Jacopo / elf_add_code

Adds another code segment to an existing program. Experimental: can take in ELF-compiled stuff.

Astonishingly flexible code injection into an ELF executable. Helps you write as little as possible to intercept the entry point.

Use add_code_32 to manipulate and ELF32 file, add_code_64 for ELF64.

Adding raw assembly code

nasm -fbin -o new_code_bin new_code.nasm
add_code_xx original_program new_code_bin [new_code_vaddr=0x16660000] > modified_program

Adding a full ELF executable

(Don't use this mode directly, it's mainly a helper for the next one.)

gcc -nostdlib -static -Wl,-Ttext=0x166000000 -Wl,-Tdata=0x166200000 -Wl,-Tbss=0x166400000 -Wl,--build-id=none -o exec_to_add something.c
add_code_xx program exec_to_add > out_program

The program's LOAD commands ("segments") will be merged into a single one. Its load address is determined by what is in exec_to_add, so make sure there is no overlap. Check with readelf -l.

Linking in an ELF object / replacing the entry point

gcc -nostdlib -static -Wl,--build-id=none -o xxx.o -c xxx.c
add_code_xx [--before-entry] program xxx.o [new_code_vaddr=0x16660000] > out_program

Will take care of linking xxx.o. You will be able to use extern uintptr_t original_entrypoint to refer to stuff in the main program.

Entrypoint replacement helper: Write a before_entry function and specify --before-entry. A helper function will be linked in and will replace the program's entry point. It saves the initial registers (access them as initial_eax, initial_ebx, ...), calls your function, restores the initial registers, and jumps to original_entrypoint.

Limitations, TODOs

• Replaces the NOTE program header. Should extend it to manipulate existing LOAD headers in case NOTE is missing or cannot be removed.
• No PIE handling. Not very problematic, at the moment, since current PIEs are always dynamic at the moment due to limitations of libc loaders (musl is working on making static PIE possible, though).
• Just exposes original_entrypoint. Should find a way to expose all original symbols.
• Can handle a single object file. Use ld -r to combine multiple ones if necessary.
• Tested on x86 and x64 only.

Note: Always make check :)