3.9 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_INTERP] [-DALIGN_STACK] [-DUSE_NX] [-DUSE_DL_FINI] \
-o nasm-output.o smol-output.asm
# -DALIGN_STACK: 64-bit only.
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
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