Simulation and Instrumentation

When you ask for a step of execution to happen in angr, something has to actually perform the step. angr uses a series of engines (subclasses of the SimEngine class) to emulate the effects that of a given section of code has on an input state. The execution core of angr simply tries all the available engines in sequence, taking the first one that is able to handle the step. The following is the default list of engines, in order:

  • The failure engine kicks in when the previous step took us to some uncontinuable state

  • The syscall engine kicks in when the previous step ended in a syscall

  • The hook engine kicks in when the current address is hooked

  • The unicorn engine kicks in when the UNICORN state option is enabled and there is no symbolic data in the state

  • The VEX engine kicks in as the final fallback.

SimSuccessors

The code that actually tries all the engines in turn is project.factory.successors(state, **kwargs), which passes its arguments onto each of the engines. This function is at the heart of state.step() and simulation_manager.step(). It returns a SimSuccessors object, which we discussed briefly before. The purpose of SimSuccessors is to perform a simple categorization of the successor states, stored in various list attributes. They are:

Attribute

Guard Condition

Instruction Pointer

Description

successors

True (can be symbolic, but constrained to True)

Can be symbolic (but 256 solutions or less; see unconstrained_successors).

A normal, satisfiable successor state to the state processed by the engine. The instruction pointer of this state may be symbolic (i.e., a computed jump based on user input), so the state might actually represent several potential continuations of execution going forward.

unsat_successors

False (can be symbolic, but constrained to False).

Can be symbolic.

Unsatisfiable successors. These are successors whose guard conditions can only be false (i.e., jumps that cannot be taken, or the default branch of jumps that must be taken).

flat_successors

True (can be symbolic, but constrained to True).

Concrete value.

As noted above, states in the successors list can have symbolic instruction pointers. This is rather confusing, as elsewhere in the code (i.e., in SimEngineVEX.process, when it’s time to step that state forward), we make assumptions that a single program state only represents the execution of a single spot in the code. To alleviate this, when we encounter states in successors with symbolic instruction pointers, we compute all possible concrete solutions (up to an arbitrary threshold of 256) for them, and make a copy of the state for each such solution. We call this process “flattening”. These flat_successors are states, each of which has a different, concrete instruction pointer. For example, if the instruction pointer of a state in successors was X+5, where X had constraints of X > 0x800000 and X <= 0x800010, we would flatten it into 16 different flat_successors states, one with an instruction pointer of 0x800006, one with 0x800007, and so on until 0x800015.

unconstrained_successors

True (can be symbolic, but constrained to True).

Symbolic (with more than 256 solutions).

During the flattening procedure described above, if it turns out that there are more than 256 possible solutions for the instruction pointer, we assume that the instruction pointer has been overwritten with unconstrained data (i.e., a stack overflow with user data). This assumption is not sound in general. Such states are placed in unconstrained_successors and not in successors.

all_successors

Anything

Can be symbolic.

This is successors + unsat_successors + unconstrained_successors.

Breakpoints

Todo

rewrite this to fix the narrative

Like any decent execution engine, angr supports breakpoints. This is pretty cool! A point is set as follows:

>>> import angr
>>> b = angr.Project('examples/fauxware/fauxware')

# get our state
>>> s = b.factory.entry_state()

# add a breakpoint. This breakpoint will drop into ipdb right before a memory write happens.
>>> s.inspect.b('mem_write')

# on the other hand, we can have a breakpoint trigger right *after* a memory write happens.
# we can also have a callback function run instead of opening ipdb.
>>> def debug_func(state):
...     print("State %s is about to do a memory write!")

>>> s.inspect.b('mem_write', when=angr.BP_AFTER, action=debug_func)

# or, you can have it drop you in an embedded IPython!
>>> s.inspect.b('mem_write', when=angr.BP_AFTER, action=angr.BP_IPYTHON)

There are many other places to break than a memory write. Here is the list. You can break at BP_BEFORE or BP_AFTER for each of these events.

Event type

Event meaning

mem_read

Memory is being read.

mem_write

Memory is being written.

address_concretization

A symbolic memory access is being resolved.

reg_read

A register is being read.

reg_write

A register is being written.

tmp_read

A temp is being read.

tmp_write

A temp is being written.

expr

An expression is being created (i.e., a result of an arithmetic operation or a constant in the IR).

statement

An IR statement is being translated.

instruction

A new (native) instruction is being translated.

irsb

A new basic block is being translated.

constraints

New constraints are being added to the state.

exit

A successor is being generated from execution.

fork

A symbolic execution state has forked into multiple states.

symbolic_variable

A new symbolic variable is being created.

call

A call instruction is hit.

return

A ret instruction is hit.

simprocedure

A simprocedure (or syscall) is executed.

dirty

A dirty IR callback is executed.

syscall

A syscall is executed (called in addition to the simprocedure event).

engine_process

A SimEngine is about to process some code.

These events expose different attributes:

Event type

Attribute name

Attribute availability

Attribute meaning

mem_read

mem_read_address

BP_BEFORE or BP_AFTER

The address at which memory is being read.

mem_read

mem_read_expr

BP_AFTER

The expression at that address.

mem_read

mem_read_length

BP_BEFORE or BP_AFTER

The length of the memory read.

mem_read

mem_read_condition

BP_BEFORE or BP_AFTER

The condition of the memory read.

mem_write

mem_write_address

BP_BEFORE or BP_AFTER

The address at which memory is being written.

mem_write

mem_write_length

BP_BEFORE or BP_AFTER

The length of the memory write.

mem_write

mem_write_expr

BP_BEFORE or BP_AFTER

The expression that is being written.

mem_write

mem_write_condition

BP_BEFORE or BP_AFTER

The condition of the memory write.

reg_read

reg_read_offset

BP_BEFORE or BP_AFTER

The offset of the register being read.

reg_read

reg_read_length

BP_BEFORE or BP_AFTER

The length of the register read.

reg_read

reg_read_expr

BP_AFTER

The expression in the register.

reg_read

reg_read_condition

BP_BEFORE or BP_AFTER

The condition of the register read.

reg_write

reg_write_offset

BP_BEFORE or BP_AFTER

The offset of the register being written.

reg_write

reg_write_length

BP_BEFORE or BP_AFTER

The length of the register write.

reg_write

reg_write_expr

BP_BEFORE or BP_AFTER

The expression that is being written.

reg_write

reg_write_condition

BP_BEFORE or BP_AFTER

The condition of the register write.

tmp_read

tmp_read_num

BP_BEFORE or BP_AFTER

The number of the temp being read.

tmp_read

tmp_read_expr

BP_AFTER

The expression of the temp.

tmp_write

tmp_write_num

BP_BEFORE or BP_AFTER

The number of the temp written.

tmp_write

tmp_write_expr

BP_AFTER

The expression written to the temp.

expr

expr

BP_BEFORE or BP_AFTER

The IR expression.

expr

expr_result

BP_AFTER

The value (e.g. AST) which the expression was evaluated to.

statement

statement

BP_BEFORE or BP_AFTER

The index of the IR statement (in the IR basic block).

instruction

instruction

BP_BEFORE or BP_AFTER

The address of the native instruction.

irsb

address

BP_BEFORE or BP_AFTER

The address of the basic block.

constraints

added_constraints

BP_BEFORE or BP_AFTER

The list of constraint expressions being added.

call

function_address

BP_BEFORE or BP_AFTER

The name of the function being called.

exit

exit_target

BP_BEFORE or BP_AFTER

The expression representing the target of a SimExit.

exit

exit_guard

BP_BEFORE or BP_AFTER

The expression representing the guard of a SimExit.

exit

exit_jumpkind

BP_BEFORE or BP_AFTER

The expression representing the kind of SimExit.

symbolic_variable

symbolic_name

BP_AFTER

The name of the symbolic variable being created. The solver engine might modify this name (by appending a unique ID and length). Check the symbolic_expr for the final symbolic expression.

symbolic_variable

symbolic_size

BP_AFTER

The size of the symbolic variable being created.

symbolic_variable

symbolic_expr

BP_AFTER

The expression representing the new symbolic variable.

address_concretization

address_concretization_strategy

BP_BEFORE or BP_AFTER

The SimConcretizationStrategy being used to resolve the address. This can be modified by the breakpoint handler to change the strategy that will be applied. If your breakpoint handler sets this to None, this strategy will be skipped.

address_concretization

address_concretization_action

BP_BEFORE or BP_AFTER

The SimAction object being used to record the memory action.

address_concretization

address_concretization_memory

BP_BEFORE or BP_AFTER

The SimMemory object on which the action was taken.

address_concretization

address_concretization_expr

BP_BEFORE or BP_AFTER

The AST representing the memory index being resolved. The breakpoint handler can modify this to affect the address being resolved.

address_concretization

address_concretization_add_constraints

BP_BEFORE or BP_AFTER

Whether or not constraints should/will be added for this read.

address_concretization

address_concretization_result

BP_AFTER

The list of resolved memory addresses (integers). The breakpoint handler can overwrite these to effect a different resolution result.

syscall

syscall_name

BP_BEFORE or BP_AFTER

The name of the system call.

simprocedure

simprocedure_name

BP_BEFORE or BP_AFTER

The name of the simprocedure.

simprocedure

simprocedure_addr

BP_BEFORE or BP_AFTER

The address of the simprocedure.

simprocedure

simprocedure_result

BP_AFTER

The return value of the simprocedure. You can also override it in BP_BEFORE, which will cause the actual simprocedure to be skipped and for your return value to be used instead.

simprocedure

simprocedure

BP_BEFORE or BP_AFTER

The actual SimProcedure object.

dirty

dirty_name

BP_BEFORE or BP_AFTER

The name of the dirty call.

dirty

dirty_handler

BP_BEFORE

The function that will be run to handle the dirty call. You can override this.

dirty

dirty_args

BP_BEFORE or BP_AFTER

The address of the dirty.

dirty

dirty_result

BP_AFTER

The return value of the dirty call. You can also override it in BP_BEFORE, which will cause the actual dirty call to be skipped and for your return value to be used instead.

engine_process

sim_engine

BP_BEFORE or BP_AFTER

The SimEngine that is processing.

engine_process

successors

BP_BEFORE or BP_AFTER

The SimSuccessors object defining the result of the engine.

These attributes can be accessed as members of state.inspect during the appropriate breakpoint callback to access the appropriate values. You can even modify these value to modify further uses of the values!

>>> def track_reads(state):
...     print('Read', state.inspect.mem_read_expr, 'from', state.inspect.mem_read_address)
...
>>> s.inspect.b('mem_read', when=angr.BP_AFTER, action=track_reads)

Additionally, each of these properties can be used as a keyword argument to inspect.b to make the breakpoint conditional:

# This will break before a memory write if 0x1000 is a possible value of its target expression
>>> s.inspect.b('mem_write', mem_write_address=0x1000)

# This will break before a memory write if 0x1000 is the *only* value of its target expression
>>> s.inspect.b('mem_write', mem_write_address=0x1000, mem_write_address_unique=True)

# This will break after instruction 0x8000, but only 0x1000 is a possible value of the last expression that was read from memory
>>> s.inspect.b('instruction', when=angr.BP_AFTER, instruction=0x8000, mem_read_expr=0x1000)

Cool stuff! In fact, we can even specify a function as a condition:

# this is a complex condition that could do anything! In this case, it makes sure that RAX is 0x41414141 and
# that the basic block starting at 0x8004 was executed sometime in this path's history
>>> def cond(state):
...     return state.eval(state.regs.rax, cast_to=str) == 'AAAA' and 0x8004 in state.inspect.backtrace

>>> s.inspect.b('mem_write', condition=cond)

That is some cool stuff!

Caution about mem_read breakpoint

The mem_read breakpoint gets triggered anytime there are memory reads by either the executing program or the binary analysis. If you are using breakpoint on mem_read and also using state.mem to load data from memory addresses, then know that the breakpoint will be fired as you are technically reading memory.

So if you want to load data from memory and not trigger any mem_read breakpoint you have had set up, then use state.memory.load with the keyword arguments disable_actions=True and inspect=False.

This is also true for state.find and you can use the same keyword arguments to prevent mem_read breakpoints from firing.