Slides for OOPSLA19 presentation "Reliable and Fast DWARF-based Stack Unwinding"
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\title[] {Reliable and Fast DWARF-based Stack Unwinding}
\author[\slidecountline]{\alert{\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}
{\small\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}
\begin{textblock*}{0.22\textwidth}[0.5,0](0.15\paperwidth,0.38\paperheight)%
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\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}
\hfill
\begin{column}{0.35\textwidth}
\end{column}
\end{columns}
\only<4->{
\begin{textblock*}{0.35\textwidth}[1,0.5](0.9\paperwidth,0.5\paperheight)%
\includegraphics[width=0.95\linewidth]{img/call_stack}
\end{textblock*}
}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Stack frames and unwinding}
\begin{frame}
\begin{columns}[c]
\begin{column}{0.55\textwidth}
\begin{center}
\large\bf
How do we get\\
the return address?
\vspace{2em}
\onslide<2>{What if we only have \reg{rsp}?}
\end{center}
\end{column}
\hfill
\begin{column}{0.35\textwidth}
\end{column}
\end{columns}
\begin{textblock*}{0.35\textwidth}[1,0.5](0.9\paperwidth,0.5\paperheight)%
\includegraphics[width=0.95\linewidth]{img/call_stack}
\end{textblock*}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{DWARF tables}
\newcolumntype{a}{>{\columncolor{RedOrange}}l}
\begin{frame}{DWARF unwinding data}
\tt \footnotesize
\begin{center}
\begin{tabular}{
>{\columncolor{YellowGreen}}l
>{\columncolor{Thistle}}l
l l l l l l
>{\columncolor{Apricot}}l}
~PC & 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}
\only<1>{\vspace{19mm}}
\begin{columns}
\begin{column}{0.50\textwidth}
\onslide<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}
\onslide<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}
\pause{}
\vfill
\begin{itemize}
\bf
\item[\textbf{$\longrightarrow$}] \alert{bytecode} for a
\alert{Turing-complete stack machine}
\item[\textbf{$\longrightarrow$}] which is \alert{interpreted on
demand at runtime}\\to reconstruct the table
\end{itemize}
\end{frame}
\begin{frame}{What does this imply?}
\begin{center}
\textbf{Your compiler generates code for \alert{two machines}:\\
your processor and the DWARF VM\@.}
\end{center}
\vfill{}
\begin{columns}
\begin{column}{0.45\textwidth}
\begin{center}
\begin{tikzpicture}
\begin{scope}[every node/.style={rectangle,thick,draw,scale=0.95}]
\node (cmd) at (0, 3.0) {
\lstbash{\$ gcc -S foo.c}
};
\node (asm) at (0, 0) {
\lstinputlisting[numbers=none, language=cfiasm]{src/main_cfi.s}
};
\end{scope}
\begin{scope}[>={Stealth[black]},
every path/.style={draw=black,very thick}]
\path [->] (cmd) -- (asm);
\end{scope}
\end{tikzpicture}
%\vspace{0.2em}
\textbf{\lstc{.cfi_*}: \alert{inline DWARF!}}
\end{center}
\end{column}
\pause{}
\begin{column}{0.55\textwidth}
\begin{itemize}
\item[$\implies$] \alert{Cumbersome} to generate for the
\alert{compiler}
\begin{itemize}
\item[$\leadsto$] might do it wrong
\item[$\leadsto$] might not do it at all
\end{itemize}
\item[$\implies$] If you write \alert{inline asm}, \alert{you} must write
inline DWARF\@!
\end{itemize}
\end{column}
\end{columns}
\vfill{}
\end{frame}
\begin{frame}
\lstinputlisting[language=bash,numbers=none,basicstyle=\tt\fontsize{8pt}{9pt}\selectfont]{src/lowlevellock_dw_extr.c}
\only<2>{
\begin{textblock*}{0.60\textwidth}[0.5,0.5](0.5\paperwidth,0.5\paperheight)%
\begin{tcolorbox}[halign=center, colframe=infoboxborder, colback=infoboxbg]
In \prog{glibc}, \prog{lowlevellock.h}:
\alert{off by one error in unwinding data}.
\lstinputlisting[language=gdb,numbers=none]{src/lowlevellock_backtrace}
\end{tcolorbox}
\end{textblock*}
}
\only<3->{
\begin{textblock*}{0.90\textwidth}[0.5,0](0.5\paperwidth,0.10\paperheight)%
\begin{tcolorbox}[halign=center, colframe=alertboxborder, colback=alertboxbg]
\bf \LARGE
Complex \,\& \,slow
\end{tcolorbox}
\end{textblock*}
}
\only<4->{
\begin{textblock*}{0.90\textwidth}[0.5,0](0.5\paperwidth,0.30\paperheight)%
\begin{tcolorbox}[halign=center, colframe=alertboxborder, colback=alertboxbg]
\LARGE
\textbf{Pervasive:}\\ relied upon by profilers, debuggers,
aaand\ldots{}
\onslide<5->{
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{Correctness by construction:\\*\textbf{synthesis of unwinding tables}}
\sectiontitleframe{}
\newcommand{\tblrowval}[4]{#1 & #2 & \only<2->{#3} & \only<2->{#4} \\}
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{\begin{block}{}
\centering\large
#1
\end{block}}
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#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*}{\textwidth}[0.5,0.5](0.5\paperwidth,0.5\paperheight)%
\begin{tcolorbox}[halign=center, colframe=alertboxborder, colback=alertboxbg]
\large
\alert{\bf Assumptions}
\vspace{0.6em}
\begin{itemize}
\item the compiler generated the unwinding data
\item we have a reliable DWARF interpreter
\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}
\end{tabularx}
\end{table}
\blknote{
\centering
\begin{overlayarea}{0.9\textwidth}{2.6em}
\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 is \alert{redundant with the binary}.}
\end{overlayarea}
}
}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data synthesis from binaries}
\begin{frame}{Synthesis strategy}
\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
the \cfa}.
\begin{itemize}
\item Heuristic to decide whether we index with \reg{rbp} or
\reg{rsp}
\end{itemize}
\item With a \alert{symbolic execution} with an abstract semantics,\\
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}
\vspace{1em}
\begin{tcolorbox}[halign=center, colframe=infoboxborder, colback=infoboxbg]
\large
Implemented on top of CMU's \prog{BAP}
\end{tcolorbox}
\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}
\begin{frame}
\begin{center}
\Huge
Unwinding data is
\textsc{slo\pause{}o\pause{}o\pause{}o\pause{}o\pause{}o\pause{}o\pause{}o\pause{}w}.
\end{center}
\vspace{2em}
\pause{}
So much that \prog{perf} cannot unwind online!
It must \alert{copy to disk the whole call stack} every few instants and
\alert{analyze it later} at report time!
\end{frame}
\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}
};
\onslide<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}
~PC & 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}
};
}
\onslide<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}: most common library for
unwinding
\bigskip{}
\item \alert{\texttt{libunwind-eh\_elf}}: modified version to support
\ehelfs{}
\item[$\leadsto$] Same API, almost \alert{``relink-and-play''} for existing projects!
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Results}
\begin{frame}{Performances}
\begin{center}
\Large\bf Unwinding speedup vs.\ libunwind:
\begin{tabular}{rl}
\alert{x15} &on \prog{\tt{}perf gzip}\\
\alert{x25} &on \prog{\tt{}perf hackbench}\\
\end{tabular}
\end{center}
\vfill
\begin{center}
\Large\bf Space overhead vs. DWARF:\\
\alert{x2.6 -- x3}
\end{center}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section*{Conclusion}
\setcounter{section}{0}
\begin{frame}{}
\vfill
\centering
\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
\Large
What's next?
\end{beamercolorbox}
\vfill
\end{frame}
\begin{frame}{}
\begin{itemize}
\item{} Synthesis + compare = verification of unwinding data!
\item{} Integrate synthesis into compilers \& debuggers\\
$\rightarrow$ support for inline assembly, fallback method, \ldots
\item{} Integrate into \prog{perf} for online unwinding
\item{} Probably many more cool projects!
\end{itemize}
\vspace{1em}
\begin{center}
Come and chat if interested! \texttt{:)}
\end{center}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section*{Extra slides}
\begin{frame}[noframenumbering]
\end{frame}
\begin{frame}[noframenumbering]{Fixpoint upon control flow merge}
\begin{columns}
\begin{column}{0.3\textwidth}
\begin{tikzpicture}
\begin{scope}[every node/.style={rectangle,thick,draw,scale=0.95}]
\node (if) at (0, 5) {\lstbash{if cnd}};
\node (then) at (-1, 3) { \lstbash{then A} };
\node (else) at (1, 3) { \lstbash{else B} };
\node (after) at (0, 1) { \lstbash{C} };
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\path [->] (if) -- (else);
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\path [->] (else) -- (after);
\end{scope}
\end{tikzpicture}
\end{column}
\begin{column}{0.6\textwidth}
\begin{center}
If eg.
\[
CFA(A) = c-48 \qquad CFA(B) = c-52
\]
no possible unwinding data for C, \alert{even for the
compiler}!
\vspace{1em}
Also, \alert{no possible clean function postlude}!
\vspace{2em}
$\implies$ $CFA(A) = CFA(B)$ and merge is immediate
\end{center}
\end{column}
\end{columns}
\end{frame}
\begin{frame}[noframenumbering]{Fixpoint upon loop control flow merge}
\begin{columns}
\begin{column}{0.3\textwidth}
\begin{tikzpicture}
\begin{scope}[every node/.style={rectangle,thick,draw,scale=0.95}]
\node (inbound) at (0, 7) {\lstbash{A}};
\node (while) at (0, 5) {\lstbash{for i in ...}};
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\node (done) at (0, 1) { \lstbash{C} };
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\node [ellipse, thick, draw, minimum width=10em, minimum height=3em, color=red] (circafter) at (0, 3) {};
\begin{scope}[>={Stealth[black]}, every path/.style={draw=black,very thick}]
\path [->] (inbound) -- (while);
\path [->] (while) edge[bend right] (do);
\path [->] (do) -- (done);
\path [->] (do) edge[bend right] (while);
\end{scope}
\end{tikzpicture}
\end{column}
\begin{column}{0.6\textwidth}
\begin{center}
{\large\alert{Variable stack frame size!}}
\vspace{1em}
We cannot hope for a simple invariant\dots\\
but the compiler cannot
either.
\vspace{1em}
{
\large\alert{$\implies$} the compiler will\\
\alert{fallback to \reg{rbp}}\\
}
even with \lstbash{--fomit-frame-pointer}
\end{center}
\end{column}
\end{columns}
\end{frame}
\end{document}