{simple,shoddy,smart} minsize-oriented linker
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3.8 KiB

smol

Shoddy minsize-oriented linker

PoC by Shiz, bugfixing and 64-bit version by PoroCYon.

Usage

./smol.py -lfoo -lbar input.o... smol-output.asm
nasm -I src/ [-DUSE_NX] [-DUSE_DL_FINI] -o nasm-output.o smol-output.asm
ld -T ld/link.ld -o binary nasm-output.o input.o...
usage: smol.py [-h] [-m TARGET] [-l LIB] [-L DIR] [--nasm NASM] [--cc CC]
               [--scanelf SCANELF] [--readelf READELF]
               input [input ...] output

positional arguments:
  input                 input object file
  output                output nasm file

optional arguments:
  -h, --help            show this help message and exit
  -m TARGET, --target TARGET
                        architecture to generate asm code for (default: auto)
  -l LIB, --library LIB
                        libraries to link against
  -L DIR, --libdir DIR  directories to search libraries in
  --nasm NASM           which nasm binary to use
  --cc CC               which cc binary to use
  --scanelf SCANELF     which scanelf binary to use
  --readelf READELF     which readelf binary to use

A minimal crt (and _start funcion) are provided in case you want to use main.

Internal workings

smol.py inspects the input object files for needed library files and symbols. It then outputs the list of needed libraries, hashes of the needed symbols and provides stubs for the external functions. This is then combined with a custom-made, small ELF header and 'runtime linker' which resolves the symbols (from the hashes) so that the function stubs are usable.

The runtime linker uses an unorthodox way of resolving the symbols (which only works for glibc): on both i386 and x86_64, the linker startup code (_dl_start_user) leaks the global struct link_map to the user code: on i386, a pointer to it is passed directly through eax:

# (eax, edx, ecx, esi) = (_dl_loaded, argc, argv, envp)
movl _rtld_local@GOTOFF(%ebx), %eax
## [ boring stuff... ]
pushl %eax
# Call the function to run the initializers.
call _dl_init
## eax still lives thanks to the ABI and calling convention
## [ boring stuff... ]
# Jump to the user's entry point.
jmp *%edi
## eax contains the pointer to the link_map!

On x86_64, it's a bit more convoluted: the contents of _rtld_local is loaded into rsi, but because of the x86_64 ABI, the caller isn't required to restore that register. However, due to the call instruction, a pointer to the instruction after the call will be placed on the stack, at _start, it's at rsp - 8. Then, the offset to the "load from _rtld_local"-instruction can be calculated, and the part of the instruction which contains the offset to _rtld_local, from the instruction after the load (of which the address is now also known), can be read, and thus the contents of that global variable are available as well.

Now the code continues with walking the "import tables" for the needed libraries (which already have been automatically parsed by ld.so), looks though their hash tables for the hashes of the imported symbols, gets their addresses, and replaces the hashes in the table with the function addresses.

However, because the struct link_map can change between glibc versions, especially the size of the l_info field (a fixed-size array, the DT_*NUM constants tend to change every few versions). To remediate this, one can note that the l_entry field comes a few bytes after l_info, that the root struct link_map is the one of the main executable, and that the contents of the l_entry field is known at compile-time. Thus, the loader scans the struct for the entry point address, and uses that as an offset for the 'far fields' of the struct link_map. ('Near' fields like l_name and l_addr are resp. 8 and 0, and will thus pretty much never change.)

Greets

auld alrj blackle breadbox faemiyah gib3&tix0 las leblane parcelshit unlord

License

WTFPL