Inside ELF: Exploring the Core of Executable Files

Set up an ARM lab to explore ELF executable structures through bare-metal programming, assemble with the GNU toolchain, and test on a Gumstick Conex ARM v5 board via an emulator.

Understand machine code and opcodes, the operation codes CPUs execute with operands in binary and hex. See how assembly and C are converted to machine code by a toolchain.

Learn how machine instructions and opcodes with operands drive ARM CPUs, updating registers like R0 and R1, as demonstrated on a flash-loaded board using an emulator.

Explore the instruction set, including arithmetic operations such as addition, subtraction, multiplication, division, and bitwise operations. Compare ARM and x86 instruction sets, opcode patterns, and immediate operands across processors.

Explore assembly language—from opcodes and hex editors to converting assembly to opcode with an assembler for ARM execution—and learn the label, instruction, and comment syntax.

Learn to write an ARM assembly program that adds two numbers using registers in a bare-metal context, with .text directives, labels, and comments.

Convert elf into a minimal binary for bare-metal execution, using objcopy to produce a 16-byte binary, overwrite flash content with the essential opcodes, and verify with a hex dump.

Explore the QEMU monitor interface to view system status, reset system, and inspect values with info resistor; dump memory with xp in hex, decimal, or octal, then disassemble with exp.

Learn to write an assembly program that sums a byte array into r3 using an end-of-array label. Observe how dot byte and align directives ensure 32-bit memory alignment.

Learn to compute a string's length in assembly with ascii z and null-terminated strings, using dollar equal and symbol table entries to map labels to addresses.

the linker resolves symbols and relocates code across object files, using symbol tables to assign addresses and exposing symbols with the global directive to access variables in ram.

Relocation changes addresses assigned to labels and patches label references, while the linker performs symbol resolution and merges same-name sections (text and data) across files.

Relocation merges text and data sections and then places them at a chosen address, patching label references accordingly.

Explore how a linker script controls relocation, section merging, and section placement, using a location counter and wildcard matching to merge text and data sections.

Demonstrates placing the sum of an array in the data section via a linker script, assigning text and data section addresses, and observing section placement for a single object file.

Explore how the linker places data in RAM, loads and sums value one and value two, and stores the result, with startup code copying data from flash to RAM.

Learn to specify load and runtime addresses in the linker script for elf executables, using the at keyword to set the load address and copy data from flash to ram.

Explain updating the linker script to define the data and Ram sections with load addresses, and show startup code that copies the data from flash to Ram at startup.

Define an arm exception vector at address zero and branch to reset, undefined instruction, and software interrupt handlers; update the linker script to place the vector before code.

Learn how C startup code prepares ARM programs by setting up the stack, copying initialized data from flash to RAM, zeroing BSS, and placing read-only data in flash.

Update startup code and the linker script to place vectors, text, data, and bss correctly in flash and ram, and explain elf creation from startup to main.

This lecture explains the ELF file structure, consisting of an ELF header, section header, program header (segments), and object code; these parts support linking and loading an executable in memory.

Demonstrates how a simple C program that returns three expands into a 64-bit elf executable containing thousands of machine instructions, and outlines elf header, section header, and program header.

Explore how the Elf header at the start of a file conveys machine type, the entry point address, and the locations of the program header and section header tables.

Explore how the ELF header identifies executables, distinguishes 32- and 64-bit formats, and uses the magic bytes and e_ident fields to reveal class, data, endianness, and Unix system V ABI.

Explore the elf header details, including file type, machine architecture, entry point, version, abi, and program and section header offsets and counts, revealing how an elf file is organized.

Demonstrates generating a core dump from an ELF executable by adjusting ulimit, locating the core file, and inspecting a 64-bit LSB core file with program headers but no section headers.

Explore what happens when you change the elf version number using a hex editor. The video demonstrates altering the version byte and observing an 'unknown' version in the executable.

Changing the elf header's architecture version demonstrates how readelf reports a mixed bitness, switching between x86_64 and i386, and can cause exec format errors when the header and code disagree.

Learn how the ELF e_entry field sets where execution begins by updating the entry point to a function not called by main, using readelf and objdump, and observe segmentation faults.

Learn to locate the entry point and trace who calls main in gdb by setting breakpoints at _start or the first instruction and using start, info files, and backtrace.

Explain how the C runtime startup code provides underscore start, calls libc start main, then main, and finally exits, with exec V launching the process.

Explore gcc options no start files and no stdlib, and see how startup code, underscore start, and library linking affect the absence of main and the C library runtime.

Construct a minimal elf header in C using elf.h, fill e_ident with magic numbers, set 64-bit little-endian amd64, and write a binary with no program or section headers.

Explore arm elf across cross-compiled c code, comparing 32-bit and 64-bit elf formats, abi versions, build ids, debug info, entry points, and program versus section headers.

Explore ELF structure by examining headers, sections, and segments. See how the section header and program header tables guide the linker and loader to merge sections and set up executables.

Discover how ELF binaries organize code and data into sections like text, data, rodata, and bss, and how headers and section tables expose these sections.

Explore the section header table in elf files, including names, types, flags, offsets, and sizes. Learn how the linker uses this information, and why the loader ignores it after generation.

Explore the dump elf command from utils to reveal elf structures in C format, including the elf header, program headers, and section headers, with data viewing via hexdump.

Discover the common elf file sections, including the text section with executable code, the data and bss sections, section headers, and the interpreter, using objdump and readelf to inspect.

Enable debug information with the -g flag to compile C code; the linker then adds dwarf debug sections (debug_info, debug_abbrev, debug_str) used by gdb, and without them gdb won't work.

Inspect the size command to reveal text, data, and BSS section sizes for object and executable files, with decimal and hexadecimal outputs.

Explore how the strings command extracts printable characters from ELF and executable files, with -S and -D options to inspect sections, data, and symbol names for reverse engineering.

Explore how elf symbol tables map symbol names to addresses, including symtab and dynamic symbol table, with string tables, dynamic linking, and runtime allocation implications.

Learn how the strip command reduces elf binaries by discarding symbol and string tables, removing global string table while keeping the dynamic symbol table needed for loading.

Discover how the strip section utility reduces an Elf file by removing non essential sections while preserving the Elf header, the program header table and its contents for execution.

Learn how to add a new section to an elf file using objcopy, copying all sections, naming the new section and setting its flags, then verify contents with objdump.

Understand how ELF program headers define memory segments for runtime loading, how section headers support linking, and how segments map to sections revealed by readelf commands.

Explore how elf program headers map sections to memory, with segments containing multiple sections, types like load, interpreter, and dynamic, and permissions distinguishing text and data.

Discover how uninitialized data is placed in the BSS section and why file size differs from memory size, as shown by examples of initialized vs uninitialized arrays in ELF builds.

The kernel loads an elf by reading elf header and program header table, loading the load-type segments into memory and handling the interpreter to load dependencies or hand over control.

Modify a program header to show how the loader maps the text section into memory; changing a header to null causes a segmentation fault, underscoring ELF loading sensitivity.

Explore building a minimal elf file that prints hello, keeping only a load segment and removing unnecessary sections to shrink size from 17 kb to about 100 bytes.

Observe how aslr randomizes memory addresses across runs, changing pid maps, text, stack, and heap, and how toggling the feature reveals or hides address reuse and memory layout security.

Compare static and dynamic ELF binaries by inspecting segments, program headers, the interpreter and maps file entries, noting C library and loader presence and the size difference.

Learn how minimal assembly with nasm for Elf 64 directly invokes system calls to print hello, reducing size from 17 kb to 8.3 kb by avoiding libraries and trimming sections.

Learn to build a minimal ELF file in C with a single load program header, no section headers, and static linking for Linux.