How do I load and execute an ELF binary executable manually?

Question

Suppose the binary is PIC, how can I load it into memory and execute the entry point? I'm doing this to get familiar with ELF so execve is not allowed.

Answer 1

These are the basic steps:

1. Read the program headers to find the LOAD directives and determine the total length of mappings you'll need, in pages.
2. Map the lowest-address LOAD directive with the total length (which may be greater than the file length), letting mmap assign you an address. This will reserve contiguous virtual address space.
3. Map the remining LOAD directives over top of parts of this mapping using MAP_FIXED.
4. Use the program headers to find the DYNAMIC vector, which will in turn give you the address of the relocation vector(s).
5. Apply the relocations. Assuming your binary was a static-linked PIE binary, they should consist entirely of RELATIVE relocations (just adding the base load address), meaning you don't have to perform any symbol lookups or anything fancy.

6. Construct an ELF program entry stack consisting of the following sequence of system-word-sized values in an array on the stack:

   ARGC ARGV[0] ARGV[1] ... ARGV[ARGC-1] 0 ENVIRON[0] ENVIRON[1] ... ENVIRON[N] 0 0
   

7. (This step requires ASM!) Point the stack pointer at the beginning of this array and jump to the loaded program's entry point address (which can be found in the program headers).