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Randomly permute C files to better match a target binary
Python C Shell
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README.md

Decomp permuter

Automatically permutes C files to better match a target binary. The permuter has two modes of operation:

  • Random: purely at random, introduce temporary variables for values, change types, put statements on the same line...
  • Manual: test all combinations of user-specified variations, using macros like PERM_TERNARY(a = , b, c, d) to try both a = b ? c : d and if (b) a = c; else a = d;.

The modes can also be combined, by using the PERM_RANDOMIZE macro.

The main target for the tool is MIPS code compiled by old compilers (IDO, possibly GCC). Getting it to work on other architectures shouldn't be too hard, however.

Usage

./permuter.py directory/ runs the permuter; see below for the meaning of the directory. Pass -h to see possible flags.

You'll first need to install a couple of prerequisites: python3 -m pip install attrs pycparser

The permuter expects as input one or more directory containing:

  • a .c file with a single function,
  • a .o file to match,
  • a .sh file that compiles the .c file.

For projects with a properly configured makefile, you should be able to set these up by running

./import.py <path/to/file.c> <path/to/file.s>

where file.c contains the function to be permuted, and file.s is its assembly in a self-contained file. Otherwise, see USAGE.md for more details.

The .c file may be modified with any of the following macros which affect manual permutation:

  • PERM_GENERAL(a, b, ...) expands to any of a, b, ...
  • PERM_TYPECAST(a, b, ...) expands to any of (a), (b), ... (empty argument for no cast at all)
  • PERM_TERNARY(prefix, a, b, c) expands to either prefix a ? b : c or if (a) prefix b; else prefix c;.
  • PERM_VAR(a, b) sets the meta-variable a to b, PERM_VAR(a) expands to the meta-variable a.
  • PERM_RANDOMIZE(code) expands to code, but allows randomization within that region.
  • PERM_LINESWAP(lines) expands to a permutation of the ordered set of non-whitespace lines (split by \n).
  • PERM_CONDNEZ(cond) expands to either cond or (cond) != 0.

Arguments are split by a commas, exluding commas inside parenthesis. (,) is a special escape sequence that resolves to ,.

Nested macros are allowed, so e.g.

PERM_VAR(delayed, )
PERM_GENERAL(stmt;, PERM_VAR(delayed, stmt;))
...
PERM_VAR(delayed)

is a valid pattern for emitting a statement either at one point or later.

FAQ

What do the scores mean? The scores are computed by taking diffs of objdump'd .o files, and giving different penalties for lines that are the same/use the same instruction/are reordered/don't match at all. See scorer.py. It's far from a perfect system, and should probably be tweaked to look at e.g. the register diff graph.

What sort of non-matchings are the permuter good at? It's generally best towards the end, when mostly regalloc changes remain. If there are reorderings or functional changes, it's often easy to resolve those by hand, and neither the scorer nor the randomizer tends to play well with them.

Should I use this instead of trying to match code by hand? Well, the manual PERM macros might speed you up if you manage to fit the permuter into your workflow. The random mode is however much more of a last ditch sort of thing. It often finds nonsensical permutations that happen to match regalloc very well by accident. Still, it's often useful in pointing out which parts of the code need to be changed to get the code nearer to matching.

Helping out

There's tons of room for helping out with the permuter! Many more randomization passes could be added, the scoring function is far from optimal, the permuter could be made easier to use, etc. etc. The GitHub Issues list has some ideas.

Ideally, mypy permuter.py and ./run-tests.sh should succeed with no errors.

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