Add first results

report/report.tex

@@ -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}

report/tables/compare_sizes_old

@@ -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}

report/tables/compare_sizes_old.cmpsz

@@ -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

report/tables/to_latex.py

@@ -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}')