ELF Cheatsheet

The repo contains the cheatsheet for ELF related, linking related commands, a template for Makefile, the bracket conventions and useful C attributes.

Build

$ make all

To make all object files and executables.

$ make TinyHelloWorld-manual

To manually link the object file with the ld script in lds/.

$ make clean

To clean up

In bin and obj, there are demo executable and object files respectively. They can be used to test the command below.

Cheatsheet

file

$ file <filename>

This command shows the ELF type (i.e. relocatable, executable, shared object, core dump), target platform.

size

$ size <filename>

This command shows the size of each section of an ELF.

objdump

$ objdump -h <filename>

This command shows the informantion of section headers, including size, vma, lma, offset, align and attributes.

$ objdump -x <filename>

This command shows the section header informantion, the symbol table, the relocation tables (one table for each section that requires relocation).

$ objdump -s <filename>

This command shows the content of each section in hexadecimal format.

$ objdump -d <filename>

This command decompiles all the sections that contain instructions.

$ objdump -t <filename>

This command shows the symbol table of an ELF.

readelf

$ readelf -h <filename>

This command shows the header of an ELF.

$ readelf -S <filename>

This command shows the information of section headers, similar to objdump -h.

$ readelf -s <filename>

This command shows the symbol table of an ELF, similar to objdump -t.

$ readelf -l <filename>

This command shows how the sections are mapped to segments and also the program header (in executable and shared files after ELF header).

nm

$ nm <filename>

This command shows the symbol table of an ELF, similar to objdump -t.

Makefile

all : a b

• $@ evaluates to all
• $< evaluates to a
• $^ evaluates to a b

Bracket Conventions

The conventiosn are defined in Ch. 12 - Utility Conventions and the gist is listed below:

• square brackets [optional option]
• angle brackets <required argument>
• curly brackets {default values}
• parenthesis (miscellaneous info)

C Attributes

__attribute__((section("<name>"))) // Place the variable or function in section "name"
__attribute__((weak)) // Define a weak symbol (covered by strong symbol). A weak symbol will have type SHN_COMMON (in common block)
__attribute__((weakref)) // Define a reference to external functions as weak reference.
__attribute__((nocommon)) // Do not mark the symbol as `SHN_COMMON`. Instead define it as a strong symbol.