report/report/report.tex

\title{DWARF debugging data, compilation and optimization}

\author{Théophile Bastian\\
Under supervision of Francesco Zappa-Nardelli\\
{\textsc{parkas}, \'Ecole Normale Supérieure de Paris}}

\date{March -- August 2018\\August 20, 2018}

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    \todo{Is there a need for an abstract, given the presence above of the
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\section{Stack unwinding data presentation}

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\subsection{Stack frames and unwinding}

On most platforms, programs make use of a \emph{call stack} to store
information about the nested function calls at the current execution point, and
keep track of their nesting. Each function call has its own \emph{stack frame},
an entry of the call stack, whose precise contents are often specified in the
Application Binary Interface (ABI) of the platform, and left to various extents
up to the compiler. Those frames are typically used for storing function
arguments, machine registers that must be restored before returning, the
function's return address and local variables.

For various reasons, it might be interesting, at some point of the execution of
a program, to glance at its program stack and be able to extract informations
from it. For instance, when running a debugger such as \prog{gdb}, a frequent
usage is to obtain a \emph{backtrace}, that is, the list of all nested function
calls at this point. This actually reads the stack to find the different stack
frames, and decode them to identify the function names, parameter values, etc.

This operation is far from trivial. Often, a stack frame will only make sense
with correct machine registers values, which can be restored from the previous
stack frame, imposing to \emph{walk} the stack, reading the entries one after
the other, instead of peeking at some frame directly. Moreover, the size of one
stack frame is often not that easy to determine when looking at some
instruction other than \texttt{return}, making it hard to extract single frames
from the whole stack.

Interpreting a frame in order to get the machine state \emph{before} this
frame, and thus be able to decode the next frame recursively, is called
\emph{unwinding} a frame. For all the reasons above and more, it is often
necessary to have additional data to perform stack unwinding. This data is
often stored among the debugging informations of a program, and one common
format of debugging data is DWARF\@.

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\subsection{Unwinding usage and frequency}

Stack unwinding is a more common operation that one might think at first. The
most commonly thought use-case is simply to get a stack trace of a program, and
provide a debugger with the information it needs: for instance, when inspecting
a stack trace in \prog{gdb}, it is quite common to jump to a previous frame:

\lstinputlisting{src/segfault/gdb_session}

To be able to do this, \texttt{gdb} must be able to restore \lstc{fct_a}'s
context, by unwinding \lstc{fct_b}'s frame.

\medskip

Yet, stack unwinding (and thus debugging data) \emph{is not limited to
debugging}.

Another common usage is profiling. A profiling tool, such as \prog{perf} under
Linux, is used to measure and analyze in which functions a program spends its
time, identify bottlenecks and find out which parts are critical to optimize.
To do so, modern profilers pause the traced program at regular, short
intervals, inspect their stack, and determine which function is currently being
run. They also often perform a stack unwinding to determine the call path to
this function, to determine which function indirectly takes time: \eg, a
function \lstc{fct_a} can call both \lstc{fct_b} and \lstc{fct_c}, which are
quite heavy; spend practically no time directly in \lstc{fct_a}, but spend a
lot of time in calls to the other two functions that were made by \lstc{fct_a}.

Exception handling also requires a stack unwinding mechanism in most languages.
Indeed, an exception is completely different from a \lstc{return}: while the
latter returns to the previous function, the former can be caught by virtually
any function in the call path, at any point of the function. It is thus
necessary to be able to unwind frames, one by one, until a suitable
\lstc{catch} block is found. The C++ language, for one, includes a
stack-unwinding library similar to \prog{libunwind} in its runtime.

In both of these two previous cases, performance \emph{can} be a problem. In
the latter, a slow unwinding directly impacts the overall program performance,
particularly if a lot of exceptions are thrown and caught far away in their
call path. In the former, profiling \emph{is} performance-heavy and often quite
slow when analyzing large programs anyway.

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\subsection{DWARF format}

The DWARF format was first standardized as the format for debugging
information of the ELF executable binaries. It is now commonly used across a
wide variety of binary formats to store debugging information. As of now, the
latest DWARF standard is DWARF 5~\cite{dwarf5std}, which is openly accessible.

The DWARF data commonly includes type information about the variables in the
original programming language, correspondence of assembly instructions with a
line in the original source file, \ldots
The format also specifies a way to represent unwinding data, as described in
the previous paragraph, in an ELF section originally called
\lstc{.debug_frame}, most often found as \ehframe.

For any binary, debugging information can easily get quite large if no
attention is payed to keeping it as compact as possible. In this matter, DWARF
does an excellent job, and everything is stored in a very compact way. This,
however, as we will see, makes it both difficult to parse correctly (with \eg{}
variable-length integers) and quite slow to interpret.

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\subsection{DWARF unwinding data}

The unwinding data, which we will call from now on the \ehframe, contains, for
each possible instruction pointer (that is, an instruction address within the
program), a set of ``registers'' that can be unwound, and a rule describing how
to do so.

The DWARF language is completely agnostic of the platform and ABI, and in
particular, is completely agnostic of a particular platform's registers. Thus,
when talking about DWARF, a register is merely a numerical identifier that is
often, but not necessarily, mapped to a real machine register by the ABI\@.

In practice, this data takes the form of a collection of tables, one table per
Frame Description Entry (FDE), which most often corresponds to a function. Each
column of the table is a register (\eg{} \reg{rsp}), with two additional
special registers, CFA (Canonical Frame Address) and RA (Return Address),
containing respectively the base pointer of the current stack frame and the
return address of the current function (\ie{} for x86\_64, the unwound value of
\reg{rip}, the instruction pointer). Each row of the table is a particular
instruction pointer, within the instruction pointer range of the tabulated FDE
(assuming a FDE maps directly to a function, this range is simply the IP range
of the given function in the \lstc{.text} section of the binary), a row being
valid from its start IP to the start IP of the next row, or the end IP of the
FDE if it is the last row.

\begin{minipage}{0.45\textwidth}
    \lstinputlisting[language=C, firstline=3, lastline=12]
        {src/fib7/fib7.c}
\end{minipage} \hfill \begin{minipage}{0.45\textwidth}
    \lstinputlisting[language=C]{src/fib7/fib7.fde}
\end{minipage}

For instance, the C source code above, when compiled with \lstbash{gcc -O0
-fomit-frame-pointer}, gives the table at its right. During the function
prelude, \ie{} for $\mhex{675} \leq \reg{rip} < \mhex{679}$, the stack frame
only contains the return address, thus the CFA is 8 bytes above \reg{rsp}
(which was the value of \reg{rsp} before the call), and the return address is
precisely at \reg{rsp}. Then, 9 integers of 8 bytes each (8 for \lstc{fibo},
one for \lstc{pos}) are allocated on the stack, which puts the CFA 80 bytes
above \reg{rsp}, and the return address still 8 bytes below the CFA\@. Then, by
the end of the function, the local variables are discarded and \reg{rsp} is
reset to its value from the first row.

However, DWARF data isn't actually stored as a table in the binary files. The
first row has the location of the first IP in the FDE, and must define at least
its CFA\@. Then, when all relevant registers are defined, it is possible to
define a new row by providing a location offset (\eg{} here $4$), and the new
row is defined as a clone of the previous one, which can then be altered (\eg{}
here by setting \lstc{CFA} to $\reg{rsp} + 80$). This means that every line is
defined \wrt{} the previous one, and that the IPs of the successive rows cannot
be determined before evaluating every row before. Thus, unwinding a frame from
an IP close to the end of the frame will require evaluating pretty much every
DWARF row in the table before reaching the relevant information, slowing down
drastically the unwinding process.

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\subsection{How big are FDEs?}
\todo{}

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\subsection{Unwinding state-of-the-art}

The most commonly used library to perform stack unwinding, in the Linux
ecosystem, is \prog{libunwind}~\cite{libunwind}. While it is very robust and
quite efficient, most of its optimization comes from fine-tuned code and good
caching mechanisms. While parsing DWARF, \prog{libunwind} is forced to parse
the relevant FDE from its start, until it finds the row it was seeking.

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\subsection{General statistics}
\todo{}


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\section{Stack unwinding data compilation}

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\subsection{Compilation: \ehelfs}
\todo{}

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\subsection{First results}
\todo{}

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\subsection{Space optimization}
\todo{}


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\section{Benchmarking}

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\subsection{Requirements}
\todo{}

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\subsection{Presentation of \prog{perf}}
\todo{}

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\subsection{Benchmarking with \prog{perf}}
\todo{}

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\subsection{Other explored methods}
\todo{}


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\section{Results}

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\subsection{Measured time performance}
\todo{}

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\subsection{Measured compactness}
\todo{}

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\subsection{Instructions coverage}
\todo{}

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