Add generated code samples, detail compilation

report/TODO

@@ -1,4 +1 @@
-- Add a sample of geenrated code?
-- Explicitly tell that we only support unwinding, and this only for a few
-  registers
 - Make consistant -ise / -ize (eg. optimize)

report/report.tex

@@ -121,10 +121,24 @@ 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
+\emph{unwinding} a frame.
-necessary to have additional data to perform stack unwinding. This data is
+
-often stored among the debugging informations of a program, and one common
+Let us consider a stack with x86\_64 calling conventions, such as shown in
-format of debugging data is DWARF\@.
+Figure~\ref{fig:call_stack}. Assuming the compiler decided here \emph{not} to
+use \reg{rbp}, and assuming the function \eg{} allocates a buffer of 8
+integers, the area allocated for local variables should be at least $32$ bytes
+long (for 4-bytes integers), and \reg{rip} will be pointing below this area.
+Left apart analyzing the assembly code produced, there is no way to find where
+the return address is stored, relatively to \reg{rsp}, at some arbitrary point
+of the function. Even when \reg{rbp} is used, there is no easy way to guess
+where each callee-saved register is stored in the stack frame, and worse, which
+callee-saved registers were saved (since it is not necessary to save a register
+that the function never touches).
+
+With this example, it seems pretty clear that 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\@.
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Unwinding usage and frequency}
@@ -574,16 +588,24 @@ machine code on the x86\_64 platform.
 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
+providing for free all the optimization passes of a modern compiler.
-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
+The generated code consists in a single monolithic function, taking as
-various machine registers) as defined in Listing~\ref{lst:unw_ctx}. An
+arguments an instruction pointer and a memory context (\ie{} the value of the
-optionally enabled parameter can be used to pass a function pointer to a
+various machine registers) as defined in Listing~\ref{lst:unw_ctx}. The
+function will then return a fresh memory context, containing the values the
+registers hold after unwinding this frame.
+
+The body of the function itself is mostly a huge switch, taking advantage of
+the non-standard ---~yet widely implemented in C compilers~--- syntax for range
+switches, in which each \lstc{case} can refer to a range. All the FDEs are
+merged together into this switch, each row of a FDE being a switch case. The
+cases then fill a context with unwound values, then return it.
+
+An optionally enabled parameter can be used to pass a function pointer to a
 dereferencing function, that conceptually does what the dereferencing \lstc{*}
 operator does 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.
 
 Unlike in the \ehframe, and unlike what should be done in a release,
 real-world-proof version of the \ehelfs, the choice was made to keep this
@@ -614,9 +636,17 @@ 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{Talk about switch structure, considered and ignored registers, etc.}
+\medskip
 
-\todo{More details here? Is it necessary or just too technical?}
+\lstinputlisting[language=C, caption={\ehelf{} for the previous example},
+                 label={lst:fib7_eh_elf_basic}]
+                 {src/fib7/fib7.eh_elf_basic.c}
+
+\note{I did not put some ASM here, as I doubt the jury will read it anyway, }\\
+\qnote{and it takes way too much space}
+
+The C code in Listing~\ref{lst:fib7_eh_elf_basic} is a part of what was
+generated for the C code in Listing~\ref{lst:ex1_c}.
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{First results}
@@ -682,20 +712,29 @@ made in order to shrink the \ehelfs.
 
 \medskip
 
-The major optimization that most reduced the output size was to use a more
+The major optimization that most reduced the output size was to use an if/else
-flexible if/else if tree implementing a binary search on the program counter
+tree implementing a binary search on the program counter relevant intervals,
-relevant intervals, instead of a huge switch. In the process, we also
+instead of a huge switch. In the process, we also \emph{outline} a lot of code,
-\emph{outline} a lot of code, that is, find out identical code blocks, move
+that is, find out identical code blocks, move them outside of the if/else tree,
-them outside of the if/else tree, identify them by a label, and jump to them
+identify them by a label, and jump to them using a \lstc{goto}, which
-using a \lstc{goto}, which de-duplicates a lot of code and contributes greatly
+de-duplicates a lot of code and contributes greatly to the shrinking. In the
-to the shrinking. In the process, we noticed that the vast majority of FDE rows
+process, we noticed that the vast majority of FDE rows are actually taken among
-are actually taken among very few ``common'' FDE rows.
+very few ``common'' FDE rows.
 
-This makes this optimisation really efficient, as seen later in
+This makes this optimization really efficient, as seen later in
 Section~\ref{ssec:results_size}, but also makes it an interesting question ---
 not investigated during this internship --- to find out whether standard DWARF
 data could be efficiently compressed in this way.
 
+\begin{minipage}{0.45\textwidth}
+    \lstinputlisting[language=C, caption={\ehelf{} for the previous example},
+                     label={lst:fib7_eh_elf_outline},
+                     lastline=18]
+                     {src/fib7/fib7.eh_elf_outline.c}
+\end{minipage} \hfill \begin{minipage}{0.45\textwidth}
+    \lstinputlisting[language=C, firstnumber=last, firstline=19]
+                     {src/fib7/fib7.eh_elf_outline.c}
+\end{minipage}
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

report/src/fib7/fib7.eh_elf_basic.c

@@ -1,21 +1,13 @@
 unwind_context_t _eh_elf(unwind_context_t ctx, uintptr_t pc) {
 	unwind_context_t out_ctx;
 	switch(pc) {
+        // [...] Previous FDEs redacted for brevity
 		case 0x615 ... 0x618:
 			out_ctx.rsp = ctx.rsp + (8);
 			out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
 			out_ctx.flags = 3u;
 			return out_ctx;
-		case 0x619 ... 0x658:
+        // [...] Further lines and FDEs redacted for brevity
-			out_ctx.rsp = ctx.rsp + (48);
-			out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
-			out_ctx.flags = 3u;
-			return out_ctx;
-		case 0x659 ... 0x659:
-			out_ctx.rsp = ctx.rsp + (8);
-			out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
-			out_ctx.flags = 3u;
-			return out_ctx;
 		default:
 			out_ctx.flags = 128u;
 			return out_ctx;

report/src/fib7/fib7.eh_elf_outline.c

@@ -0,0 +1,33 @@
+unwind_context_t _eh_elf(unwind_context_t ctx, uintptr_t pc) {
+	unwind_context_t out_ctx;
+    if(pc < 0x619) {
+        // IP=0x615 ... 0x618
+        goto _factor_3;
+    } else {
+        if(pc < 0x659) {
+            // IP=0x619 ... 0x658
+            goto _factor_4;
+        } else {
+            // IP=0x659 ... 0x659
+            goto _factor_3;
+        }
+    }
+
+	_factor_default:
+	out_ctx.flags = 128u;
+	return out_ctx;
+
+	/* ===== LABELS  ======== */
+
+	_factor_4:
+	out_ctx.rsp = ctx.rsp + (48);
+	out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
+	out_ctx.flags = 3u;
+	return out_ctx;
+
+	_factor_3:
+	out_ctx.rsp = ctx.rsp + (8);
+	out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
+	out_ctx.flags = 3u;
+	return out_ctx;
+}