ELF Tutorial: Difference between revisions

==ELF Data Types==

 

# include <stdint.h>

 

typedef uint32_t Elf32_Word; // Unsigned int

typedef int32_t Elf32_Sword; // Signed int

 

The ELF file format is made to function on a number of different architectures, many of which support different data widths. For support across multiple machine types, the ELF format provides a set of guidelines for fixed width types that make up the layout of the section and data represented within object files. You may choose to name your types differently or use types defined in stdint.h directly, but they should conform to those shown above.

While the BSS is one specific example, any section that is of type '''SHT_NOBITS''' and has the attribute '''SHF_ALLOC''' should be allocated early on during program loading. Since this tutorial is intended to be general and unspecific, the example below will follow the trend and use the simplest example for allocating sections.

 

static int elf_load_stage1(Elf32_Ehdr *hdr) {

Elf32_Shdr *shdr = elf_sheader(hdr);

return 0;

}

 

The example above allocates as much memory as necessary for the section, described by the '''sh_size''' field of the section's header. Although the function in the example only seeks out sections that needs to be allocated, it can be modified to perform other operation that should be performed early on into the loading process.

Loading a relocatable ELF file entails processing all relocation entries present in the file (Remember to alloc allocate all '''SHT_NOBITS''' sections first!). This process starts with finding all the relocation tables in the file, which is done in the example code below.

 

# define ELF_RELOC_ERR -1

 

return 0;

}

 

Note that the code above only processes '''Elf32_Rel''' entries, but it can be modified to process entries with explicit addends as well. The code also relies on a function called '''elf_do_reloc''' which will be shown in the next example. This example function stops, displays an error message, and returns an error code if it's unable to process a relocation.