talk-2018-11-uppsala/slides.tex

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% vim: spell spelllang=en
\documentclass[11pt,xcolor={usenames,dvipsnames}]{beamer}
\usetheme{Warsaw}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
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\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{booktabs}
\usepackage{makecell}
\usepackage{ifthen}
\usepackage{colortbl}
\usepackage{tabularx}
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\usepackage{texlib/my_listings}
\usepackage{texlib/specific}
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{}
{\insertframenumber/\inserttotalframenumber}}
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\begin{frame}
\vfill
\centering
\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
\usebeamerfont{title}\insertsectionhead\par%
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\lstdefinelanguage{gdb}{
morekeywords={gdb},
sensitive=false,
}
\newcolumntype{b}{X}
\newcolumntype{s}{>{\hsize=.43\hsize}X}
\newcommand{\lstinl}
{\lstinline[language=C, keepspaces=true, basicstyle=\ttfamily]}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\title[\sectionline] {Growing the DWARF tougher:\\
synthesis, validation and compilation}
\author[\slidecountline]{\textbf{Théophile Bastian} \\ \vspace{0.5em}
{{\footnotesize{}Based on work done with}\\
\textbf{Francesco Zappa Nardelli},
\textbf{Stephen Kell},
\textbf{Simon Ser}}}
\date{}
%\subject{}
%\logo{}
\institute{ENS Paris, INRIA}
\begin{document}
\begin{frame}
\addtocounter{framenumber}{-1}
\titlepage{}
\vspace{-2em}
\begin{center}
\begin{align*}
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\text{Slides: } &\text{\url{https://tobast.fr/files/sweden18.pdf}} \\
\end{align*}
\end{center}
\end{frame}
\begin{frame}{~}
\addtocounter{framenumber}{-1}
\tableofcontents[hideallsubsections]
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{DWARF and stack unwinding data}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Introduction}
\begin{frame}[fragile]{We often use stack unwinding!}
\begin{columns}[c]
\begin{column}{0.70\textwidth}
\begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
Program received signal SIGSEGV.
0x54625 in fct_b at segfault.c:5
5 printf("%l\n", *b);
|\pause| (gdb) backtrace
#0 0x54625 in fct_b at segfault.c:5
#1 0x54663 in fct_a at segfault.c:10
#2 0x54674 in main at segfault.c:14
|\pause| (gdb) frame 1
#1 0x54663 in fct_a at segfault.c:10
10 fct_b((int*) a);
|\pause| (gdb) print a
$1 = 84
\end{lstlisting}
\vspace{-1em}
\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 grandparent RA\@?
\medskip
Isn't it as trivial as \texttt{pop()}?
\vspace{2em}
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\onslide<2>{We only have \reg{rsp} and \reg{rip}.}
\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}
\end{frame}
\begin{frame}{Difficult to generate}
Major concern with DWARF\@: it is \alert{difficult to generate}
(correctly).
\begin{itemize}
\item{} \alert{Hard to generate}: each compiler pass must keep it in
sync
\item{} Most of it is \alert{seldom used} (\eg{} unwinding data of
dusty code), and thus \alert{seldom tested}
\end{itemize}
\vspace{1em}
Yields to
\begin{itemize}
\item{} unreliable DWARF\@: can cause headaches when debugging
\item{} or not generated at all (eg. OCaml until recently) \todo{Check
this}
\end{itemize}
\vspace{1em}
\begin{center}
\Large\bf
$\leadsto$ Complex, buggy, untested
\end{center}
\end{frame}
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\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}.''
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}
\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{Ill reconsider}.''
2018-11-08 19:37:04 +01:00
}
\newcommand{\LinusSource}{
\hfill ---~Linus Torvalds, Kernel mailing list, 2012
}
\begin{frame}{A debugging hell: Linux kernel}
\LinusMailOne{}
\vspace{1em}
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\LinusSource{}
\pause{}\vspace{1em}
\begin{center}
\Large\bf
\alert{This is where we still are!}
\end{center}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data validation}
\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}}
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\begin{frame}{Validating 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}
\rowonly<9>{\tblhl{}} \tblrowval{cmp}{\$0x1,\%edi}{rsp+160}{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})} (ABI)}
\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 can actually be seen as\\
an \alert{abstract interpretation} of the code\ldots}
\only<8>{\ldots and thus, for a given run, be
\alert{re-computed for verification}}
\end{overlayarea}
}
}
\only<9->{
\begin{table}
\ttfamily\large
\begin{tabularx}{0.9\linewidth}{
l
b
>{\columncolor{SkyBlue}}s
>{\columncolor{SkyBlue}}s
}
\firsttblrows{}%
\end{tabularx}
\end{table}
\vspace{-0.8em}
\only<9>{\blklnote{If, within an execution,
\begin{itemize}
\item ra = \texttt{*(0xFFFF1098)}
\item \reg{rsp} = \texttt{0xFFFF1000}
\end{itemize}
We can \alert{evaluate both expressions} and \alert{compare}
}}
}
\end{frame}
\begin{frame}{Dynamic validation}
\textbf{Abstract state}
\begin{itemize}
\item \alert{Stack} of actual \alert{addresses} where
\alert{return addresses} are stored
\end{itemize}
\vspace{2em}\pause{}
\textbf{Abstract instruction semantics}
\begin{itemize}
\item[\alert{\texttt{call}}] push \alert{\reg{rsp}} on the stack
\item[\alert{\texttt{ret}}] pop from the stack
\end{itemize}
\vspace{2em}\pause{}
\textbf{Validation of each instruction}
\begin{itemize}
\item Evaluate the return address provided by DWARF
\item Compare it with the value at the top of the stack
\end{itemize}
\end{frame}
\begin{frame}{In practice: \texttt{eh\_frame\_check}}
Strategy implemented and working: \alert{\texttt{eh\_frame\_check}}
\begin{itemize}
\item \texttt{gdb} allows for Python instrumentation
\vspace{1em}\pause{}
\item Parse ELF and DWARF data (\texttt{pyelftools})
\item Run the binary inside gdb
\item Pause at each (assembly) step
\item Jointly evaluate DWARF data and the abstract stack
\item Report upon error
\end{itemize}
\vspace{1em}
Works, but… \alert{Python is slow}!
A few thousand of ASM instructions/second (good enough)
\end{frame}
\begin{frame}{A real bug!}
\begin{columns}[c]
\begin{column}{0.65\textwidth}
\lstinputlisting[language=C]{src/llvm_bug.c}
\end{column} \begin{column}{0.35\textwidth}
\textbf{\texttt{CSmith}\\+ \texttt{Creduce}\\+
\texttt{eh\_frame\_check}}
\vspace{2em}$\leadsto$ \alert{\bf LLVM (3.8) bug!}
\end{column}
\end{columns}
\end{frame}
\renewcommand{\tblrowval}[5]{#1 & #2 & #3 & #4 & #5 \\}
\begin{frame}{A real bug!}
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\begin{columns}[c]
\column{0.7\textwidth}
\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}
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{\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}
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\rowonly<9->{\tblhl{}} \tblrowval{40061e}{retq}{}{rsp+16}{c-8}
\end{tabularx}
\end{table}
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\begin{center}
\bf\Large %
\onslide<11>{$\leadsto$ LLVM bug \#13161}
\end{center}
\end{frame}
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\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}
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\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}
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\item We had a working strategy for a \alert{linear execution}
\item We still have to handle
\begin{itemize}
\item \alert{\cfa{} expression}
\item \alert{control flow graph}
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}{\cfa{} expression}
Two possibilities:
\begin{itemize}
\item Either \reg{rbp} is used as base pointer \pause{}
\item Or we must track \cfa{} wrt. \reg{rsp}
\begin{itemize}
\item And update it after each instruction if needed
\end{itemize}
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\end{itemize}
\end{frame}
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\begin{frame}{Control flow graph}
\begin{columns}[c]
\column{0.4\textwidth}
\lstinputlisting[language=C]{src/cfg/cfg.c}
\pause{}
\column{0.30\textwidth}
\begin{figure}
\centering
\includegraphics[width=\textwidth]{src/cfg/cfg.png}
\end{figure}
\end{columns}
\pause{}
\begin{itemize}
\item \alert{Upon split} (eg. \texttt{X})\alert{:} nothing special,
propagate end state of X to children nodes A and B
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\item \alert{Upon join} (eg. \texttt{while\_end})\alert{:} check
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consistency of both input states
\begin{itemize}
\item If tricky, \texttt{gcc} will have used \reg{rbp}, even
with \texttt{-fomit-frame-pointer}.
\end{itemize}
\end{itemize}
\end{frame}
\renewcommand{\tblrowval}[4]{#1 & #2 & #3 & #4 \\}
\begin{frame}{Trust the compiler to avoid tricky unwinding}
\begin{columns}[c]
\column{0.55\textwidth}
\lstinputlisting[language=C,firstline=3,lastline=7]
{src/use_rbp/use_rbp.c}
\pause{}
\begin{itemize}
\item At each loop cycle, \alert{\texttt{y} is larger} and
\alert{allocated on the stack}
\item Thus, \reg{rsp} is \alert{farther from \cfa} at each cycle: no
constant rule $\cfa = \reg{rsp} + k$.
\item A complex DWARF expression is possible, but
\alert{the compiler won't}.
\end{itemize}
\pause{}
\column{0.45\textwidth}
\lstbash{\$ gcc -O0 -g -c src.c -fomit-frame-pointer}
\vspace{1em}
\begin{center}
\tt
\begin{tabular}{l l l l}
\tblrowval{LOC} {CFA} {rbp} {ra}
\tblrowval{000} {rsp+8} {u} {c-8}
\tblrowval{001} {rsp+16} {c-16} {c-8}
\tblrowval{004} {\alert{rbp}+16} {c-16} {c-8}
\tblrowval{010} {\alert{rbp}+16} {c-16} {c-8}
\tblrowval{0ce} {rsp+8} {c-16} {c-8}
\end{tabular}
\end{center}
\end{columns}
\end{frame}
2018-11-08 20:35:15 +01:00
\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}
\subsection{Compilation ahead-of-time}
2018-11-08 20:35:15 +01:00
\begin{frame}{Why compiling?}
\begin{itemize}
\item Remember that \alert{DWARF is slow}!
\item \alert{Bytecode} interpreted \alert{on the fly} to
generate the data tables
\item Done so for \alert{extreme compacity}
\end{itemize}
\pause{}
\vspace{1em}
\begin{itemize}
\item Goal: \alert{reasonable time-space trade-off} to speed up DWARF
\item Tables are now \alert{compiled functions} returning the requested
DAWRF row
\end{itemize}
\end{frame}
\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}
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\item Most of the rows boil down to \alert{a few common rows}.
\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}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\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}
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\textbf{\alert{perf} is a 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}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
2018-11-08 19:37:04 +01:00
\section*{Conclusion}
\setcounter{section}{0}
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\begin{frame}{}
\LinusMailOne{}
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\only<1>{\LinusSource{}}
\only<2->\vspace{1em}
\pause{}
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\LinusMailTwo{}
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\LinusSource{}
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\pause{}
\vspace{1em}
\begin{center}
\bf
Give us a few months to make Linus reconsider \texttt{;)}
\end{center}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}
\vspace{5mm}
\includegraphics[width=\linewidth]{img/keep_breathing}
\vspace{-1cm}
\begin{center}
\large
\begin{align*}
2018-11-08 19:37:04 +01:00
\textbf{Slides: } &\text{\url{https://tobast.fr/files/sweden18.pdf}} \\
\end{align*}
\end{center}
\end{frame}
\end{document}