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use binaryninja::{
architecture::{Architecture as _, CoreRegister, Register as _, RegisterInfo as _},
binary_view::BinaryView,
low_level_il::{
expression::{ExpressionHandler, LowLevelILExpressionKind},
function::{LowLevelILFunction, Mutable, NonSSA},
instruction::{
InstructionHandler, LowLevelILInstruction, LowLevelILInstructionKind,
LowLevelInstructionIndex,
},
lifting::LowLevelILLabel,
LowLevelILRegisterKind,
},
variable::PossibleValueSet,
workflow::AnalysisContext,
};
use crate::{error::ILLevel, metadata::GlobalState, Error};
// TODO: We should also handle `objc_retain_x` / `objc_release_x` variants
// that use a custom calling convention.
const IGNORABLE_MEMORY_MANAGEMENT_FUNCTIONS: &[&[u8]] = &[
b"_objc_autorelease",
b"_objc_autoreleaseReturnValue",
b"_objc_release",
b"_objc_retain",
b"_objc_retainAutorelease",
b"_objc_retainAutoreleaseReturnValue",
b"_objc_retainAutoreleasedReturnValue",
b"_objc_retainBlock",
b"_objc_unsafeClaimAutoreleasedReturnValue",
];
fn is_call_to_ignorable_memory_management_function<'func>(
view: &binaryninja::binary_view::BinaryView,
instr: &'func LowLevelILInstruction<'func, Mutable, NonSSA>,
) -> bool {
let target = match instr.kind() {
LowLevelILInstructionKind::Call(call) | LowLevelILInstructionKind::TailCall(call) => {
match call.target().possible_values() {
PossibleValueSet::ConstantValue { value }
| PossibleValueSet::ConstantPointerValue { value }
| PossibleValueSet::ImportedAddressValue { value } => value as u64,
_ => return false,
}
}
LowLevelILInstructionKind::Goto(target) => target.address(),
_ => return false,
};
let Some(symbol) = view.symbol_by_address(target) else {
return false;
};
let symbol_name = symbol.full_name();
let symbol_name = symbol_name.to_bytes();
// Remove any j_ prefix that the shared cache workflow adds to stub functions.
let symbol_name = symbol_name.strip_prefix(b"j_").unwrap_or(symbol_name);
IGNORABLE_MEMORY_MANAGEMENT_FUNCTIONS.contains(&symbol_name)
}
fn process_instruction(
bv: &BinaryView,
llil: &LowLevelILFunction<Mutable, NonSSA>,
insn: &LowLevelILInstruction<Mutable, NonSSA>,
link_register: LowLevelILRegisterKind<CoreRegister>,
link_register_size: usize,
) -> Result<bool, &'static str> {
if !is_call_to_ignorable_memory_management_function(bv, insn) {
return Ok(false);
}
// TODO: Removing calls to `objc_release` can sometimes leave behind a load of a struct field
// that appears to be unused. It's not clear whether we should be trying to detect and remove
// those here, or if some later analysis pass should be cleaning them up but isn't.
match insn.kind() {
LowLevelILInstructionKind::TailCall(_) => unsafe {
llil.set_current_address(insn.address());
llil.replace_expression(
insn.expr_idx(),
llil.ret(llil.reg(link_register_size, link_register)),
);
},
LowLevelILInstructionKind::Call(_) => unsafe {
// The memory management functions that are currently supported either return void
// or return their first argument. For arm64, the first argument is passed in `x0`
// and results are returned in `x0`, so we can replace the call with a nop. We'll need
// to revisit this to support other architectures, and to support the `objc_retain_x`
// `objc_release_x` functions that accept their argument in a different register.
llil.set_current_address(insn.address());
llil.replace_expression(insn.expr_idx(), llil.nop());
},
LowLevelILInstructionKind::Goto(_) if insn.index.0 == 0 => unsafe {
// If the `objc_retain` is the first instruction in the function, this function
// can only contain the call to the memory management function since when the
// memory management function returns, it will return to this function's caller.
llil.set_current_address(insn.address());
llil.replace_expression(
insn.expr_idx(),
llil.ret(llil.reg(link_register_size, link_register)),
);
},
LowLevelILInstructionKind::Goto(_) => {
// The shared cache workflow inlines calls to stub functions, which causes them
// to show up as a `lr = <next instruction>; goto <stub function instruction>;`
// sequence. We need to remove the load of `lr` and update the `goto` to jump
// to the next instruction.
let Some(prev) =
llil.instruction_from_index(LowLevelInstructionIndex(insn.index.0 - 1))
else {
return Ok(false);
};
let target = match prev.kind() {
LowLevelILInstructionKind::SetReg(op) if op.dest_reg() == link_register => {
let LowLevelILExpressionKind::ConstPtr(value) = op.source_expr().kind() else {
return Ok(false);
};
value.value()
}
_ => return Ok(false),
};
let Some(LowLevelInstructionIndex(target_idx)) = llil.instruction_index_at(target)
else {
return Ok(false);
};
// TODO: Manually creating a label like this is fragile and relies on a) knowledge of
// how labels are used by core, and b) that the target is the first instruction in
// a basic block. We should do this differently.
let mut label = LowLevelILLabel::new();
label.operand = target_idx;
unsafe {
llil.set_current_address(prev.address());
llil.replace_expression(prev.expr_idx(), llil.nop());
llil.set_current_address(insn.address());
llil.replace_expression(insn.expr_idx(), llil.goto(&mut label));
}
}
_ => return Ok(false),
}
Ok(true)
}
pub fn process(ac: &AnalysisContext) -> Result<(), Error> {
let view = ac.view();
if GlobalState::should_ignore_view(&view) {
return Ok(());
}
let func = ac.function();
let Some(link_register) = func.arch().link_reg() else {
return Ok(());
};
let link_register_size = link_register.info().size();
let link_register = LowLevelILRegisterKind::Arch(link_register);
let Some(llil) = (unsafe { ac.llil_function() }) else {
return Err(Error::MissingIL {
level: ILLevel::Low,
func_start: func.start(),
});
};
let mut function_changed = false;
for block in llil.basic_blocks().iter() {
for insn in block.iter() {
match process_instruction(&view, &llil, &insn, link_register, link_register_size) {
Ok(true) => function_changed = true,
Ok(_) => {}
Err(err) => {
tracing::error!(
"Error processing instruction at {:#x}: {}",
insn.address(),
err
);
continue;
}
}
}
}
if function_changed {
// Regenerate SSA form after modifications
llil.generate_ssa_form();
}
Ok(())
}
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