m2-internship
/
report
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

Add first results

This commit is contained in:
Théophile Bastian 2018-08-03 01:04:38 +02:00
parent 122c69b0ca
commit 2e656c44d9
4 changed files with 132 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

91
report/report.tex
View File

@ -15,6 +15,8 @@ Under supervision of Francesco Zappa-Nardelli\\
\usepackage{mathtools}
\usepackage{indentfirst}
\usepackage[utf8]{inputenc}
\usepackage{makecell}
\usepackage{booktabs}
%\usepackage[backend=biber,style=alphabetic]{biblatex}
\usepackage[backend=biber]{biblatex}
@ -26,6 +28,11 @@ Under supervision of Francesco Zappa-Nardelli\\
\addbibresource{../shared/report.bib}
\renewcommand\theadalign{c}
\renewcommand\theadfont{\bfseries}
%\renewcommand\theadgape{\Gape[4pt]}
%\renewcommand\cellgape{\Gape[4pt]}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
@ -227,7 +234,7 @@ the relevant FDE from its start, until it finds the row it was seeking.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{DWARF semantics}
\section{DWARF semantics}\label{sec:semantics}
We will now define semantics covering most of the operations used for
CFI\footnote{To be defined elsewhere in the report} described in the DWARF
@ -507,13 +514,91 @@ and $\semR{\bullet}$ is defined as
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Stack unwinding data compilation}
The tentative approach that was chosen to try to get better unwinding speeds at
a reasonable space loss was to compile directly the \ehframe{} into native
machine code on the x86\_64 platform.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Compilation: \ehelfs}
\todo{}
The rough idea of the compilation is to produce, out of the \ehframe{} section
of a binary, C code that resembles the code shown in the DWARF semantics from
Section~\ref{sec:semantics} above. This C code is then compiled by GCC,
providing for free all the optimisation passes of a modern compiler. This code
is compiled as a shared library, containing a single function, taking as
argument an instruction pointer and a memory context (\ie{} the value of the
various machine registers). An optionally enabled parameter can be used to pass
a function pointer to a dereferencing function, that conceptually does what the
dereferencing \lstc{*} operator on a pointer, and is used to unwind a process
that is not the currently running process, and thus not sharing the same
address space. A call to this function returns a fresh memory context,
containing the values the registers hold after unwinding this frame.
This generated data is stored in separate shared object files, which we call
\ehelfs. It would have been possible to alter the original ELF file to embed
this data as a new section, but it getting it to be executed just as any
portion of the \lstc{.text} section would probably have been painful, and
keeping it separated during the experimental phase is quite convenient. It is
possible to have multiple versions of \ehelfs{} files in parallel, with various
options turned on or off, and it doesn't require to alter the base system by
editing \eg{} \texttt{/usr/lib/libc-*.so}. Instead, when the \ehelf{} data is
required, those files can simply be \lstc{dlopen}'d.
\todo{More details here? Is it necessary or just too technical?}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{First results}
\todo{}
Without any particular care to efficiency or compactness, it is already
possible to produce a compiled version very close to the one described in
Section~\ref{sec:semantics}. Although the unwinding speed cannot yet be
actually benchmarked, it is already possible to write in a few hundreds of line
of C a simple stack walker printing the functions traversed. It already works
without any problem on the easily tested cases, since corner cases are mostly
found in standard and highly optimal libraries, and it is not that easy to get
the program to stop and print a stack trace from within a system library
without using a debugger.
The major drawback of this approach, without any particular care taken, is the
space waste.
\begin{table}[h]
\begin{tabular}{r r r r r r}
\toprule
\thead{Shared object} & \thead{Original \\ program size}
& \thead{Original \\ \lstc{.eh\_frame}}
& \thead{Generated \\ \ehelf{} \lstc{.text}}
& \thead{\% of original \\ program size}
& \thead{Growth \\ factor} \\
\midrule
libc-2.27.so & 1.4 MiB & 130.1 KiB & 914.9 KiB & 63.92 & 7.03 \\
libpthread-2.27.so & 58.1 KiB & 11.6 KiB & 70.5 KiB & 121.48 & 6.09 \\
ld-2.27.so & 129.6 KiB & 9.6 KiB & 71.7 KiB & 55.34 & 7.44 \\
hackbench & 2.9 KiB & 568.0 B & 2.1 KiB & 74.78 & 3.97 \\
Total & 1.6 MiB & 151.8 KiB & 1.0 MiB & 65.32 & 6.98 \\
\bottomrule
\end{tabular}
\caption{Basic \ehelfs{} space usage}\label{table:basic_eh_elf_space}
\end{table}
The space taken by those tentative \ehelfs{} is analyzed in
Table~\ref{table:basic_eh_elf_space} for \prog{hackbench}, a small program
introduced later in Section~\qtodo{Add a reference}, and the libraries on which
it depends.
The first column only includes the sizes of the ELF sections \lstc{.text} (the
program itself) and \lstc{.rodata}, the read-only data (such as static strings,
etc.). Only the weight of the \lstc{.text} section of the generated \ehelfs{}
is considered, because it is self-consistent (few data or none is stored in
\lstc{.rodata}), and the other sections could be removed if the \ehelfs{}
\lstc{.text} was somehow embedded in the original shared object.
This first tentative version of \ehelfs{} is roughly 7 times heavier than the
original \lstc{.eh_frame}, and represents a far too significant proportion of
the original program size.
\todo{more in-depth analysis?}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Space optimization}

8
report/tables/compare_sizes_old Normal file
View File

@ -0,0 +1,8 @@
\begin{tabular}{r r r r r r r }
\leftcell{Shared object} & \leftcell{Orig prog size} & \leftcell{Orig eh_frame} & \leftcell{Gen eh_elf .text} & \leftcell{+ .rodata} & \leftcell{% of prog size} & \leftcell{Growth} \\
libc-2.27.so & 1.4 MiB & 130.1 KiB & 914.9 KiB & 914.9 KiB & 63.92 & 7.03 \\
libpthread-2.27.so & 58.1 KiB & 11.6 KiB & 70.5 KiB & 70.5 KiB & 121.48 & 6.09 \\
ld-2.27.so & 129.6 KiB & 9.6 KiB & 71.7 KiB & 71.7 KiB & 55.34 & 7.44 \\
hackbench & 2.9 KiB & 568.0 B & 2.1 KiB & 2.2 KiB & 74.78 & 3.97 \\
Total & 1.6 MiB & 151.8 KiB & 1.0 MiB & 1.0 MiB & 65.32 & 6.98 \\
\end{tabular}

6
report/tables/compare_sizes_old.cmpsz Normal file
View File

@ -0,0 +1,6 @@
Shared object Orig prog size Orig eh_frame Gen eh_elf .text + .rodata % of prog size Growth
libc-2.27.so 1.4 MiB 130.1 KiB 914.9 KiB 914.9 KiB 63.92 7.03
libpthread-2.27.so 58.1 KiB 11.6 KiB 70.5 KiB 70.5 KiB 121.48 6.09
ld-2.27.so 129.6 KiB 9.6 KiB 71.7 KiB 71.7 KiB 55.34 7.44
hackbench 2.9 KiB 568.0 B 2.1 KiB 2.2 KiB 74.78 3.97
Total 1.6 MiB 151.8 KiB 1.0 MiB 1.0 MiB 65.32 6.98

30
report/tables/to_latex.py Normal file
View File

@ -0,0 +1,30 @@
import sys
def split_line(line):
spl = line.strip().split(' ')
cleaned = map(lambda x: x.strip(), (filter(lambda x: x.strip(), spl)))
return list(cleaned)
def latexify_row(row):
return ((' & '.join(row)) + r' \\')
lines = sys.stdin.readlines()
table = list(map(split_line, lines))
cols = len(table[0])
### Formatting
print('\\begin{tabular}{' + ('r ' * cols) + '}')
## Head row
print('\t'
+ latexify_row(list(map(lambda x: r'\leftcell{' + x + r'}', table[0]))))
for row in table[1:]:
print('\t' + latexify_row(row))
print(r'\end{tabular}')