Start describing DWARF

report/report.tex

@@ -55,14 +55,55 @@ Under supervision of Francesco Zappa-Nardelli\\
 
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 \subsection{Stack frames and unwinding}
-\todo{}
+
+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{DWARF format}
-\todo{}
+
+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 \lstc{.eh_frame}.
 
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-\subsection{DWARF functioning}
+\subsection{DWARF unwinding data}
 \todo{}
 
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