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Auto merge of rust-lang#98112 - saethlin:mir-alignment-checks, r=oli-obk
Insert alignment checks for pointer dereferences when debug assertions are enabled Closes rust-lang#54915 - [x] Jake tells me this sounds like a place to use `MirPatch`, but I can't figure out how to insert a new basic block with a new terminator in the middle of an existing basic block, using `MirPatch`. (if nobody else backs up this point I'm checking this as "not actually a good idea" because the code looks pretty clean to me after rearranging it a bit) - [x] Using `CastKind::PointerExposeAddress` is definitely wrong, we don't want to expose. Calling a function to get the pointer address seems quite excessive. ~I'll see if I can add a new `CastKind`.~ `CastKind::Transmute` to the rescue! - [x] Implement a more helpful panic message like slice bounds checking. r? `@oli-obk`
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,227 @@ | ||
| use crate::MirPass; | ||
| use rustc_hir::def_id::DefId; | ||
| use rustc_index::vec::IndexVec; | ||
| use rustc_middle::mir::*; | ||
| use rustc_middle::mir::{ | ||
| interpret::{ConstValue, Scalar}, | ||
| visit::{PlaceContext, Visitor}, | ||
| }; | ||
| use rustc_middle::ty::{Ty, TyCtxt, TypeAndMut}; | ||
| use rustc_session::Session; | ||
|
|
||
| pub struct CheckAlignment; | ||
|
|
||
| impl<'tcx> MirPass<'tcx> for CheckAlignment { | ||
| fn is_enabled(&self, sess: &Session) -> bool { | ||
| sess.opts.debug_assertions | ||
| } | ||
|
|
||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| let basic_blocks = body.basic_blocks.as_mut(); | ||
| let local_decls = &mut body.local_decls; | ||
|
|
||
| for block in (0..basic_blocks.len()).rev() { | ||
| let block = block.into(); | ||
| for statement_index in (0..basic_blocks[block].statements.len()).rev() { | ||
| let location = Location { block, statement_index }; | ||
| let statement = &basic_blocks[block].statements[statement_index]; | ||
| let source_info = statement.source_info; | ||
|
|
||
| let mut finder = PointerFinder { | ||
| local_decls, | ||
| tcx, | ||
| pointers: Vec::new(), | ||
| def_id: body.source.def_id(), | ||
| }; | ||
| for (pointer, pointee_ty) in finder.find_pointers(statement) { | ||
| debug!("Inserting alignment check for {:?}", pointer.ty(&*local_decls, tcx).ty); | ||
|
|
||
| let new_block = split_block(basic_blocks, location); | ||
| insert_alignment_check( | ||
| tcx, | ||
| local_decls, | ||
| &mut basic_blocks[block], | ||
| pointer, | ||
| pointee_ty, | ||
| source_info, | ||
| new_block, | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl<'tcx, 'a> PointerFinder<'tcx, 'a> { | ||
| fn find_pointers(&mut self, statement: &Statement<'tcx>) -> Vec<(Place<'tcx>, Ty<'tcx>)> { | ||
| self.pointers.clear(); | ||
| self.visit_statement(statement, Location::START); | ||
| core::mem::take(&mut self.pointers) | ||
| } | ||
| } | ||
|
|
||
| struct PointerFinder<'tcx, 'a> { | ||
| local_decls: &'a mut LocalDecls<'tcx>, | ||
| tcx: TyCtxt<'tcx>, | ||
| def_id: DefId, | ||
| pointers: Vec<(Place<'tcx>, Ty<'tcx>)>, | ||
| } | ||
|
|
||
| impl<'tcx, 'a> Visitor<'tcx> for PointerFinder<'tcx, 'a> { | ||
| fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) { | ||
| if let PlaceContext::NonUse(_) = context { | ||
| return; | ||
| } | ||
| if !place.is_indirect() { | ||
| return; | ||
| } | ||
|
|
||
| let pointer = Place::from(place.local); | ||
| let pointer_ty = pointer.ty(&*self.local_decls, self.tcx).ty; | ||
|
|
||
| // We only want to check unsafe pointers | ||
| if !pointer_ty.is_unsafe_ptr() { | ||
| trace!("Indirect, but not an unsafe ptr, not checking {:?}", pointer_ty); | ||
| return; | ||
| } | ||
|
|
||
| let Some(pointee) = pointer_ty.builtin_deref(true) else { | ||
| debug!("Indirect but no builtin deref: {:?}", pointer_ty); | ||
| return; | ||
| }; | ||
| let mut pointee_ty = pointee.ty; | ||
| if pointee_ty.is_array() || pointee_ty.is_slice() || pointee_ty.is_str() { | ||
| pointee_ty = pointee_ty.sequence_element_type(self.tcx); | ||
| } | ||
|
|
||
| if !pointee_ty.is_sized(self.tcx, self.tcx.param_env_reveal_all_normalized(self.def_id)) { | ||
| debug!("Unsafe pointer, but unsized: {:?}", pointer_ty); | ||
| return; | ||
| } | ||
|
|
||
| if [self.tcx.types.bool, self.tcx.types.i8, self.tcx.types.u8, self.tcx.types.str_] | ||
| .contains(&pointee_ty) | ||
| { | ||
| debug!("Trivially aligned pointee type: {:?}", pointer_ty); | ||
| return; | ||
| } | ||
|
|
||
| self.pointers.push((pointer, pointee_ty)) | ||
| } | ||
| } | ||
|
|
||
| fn split_block( | ||
| basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>, | ||
| location: Location, | ||
| ) -> BasicBlock { | ||
| let block_data = &mut basic_blocks[location.block]; | ||
|
|
||
| // Drain every statement after this one and move the current terminator to a new basic block | ||
| let new_block = BasicBlockData { | ||
| statements: block_data.statements.split_off(location.statement_index), | ||
| terminator: block_data.terminator.take(), | ||
| is_cleanup: block_data.is_cleanup, | ||
| }; | ||
|
|
||
| basic_blocks.push(new_block) | ||
| } | ||
|
|
||
| fn insert_alignment_check<'tcx>( | ||
| tcx: TyCtxt<'tcx>, | ||
| local_decls: &mut LocalDecls<'tcx>, | ||
| block_data: &mut BasicBlockData<'tcx>, | ||
| pointer: Place<'tcx>, | ||
| pointee_ty: Ty<'tcx>, | ||
| source_info: SourceInfo, | ||
| new_block: BasicBlock, | ||
| ) { | ||
| // Cast the pointer to a *const () | ||
| let const_raw_ptr = tcx.mk_ptr(TypeAndMut { ty: tcx.types.unit, mutbl: Mutability::Not }); | ||
| let rvalue = Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(pointer), const_raw_ptr); | ||
| let thin_ptr = local_decls.push(LocalDecl::with_source_info(const_raw_ptr, source_info)).into(); | ||
| block_data | ||
| .statements | ||
| .push(Statement { source_info, kind: StatementKind::Assign(Box::new((thin_ptr, rvalue))) }); | ||
|
|
||
| // Transmute the pointer to a usize (equivalent to `ptr.addr()`) | ||
| let rvalue = Rvalue::Cast(CastKind::Transmute, Operand::Copy(thin_ptr), tcx.types.usize); | ||
| let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into(); | ||
| block_data | ||
| .statements | ||
| .push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) }); | ||
|
|
||
| // Get the alignment of the pointee | ||
| let alignment = | ||
| local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into(); | ||
| let rvalue = Rvalue::NullaryOp(NullOp::AlignOf, pointee_ty); | ||
| block_data.statements.push(Statement { | ||
| source_info, | ||
| kind: StatementKind::Assign(Box::new((alignment, rvalue))), | ||
| }); | ||
|
|
||
| // Subtract 1 from the alignment to get the alignment mask | ||
| let alignment_mask = | ||
| local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into(); | ||
| let one = Operand::Constant(Box::new(Constant { | ||
| span: source_info.span, | ||
| user_ty: None, | ||
| literal: ConstantKind::Val( | ||
| ConstValue::Scalar(Scalar::from_target_usize(1, &tcx)), | ||
| tcx.types.usize, | ||
| ), | ||
| })); | ||
| block_data.statements.push(Statement { | ||
| source_info, | ||
| kind: StatementKind::Assign(Box::new(( | ||
| alignment_mask, | ||
| Rvalue::BinaryOp(BinOp::Sub, Box::new((Operand::Copy(alignment), one))), | ||
| ))), | ||
| }); | ||
|
|
||
| // BitAnd the alignment mask with the pointer | ||
| let alignment_bits = | ||
| local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into(); | ||
| block_data.statements.push(Statement { | ||
| source_info, | ||
| kind: StatementKind::Assign(Box::new(( | ||
| alignment_bits, | ||
| Rvalue::BinaryOp( | ||
| BinOp::BitAnd, | ||
| Box::new((Operand::Copy(addr), Operand::Copy(alignment_mask))), | ||
| ), | ||
| ))), | ||
| }); | ||
|
|
||
| // Check if the alignment bits are all zero | ||
| let is_ok = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into(); | ||
| let zero = Operand::Constant(Box::new(Constant { | ||
| span: source_info.span, | ||
| user_ty: None, | ||
| literal: ConstantKind::Val( | ||
| ConstValue::Scalar(Scalar::from_target_usize(0, &tcx)), | ||
| tcx.types.usize, | ||
| ), | ||
| })); | ||
| block_data.statements.push(Statement { | ||
| source_info, | ||
| kind: StatementKind::Assign(Box::new(( | ||
| is_ok, | ||
| Rvalue::BinaryOp(BinOp::Eq, Box::new((Operand::Copy(alignment_bits), zero.clone()))), | ||
| ))), | ||
| }); | ||
|
|
||
| // Set this block's terminator to our assert, continuing to new_block if we pass | ||
| block_data.terminator = Some(Terminator { | ||
| source_info, | ||
| kind: TerminatorKind::Assert { | ||
| cond: Operand::Copy(is_ok), | ||
| expected: true, | ||
| target: new_block, | ||
| msg: AssertKind::MisalignedPointerDereference { | ||
| required: Operand::Copy(alignment), | ||
| found: Operand::Copy(addr), | ||
| }, | ||
| cleanup: None, | ||
| }, | ||
| }); | ||
| } |
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