elfloader is a super simple loader for ELF files that generates a flat in-memory representation of the ELF.
Pair this with Rust and now you can write your shellcode in a proper, safe, high-level language. Any target that LLVM can target can be used, including custom target specifications for really exotic platforms and ABIs. Enjoy using things like
u64s on 32-bit systems, bounds checked arrays, drop handling of allocations, etc :)
It simply concatenates all
LOAD sections together, using zero-padding if there are gaps, into one big flat file.
This file includes zero-initialization of
.bss sections, and thus can be used directly as a shellcode payload.
If you don't want to waste time with fail-open linker scripts, this is probably a great way to go.
This doesn't handle any relocations, it's on you to make sure the original ELF is based at the address you want it to be at.
To use this tool, simply:
Usage: elfloader [--binary] [--base=<addr>] <input ELF> <output> --binary - Don't output a FELF, output the raw loaded image with no metadata --base=<addr> - Force the output to start at `<addr>`, zero padding from the base to the start of the first LOAD segment if needed. `<addr>` is default hex, can be overrided with `0d`, `0b`, `0x`, or `0o` prefixes. Warning: This does not _relocate_ to base, it simply starts the output at `<addr>` (adding zero bytes such that the output image can be loaded at `<addr>` instead of the original ELF base) <input ELF> - Path to input ELF <output> - Path to output file
To install this tool run:
cargo install --path .
Now you can use
elfloader from anywhere in your shell!
There's an example in
example_small_program, simply run
nmake and this should generate an
example.bin which is 8 bytes.
[email protected] ~/elfloader/example_small_program $ make cargo build --release Finished release [optimized] target(s) in 0.03s elfloader --binary target/aarch64-unknown-none/release/example_small_program example.bin [email protected] ~/elfloader/example_small_program $ ls -l ./example.bin -rw-r--r-- 1 pleb pleb 8 Nov 8 12:27 ./example.bin [email protected] ~/elfloader/example_small_program $ objdump -d target/aarch64-unknown-none/release/example_small_program target/aarch64-unknown-none/release/example_small_program: file format elf64-littleaarch64 Disassembly of section .text: 00000000133700b0 <_start>: 133700b0: 8b000020 add x0, x1, x0 133700b4: d65f03c0 ret
Now you can write your shellcode in Rust, and you don't have to worry about whether you emit
.bss, etc. This will handle it all for you!
There's also an example with
[email protected] ~/elfloader/example_program_with_data $ make cargo build --release Finished release [optimized] target(s) in 0.04s elfloader --binary target/aarch64-unknown-none/release/example_program_with_data example.bin [email protected] ~/elfloader/example_program_with_data $ ls -l ./example.bin -rw-r--r-- 1 pleb pleb 29 Nov 8 12:39 ./example.bin [email protected] ~/elfloader/example_program_with_data $ objdump -d target/aarch64-unknown-none/release/example_program_with_data target/aarch64-unknown-none/release/example_program_with_data: file format elf64-littleaarch64 Disassembly of section .text: 0000000013370124 <_start>: 13370124: 90000000 adrp x0, 13370000 <_start-0x124> 13370128: 90000008 adrp x8, 13370000 <_start-0x124> 1337012c: 52800029 mov w9, #0x1 // #1 13370130: 91048000 add x0, x0, #0x120 13370134: 3904f109 strb w9, [x8, #316] 13370138: d65f03c0 ret [email protected] ~/elfloader/example_program_with_data $ readelf -l target/aarch64-unknown-none/release/example_program_with_data Elf file type is EXEC (Executable file) Entry point 0x13370124 There are 4 program headers, starting at offset 64 Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flags Align LOAD 0x0000000000000120 0x0000000013370120 0x0000000013370120 0x0000000000000004 0x0000000000000004 R 0x1 LOAD 0x0000000000000124 0x0000000013370124 0x0000000013370124 0x0000000000000018 0x0000000000000018 R E 0x4 LOAD 0x000000000000013c 0x000000001337013c 0x000000001337013c 0x0000000000000000 0x0000000000000001 RW 0x4 GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 RW 0x0 Section to Segment mapping: Segment Sections... 00 .rodata 01 .text 02 .bss 03
This tool doesn't care about anything except for
LOAD sections. It determines the endianness (little vs big) and bitness (32 vs 64) from the ELF header, and from there it creates a flat image based on program header virtual addresses (where it's loaded), file size (number of initialized bytes) and mem size (size of actual memory region). The bytes are initialized from the file based on the offset and file size, and this is then extended with zeros until mem size (or truncated if mem size is smaller than file size).
LOAD sections are then concatenated together with zero-byte padding for gaps.
This is designed to be incredibly simple, and agnostic to the ELF input. It could be an executable, object file, shared object, core dump, etc, doesn't really care. It'll simply give you the flat representation of the memory, nothing more.
This allows you to turn any ELF into shellcode, or a simpler file format that is easier to load in hard-to-reach areas, like embedded devices. Personally, I developed this for my MIPS NT 4.0 loader which allows me to run Rust code.
This tool by default generates a FELF file format. This is a Falk ELF. This is a simple file format:
FELF0001 - Magic header entry - 64-bit little endian integer of the entry point address base - 64-bit little endian integer of the base address to load the image <image> - Rest of the file is the raw image, to be loaded at `base` and jumped into at `entry`