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 = 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            Operand::Constant(Box::new(ConstOperand {
64                span: source_info.span,
65                user_ty: None,
66                const_: Const::from_bool(tcx, true),
67            }))
68        }
69        // Other usages of null pointers only are UB if the pointee is not a ZST.
70        _ => {
71            let size_of = tcx.require_lang_item(LangItem::SizeOf, source_info.span);
72            let size_of =
73                Operand::unevaluated_constant(tcx, size_of, &[pointee_ty.into()], source_info.span);
74
75            let pointee_should_be_checked =
76                local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
77            let rvalue = Rvalue::BinaryOp(BinOp::Ne, Box::new((size_of, zero.clone())));
78            stmts.push(Statement::new(
79                source_info,
80                StatementKind::Assign(Box::new((pointee_should_be_checked, rvalue))),
81            ));
82            Operand::Copy(pointee_should_be_checked.into())
83        }
84    };
85
86    // Check whether the pointer is null.
87    let is_null = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
88    stmts.push(Statement::new(
89        source_info,
90        StatementKind::Assign(Box::new((
91            is_null,
92            Rvalue::BinaryOp(BinOp::Eq, Box::new((Operand::Copy(addr), zero))),
93        ))),
94    ));
95
96    // We want to throw an exception if the pointer is null and the pointee is not unconditionally
97    // allowed (which for all non-borrow place uses, is when the pointee is ZST).
98    let should_throw_exception =
99        local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
100    stmts.push(Statement::new(
101        source_info,
102        StatementKind::Assign(Box::new((
103            should_throw_exception,
104            Rvalue::BinaryOp(
105                BinOp::BitAnd,
106                Box::new((Operand::Copy(is_null), pointee_should_be_checked)),
107            ),
108        ))),
109    ));
110
111    // The final condition whether this pointer usage is ok or not.
112    let is_ok = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
113    stmts.push(Statement::new(
114        source_info,
115        StatementKind::Assign(Box::new((
116            is_ok,
117            Rvalue::UnaryOp(UnOp::Not, Operand::Copy(should_throw_exception)),
118        ))),
119    ));
120
121    // Emit a PointerCheck that asserts on the condition and otherwise triggers
122    // a AssertKind::NullPointerDereference.
123    PointerCheck {
124        cond: Operand::Copy(is_ok),
125        assert_kind: Box::new(AssertKind::NullPointerDereference),
126    }
127}
rustc_mir_transform/check_null.rs

rustc_mir_transform/
check_null.rs
