[ObjC] Add support for removing runtime calls that perform retain count operations

A new activity is added that will remove calls to `objc_retain`,
`objc_release`, `objc_autorelease`, and related functions. It is
disabled by default due to the fact it changes the semantics of the
code. It can be enabled on a per-function basis via the Function
Settings context menu or command palette entry. Alternatively, it can be
enabled in Open with Options or in user settings if a user would prefer
it be on for an entire file or for all files they open.

For now the activity is only eligible within arm64 binaries. Supporting
x86_64 will require matching some slightly different IL patterns.

3 files changed, 212 insertions, 0 deletions

diff --git a/plugins/workflow_objc/src/activities/mod.rs b/plugins/workflow_objc/src/activities/mod.rs
index f1132d03..98c9770f 100644
--- a/plugins/workflow_objc/src/activities/mod.rs
+++ b/plugins/workflow_objc/src/activities/mod.rs
@@ -1,3 +1,4 @@
 pub mod inline_stubs;
 pub mod objc_msg_send_calls;
+pub mod remove_memory_management;
 pub mod super_init;
diff --git a/plugins/workflow_objc/src/activities/remove_memory_management.rs b/plugins/workflow_objc/src/activities/remove_memory_management.rs
new file mode 100644
index 00000000..470c3152
--- /dev/null
+++ b/plugins/workflow_objc/src/activities/remove_memory_management.rs
@@ -0,0 +1,192 @@
+use binaryninja::{
+    architecture::{Architecture as _, CoreRegister, Register as _, RegisterInfo as _},
+    binary_view::{BinaryView, BinaryViewExt as _},
+    low_level_il::{
+        expression::{ExpressionHandler, LowLevelILExpressionKind},
+        function::{LowLevelILFunction, Mutable, NonSSA},
+        instruction::{
+            InstructionHandler, LowLevelILInstruction, LowLevelILInstructionKind,
+            LowLevelInstructionIndex,
+        },
+        lifting::LowLevelILLabel,
+        LowLevelILRegisterKind,
+    },
+    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) => {
+            let LowLevelILExpressionKind::ConstPtr(address) = call.target().kind() else {
+                return false;
+            };
+            address.value()
+        }
+        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) => {
+                    log::error!(
+                        "Error processing instruction at {:#x}: {}",
+                        insn.address(),
+                        err
+                    );
+                    continue;
+                }
+            }
+        }
+    }
+
+    if function_changed {
+        // Regenerate SSA form after modifications
+        llil.generate_ssa_form();
+    }
+    Ok(())
+}
diff --git a/plugins/workflow_objc/src/workflow.rs b/plugins/workflow_objc/src/workflow.rs
index f411951b..f1054411 100644
--- a/plugins/workflow_objc/src/workflow.rs
+++ b/plugins/workflow_objc/src/workflow.rs
@@ -69,9 +69,28 @@ pub fn register_activities() -> Result<(), WorkflowRegistrationError> {
         run(activities::super_init::process),
     );
 
+    let remove_memory_management_activity = Activity::new_with_action(
+        activity::Config::action(
+            "core.function.objectiveC.removeMemoryManagement",
+            "Obj-C: Remove reference counting calls",
+            "Remove calls to objc_retain / objc_release / objc_autorelease to simplify the resulting higher-level ILs",
+        )
+        .eligibility(
+            activity::Eligibility::auto_with_default(false).matching_all_predicates(&[
+                activity::ViewType::in_(["Mach-O", "DSCView"]).into(),
+                activity::Platform::in_(["mac-aarch64", "ios-aarch64"]).into()
+            ])
+        ),
+        run(activities::remove_memory_management::process),
+    );
+
     workflow
         .activity_after(&inline_stubs_activity, "core.function.translateTailCalls")?
         .activity_after(&objc_msg_send_calls_activity, &inline_stubs_activity.name())?
+        .activity_before(
+            &remove_memory_management_activity,
+            "core.function.generateMediumLevelIL",
+        )?
         .activity_after(&super_init_activity, "core.function.generateMediumLevelIL")?
         .register_with_config(WORKFLOW_INFO)?;