counters} inside perf
|
||
\bigskip{}
|
||
|
||
\item{} Use perf on \alert{hackbench}, a kernel stress-test program
|
||
\begin{itemize}
|
||
\item Small program
|
||
\item Lots of calls
|
||
\item Relies on libc, libpthread
|
||
\end{itemize}
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\subsection{Results}
|
||
|
||
\begin{frame}{Time performance}
|
||
\small
|
||
\centering
|
||
\begin{tabular}{l r r r r r}
|
||
\toprule
|
||
\thead{Unwinding method} & \thead{Frames \\ unwound}
|
||
& \thead{Tot.\ time \\ ($\mu s$)}
|
||
& \thead{Avg. \\ time / frame \\ ($ns$)}
|
||
& \thead{Time \\ ratio} \\
|
||
\midrule
|
||
\alert{\ehelfs{}}
|
||
& 23506 % Frames unwound
|
||
& 14837 % Total time
|
||
& 631 % Avg time
|
||
& 1
|
||
\\
|
||
\prog{libunwind}, \alert{cached}
|
||
& 27058 % Frames unwound
|
||
& 441601 % Total time
|
||
& 16320 % Avg time
|
||
& \alert{25.9}
|
||
\\
|
||
\prog{libunwind}, \alert{uncached}
|
||
& 27058 % Frames unwound
|
||
& 671292 % Total time
|
||
& 24809 % Avg time
|
||
& \alert{39.3}
|
||
\\
|
||
\bottomrule
|
||
\end{tabular}
|
||
\end{frame}
|
||
|
||
\begin{frame}{Space performance}
|
||
\begin{center}
|
||
\begin{tabular}{r r r r r r}
|
||
\toprule
|
||
\thead{Object}
|
||
& \thead{\% of binary size}
|
||
& \thead{Growth factor} \\
|
||
\midrule
|
||
libc
|
||
& 21.88 & 2.41 \\
|
||
libpthread
|
||
& 43.71 & 2.19 \\
|
||
ld
|
||
& 22.09 & 2.97 \\
|
||
hackbench
|
||
& 93.87 & 4.99 \\
|
||
\midrule
|
||
Total
|
||
& 22.81 & \alert{2.44} \\
|
||
\bottomrule
|
||
\end{tabular}
|
||
\end{center}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\section*{}
|
||
\setcounter{section}{0}
|
||
|
||
\begin{frame}{What next?}
|
||
\begin{itemize}
|
||
\item Implement a release-ready, packageable, easy to use version of
|
||
perf with \ehelfs{} and submit it for inclusion
|
||
|
||
\item{} Measure \alert{C++ exceptions overhead} precisely in common
|
||
software
|
||
|
||
\item{} Implement \alert{\ehelfs{}} support for \alert{C++ runtime}
|
||
exception handling, and other systems where unwinding is a
|
||
performance bottleneck
|
||
|
||
\medskip
|
||
|
||
\item \alert{Outlining} was effective for
|
||
compactness\ldots{} Try outlining DWARF bytecode\@?
|
||
|
||
\end{itemize}
|
||
\end{frame}
|
||
|
||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||
\begin{frame}
|
||
\vspace{5mm}
|
||
\includegraphics[width=\linewidth]{img/keep_breathing}
|
||
\vspace{-1cm}
|
||
|
||
\begin{center}
|
||
\large
|
||
\begin{align*}
|
||
\textbf{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
|
||
\textbf{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
|
||
\end{align*}
|
||
\end{center}
|
||
|
||
\end{frame}
|
||
|
||
\end{document}
|