rustc_mir_transform/
check_null.rs

1use rustc_hir::LangItem;
2use rustc_index::IndexVec;
3use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext};
4use rustc_middle::mir::*;
5use rustc_middle::ty::{Ty, TyCtxt};
6use rustc_session::Session;
7
8use crate::check_pointers::{BorrowedFieldProjectionMode, PointerCheck, check_pointers};
9
10pub(super) struct CheckNull;
11
12impl<'tcx> crate::MirPass<'tcx> for CheckNull {
13    fn is_enabled(&self, sess: &Session) -> bool {
14        sess.ub_checks()
15    }
16
17    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
18        check_pointers(
19            tcx,
20            body,
21            &[],
22            insert_null_check,
23            BorrowedFieldProjectionMode::NoFollowProjections,
24        );
25    }
26
27    fn is_required(&self) -> bool {
28        true
29    }
30}
31
32fn insert_null_check<'tcx>(
33    tcx: TyCtxt<'tcx>,
34    pointer: Place<'tcx>,
35    pointee_ty: Ty<'tcx>,
36    context: PlaceContext,
37    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
38    stmts: &mut Vec<Statement<'tcx>>,
39    source_info: SourceInfo,
40) -> PointerCheck<'tcx> {
41    // Cast the pointer to a *const ().
42    let const_raw_ptr = Ty::new_imm_ptr(tcx, tcx.types.unit);
43    let rvalue = Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(pointer), const_raw_ptr);
44    let thin_ptr = local_decls.push(LocalDecl::with_source_info(const_raw_ptr, source_info)).into();
45    stmts.push(Statement::new(source_info, StatementKind::Assign(Box::new((thin_ptr, rvalue)))));
46
47    // Transmute the pointer to a usize (equivalent to `ptr.addr()`).
48    let rvalue = Rvalue::Cast(CastKind::Transmute, Operand::Copy(thin_ptr), tcx.types.usize);
49    let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
50    stmts.push(Statement::new(source_info, StatementKind::Assign(Box::new((addr, rvalue)))));
51
52    let zero = Operand::Constant(Box::new(ConstOperand {
53        span: source_info.span,
54        user_ty: None,
55        const_: Const::Val(ConstValue::from_target_usize(0, &tcx), tcx.types.usize),
56    }));
57
58    let (pointee_should_be_checked, assert_kind) = match context {
59        // Borrows pointing to "null" are UB even if the pointee is a ZST.
60        PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
61        | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
62            // Pointer should be checked unconditionally.
63            (
64                Operand::Constant(Box::new(ConstOperand {
65                    span: source_info.span,
66                    user_ty: None,
67                    const_: Const::from_bool(tcx, true),
68                })),
69                AssertKind::NullReferenceConstructed,
70            )
71        }
72        // Other usages of null pointers only are UB if the pointee is not a ZST.
73        _ => {
74            let size_of = tcx.require_lang_item(LangItem::SizeOf, source_info.span);
75            let size_of =
76                Operand::unevaluated_constant(tcx, size_of, &[pointee_ty.into()], source_info.span);
77
78            let pointee_should_be_checked =
79                local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
80            let rvalue = Rvalue::BinaryOp(BinOp::Ne, Box::new((size_of, zero.clone())));
81            stmts.push(Statement::new(
82                source_info,
83                StatementKind::Assign(Box::new((pointee_should_be_checked, rvalue))),
84            ));
85            (Operand::Copy(pointee_should_be_checked), AssertKind::NullPointerDereference)
86        }
87    };
88
89    // Check whether the pointer is null.
90    let is_null = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
91    stmts.push(Statement::new(
92        source_info,
93        StatementKind::Assign(Box::new((
94            is_null,
95            Rvalue::BinaryOp(BinOp::Eq, Box::new((Operand::Copy(addr), zero))),
96        ))),
97    ));
98
99    // We want to throw an exception if the pointer is null and the pointee is not unconditionally
100    // allowed (which for all non-borrow place uses, is when the pointee is ZST).
101    let should_throw_exception =
102        local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
103    stmts.push(Statement::new(
104        source_info,
105        StatementKind::Assign(Box::new((
106            should_throw_exception,
107            Rvalue::BinaryOp(
108                BinOp::BitAnd,
109                Box::new((Operand::Copy(is_null), pointee_should_be_checked)),
110            ),
111        ))),
112    ));
113
114    // The final condition whether this pointer usage is ok or not.
115    let is_ok = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
116    stmts.push(Statement::new(
117        source_info,
118        StatementKind::Assign(Box::new((
119            is_ok,
120            Rvalue::UnaryOp(UnOp::Not, Operand::Copy(should_throw_exception)),
121        ))),
122    ));
123
124    // Emit a PointerCheck that asserts on the condition and otherwise triggers
125    // the chosen AssertKind.
126    PointerCheck { cond: Operand::Copy(is_ok), assert_kind: Box::new(assert_kind) }
127}
rustc_mir_transform/check_null.rs

rustc_mir_transform/
check_null.rs
