Symbol Table

A key value mapping


relocation Relocations are “fixups” to the binary. There is a list of possible ones. The name comes from relocatable values or something. A memory address you don’t currently know. But the mechanism can get used for more things. Some relocations look for the keys coming in from the symbol table and do something with the corresponding value.

understanding relocations

Dynamic Linking

GOT global offset table - table holding global variables PLT - procedure linkage table - table holding function pointers got.plt

DSO - dynamic shared object

how to write shared libraries

Better understanding Linux secondary dependencies solving with examples Shared Libraries: Understanding Dynamic Loading

The .dynamic section of the elf file is symbols to look for at runtime. readelf -d and objdump -T If the library file found during the ocmpile process has .so it is linked at load time.

ldd lists dynamic dependencies recursively.

For some purposes you really link during program execuction. For example if you’re JIT compiling this might be a way to do it. #include <dlfcn.h>

  • dlopen(libname, flags)
  • dlsym(handle, symbolname) looks up symbol name by string dlmopen

  • LD_DEBUG=help cat This is crazy. This exists?
  • LD_PRELOAD ldconfig

    Compilation Units

Link Time Optimization



Not linking persay. But some useful stuff for manipulating object files

LMA vs VMA load memoery address vs virtual memory address. Can differe when stored in romvs ram for example


Debug info unwind tables Reliable and Fast DWARF-Based Stack Unwinding How debuggers work: Part 3 - Debugging information

gimli a rust library for reading and writing dwarf

C++ uses drawf unwind tables to implement exceptions

  • .debug_line maps addresses to source code lines
  • .debug_info maps source variables to registers and stack locations
  • .eh_frame maps address location return address and callee saved registers info. “exception handling”
  • DWARF encodes using bytecode. CFA - canonical frame address FDE - frame description entity I suppose the dwarf bytecode is kind of a uniform abstraction of the code itself? Only replicating instructions that change frame information?

I don’t know what these are

Exploitng the hard working dwarf video

dwarf debug format .loc directive outputs into debug_lines table

pyelftools can read dwarf data

Introduction to the DWARF Debugging Format

dwarf standard

DIE - debugging information entry .debug_info field Tons of possible tags

Dwarf expressions are stack machine based. They give the ability to compute values based on machine state. The mapping of dwarf registers to machine registers is archtecture specific

Each DIE has a tag and attributes

Chapter 4

  • DW_TAG_variable
  • DW_TAG_formal_parameter function call parameters
  • DW_TAG_constant

These have attribtures DW_AT_name DW_AT_type DW_AT_location which are the most interesting

You can get line number and column info from dwarf

gcc flags related to debugging

  • -Og


    ld - linker editor

oracle linker guide

armlink user guide A different flavor. Some interesting optimization options “veneer” as chunks of linker produced code.

mold talks about arm thunks for long jumps veneers, risc v does opposite, shrinks long jumps into short jumps. rv5 abi allows this in ways arm doesnt? What does that mean veneer linker relacation on rv5

mold design notes very interesting. performance tricks, history of linker features The teensy files - writing elf by hand On ELF, Part 2 - a ctrique of elf for being too complex compared to previous formats and for little gain. Counterpoints Assembler and loaders - David Salomon compcert linking file This file follows “approach A” from the paper “Lightweight Verification of Separate Compilation” by Kang, Kim, Hur, Dreyer and Vafeiadis, POPL 2016. Cody tweeting about it

APE - Actually portable executable, Cosmopolitan library geometry of interaction stuff. Neel Krinshnasawmi

Dwarf - it’s like complicated. But there is a lot in there.

addr2line and other binutils libdwarf. dwarfdump - lots of interesting stuff on linkers, elf, other - his small linker. Uses ocaml? - generates ocaml?

ld_preload hacking Hmm. So one thing you can do is mock out functions for testing. That’s nice referenced guide on how to write dynamic linker

ryan oneill elfmaster silvio text segement padding SCOP padding secure cide oartitioning PT_LOAD insertion - “learning linux binary analysis” book. reverse text extension techniuqe github elfmaster - skeski virus, dt data segment insertion - “learning linux binary analysis” dt_needed override dt_infect. dynamic segment .got.plt infection ymbol string redirection - global data in the .got elf binary control flow hooks, function trampolines, function pointer hooks, breakpoint hnadlerh ook, tls resolver, sigill handler

ptrace , injecting a loader

  • the author of gold, Ian Lance Taylor’s 20 part Linker posts V good

Symbols - Types and values? Relocations - Bits of code that runs in general? Kind of bytecode?

Executable config files - dhall. Others. Kaitai has executable aspects. if then else, others.

Functional models of linking

type fixups/relocs = Mov Add ?

{ relocs: fixups symbols: {name : string ; typ : typ; value : value}list contents: { section : name; contents : string } }

bap’s model

type name = [
  | `tid of tid
  | `addr of addr
  | `symbol of string
] [@@deriving bin_io, compare, sexp]

type exn += Unbound_name of name

module type Code = functor (Machine : Machine) -> sig
  val exec : unit Machine.t

type code = (module Code)

(* the same piece of code can be referred via multiple names *)
type refs = {
  term : tid option;
  name : string option;
  addr : addr option;

type t = {
  codes : code Int.Map.t;
  alias : refs Int.Map.t;
  terms : int Tid.Map.t;
  names : int String.Map.t;
  addrs : int Addr.Map.t;
} This is the ocaml data format of cmo files

JT’s notes

ld has default linker script ld --verbose to see -T linker script (manual)

SECTION : set current address. tell what sections from different files to include.


So could I use a linker script to build a computation? By wiring together input and output cells?

Here’s an interesting challenge: linking verification Verify a relational property that after linking the program still does the same thing. Requires a model of linking and patching requires fixups maybe you could have the loader actually do its work and pop in by peeking at /proc/mem Or maybe ld_preload ? You’d want a total program verification. In principle the problem is easy? There is link level modulairty there. So maybe that’s good