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Using revamb with Python: a simple instrumenation

In this document we'll guide the user through using revamb's output from Python. Amomng the many possibilities that arouse from the LLVM IR provided by revamb, in this document we'll show how it's possible to perform a simple instrumentation of an existing binary, by injecting some code in the generated LLVM IR and recompiling it.

To manipulate LLVM IR from Python we'll use llvmcpy, a lightweight wrapper around the LLVM-C API for Python.

An Hello World for ARM

Let's first of all create a simple program in C called hello.c:

#include <stdio.h>

int main(int argc, char *argv[]) {
  printf("Hello world!\n");
  return 0;
}

We can compile hello.c for ARM and link it statically:

armv7a-hardfloat-linux-uclibceabi-gcc hello.c -o hello -static

Using the translate tool we can have revamb produce the LLVM IR and recompile it for us. The output should be a working hello.translated program for x86-64 (our host architecture):

$ translate hello
$ ./hello.translated
Hello world!

All good!

Tracing all the syscall invocations

For this example, we'll write a simple Python script (instrument.py) which takes in input the revamb generated LLVM IR, identifies all the syscalls and instrument them injecting the code to print the number of syscall to be performed.

In the ARM architecture, syscalls are expressed as calls to a function whose name begins with helper_exception_with_syndrome_. After this prefix there is an index identifying the specialization of the function, as explained in GeneratedIRReference.rst, but this is not interesting for us in this case.

Once we identified all the calls to the said functions, we will simply load the value of the r7 register, which holds the number of the syscall, and print it to stderr using the dprintf function.

First of all we need to import llvmcpy,obtain the default LLVMContext object and load the input LLVM IR:

from llvmcpy import llvm
context = llvm.get_global_context()
buffer = llvm.create_memory_buffer_with_contents_of_file(sys.argv[1])
module = context.parse_ir(buffer)

Now that we a reference to the module produced by revamb we can collect the objects required to perform the dprintf call, i.e., the function itself, the CSV representing the register r7, a constant integer representing stderr and the format string for dprintf:

r7 = module.get_named_global("r7")

dprintf = module.get_named_function("dprintf")

two = context.int32_type().const_int(2, True)

message_str = context.const_string("%d\n", 4, True)
message = module.add_global(message_str.type_of(), "message")
message.set_initializer(message_str)
message_ptr = message.const_bit_cast(context.int8_type().pointer(0))

Note that to build the format string we first have to create a new global variable, then set its initializer with the constant string and finally cast it to int8 * so that can be passed to dprintf, whose prototype is:

int dprintf(int fd, const char *format, ...);

At this point we have to iterate over all the instructions of the function containing the generated code (root):

root_function = module.get_named_function("root")
for basic_block in root_function.iter_basic_blocks():
    for instruction in basic_block.iter_instructions():
        # ...

However, we are not interested in all instructions, but only in calls to helper_exception_with_syndrome_* functions. Therefore, we check the opcode of the instruction, and, if it's a call, we consider the last operand (which represents the called function) and check it's name:

if instruction.instruction_opcode == llvm.Call:

last_operand_index = instruction.get_num_operands() - 1
    callee = instruction.get_operand(last_operand_index)

    if not callee.name:
        assert(callee.get_num_operands() == 1)
        callee = callee.get_operand(0)

    if callee.name.startswith("helper_exception_with_syndrome_"):
        # ...

Note that the called function is often casted to a slightly different function type, but we are not interested in this cast. The if not callee.name: block handles this situation by moving to the first operand of the cast instruction.

Finally, we've found a location where we want to insert our instrumentation. To do this, we create a builder object, position it right before the call instruction, emit an instruction to load r7, prepare the other arguments and, finally, emit the call to dprintf:

builder = context.create_builder()
builder.position_builder_before(instruction)
load_r7 = builder.build_load(r7, "")
builder.build_call(dprintf, [two, message_ptr, load_r7], "")

That's all. The last thing left to do is to serialize the new IR to file:

module.print_module_to_file(sys.argv[2])

Let's now run our script and recompile the code:

$ mv hello.ll hello.ll.original
$ python instrument.py hello.ll.original hello.ll
$ translate -s hello
$ ./hello.translated
45
45
983045
5
3
6
54
54
4
Hello world!
248

We can compare the result with a QEMU run of the original program:

$ qemu-arm -strace hello
7346 brk(NULL) = 0x00039000
7346 brk(0x000394b0) = 0x000394b0
7346 open("/dev/urandom",O_RDONLY) = 3
7346 read(3,0xf6ffde84,4) = 4
7346 close(3) = 0
7346 ioctl(0,21505,-151003688,0,221184,0) = 0
7346 ioctl(1,21505,-151003688,1,221184,0) = 0
7346 write(1,0x372a8,13)Hello world!
 = 13
7346 exit_group(0)

The complete instrument.py script is available in docs/instrument.py.

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