ELF File Format

Question

I'm attempting to manually load the hexdump of an elf file that I compiled using g++ into a processor simulation I designed. There are 30 sections to a standard elf file and I am loading all 30 segments with their proper memory location offset taken into account. I then start my program counter at the beginning of the .text section (00400130) but it seems that the program isn't running correctly. I have verified my processor design relatively thoroughly using SPIM as a gold standard. The strange thing is that, if I load an assembly file into SPIM, and then take the disassembled .text and .data sections that are generated by the software, load them into my processor's memory, the programs work. This is different from what I want to do because I want to:

• write a c++ program
• compile it using mipseb-linux-g++ (cross compiler)
• hex dump all sections into their own file
• read files and load contents into processor "memory"
• run program

Where in the ELF file should I place my program counter initially? I have it at the beginning of .text right now. Also, do I only need to include .text and .data for my program to work correctly? What am I doing wrong here?

Answer 1

The ELF header should include the entry address, which is not necessarily the same as the first address in the .text region. Use objdump -f to see what the entry point of the file is -- it'll be called the "start address".

The format is described here - you should be using the program headers rather than the section headers for loading the ELF image into memory (I doubt that there are 30 program headers), and the entry point will be described by the e_entry field in the ELF header.

Answer 2

Use the e_entry field of the ELF header to determine where to set the Program Counter.

Answer 3

Look into Elf32_Ehdr.e_entry (or Elf64_Ehdr.e_entry if you are on 64-bit platform). You should at least also include the .bss section, which is empty, but has "in-memory" size in the disk ELF image.

Wikipedia will lead you to all necessary documentation.

Edit:

Here's from objdump -h /usr/bin/vim on my current box:

Sections:
Idx Name         Size      VMA               LMA               File off  Algn
...
22 .bss          00009628  00000000006df760  00000000006df760  001df760  2**5
                 ALLOC
23 .comment      00000bc8  0000000000000000  0000000000000000  001df760  2**0
                 CONTENTS, READONLY

Note the File off is the same for both .bss and .comment, which means .bss is empty in the disk file, but should be 0x9628 bytes in memory.