rustc_mir_transform/
check_null.rs

1use rustc_hir::attrs::lang_items::LangItem;
2use rustc_index::IndexVec;
3use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext};
4use rustc_middle::mir::*;
5use rustc_middle::ty::{Ty, TyCtxt};
6
7use crate::PassPolicy;
8use crate::check_pointers::{BorrowedFieldProjectionMode, PointerCheck, check_pointers};
9
10pub(super) struct CheckNull;
11
12impl<'tcx> crate::MirPass<'tcx> for CheckNull {
13    fn policy(&self, ctx: &crate::PassCtx<'_>) -> PassPolicy {
14        // When UB checks are enabled this is part of their semantics, not an optimization.
15        PassPolicy::optional(ctx.ub_checks())
16    }
17
18    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
19        check_pointers(
20            tcx,
21            body,
22            &[],
23            insert_null_check,
24            BorrowedFieldProjectionMode::NoFollowProjections,
25        );
26    }
27}
28
29fn insert_null_check<'tcx>(
30    tcx: TyCtxt<'tcx>,
31    pointer: Place<'tcx>,
32    pointee_ty: Ty<'tcx>,
33    context: PlaceContext,
34    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
35    stmts: &mut Vec<Statement<'tcx>>,
36    source_info: SourceInfo,
37) -> PointerCheck<'tcx> {
38    // Cast the pointer to a *const ().
39    let const_raw_ptr = Ty::new_imm_ptr(tcx, tcx.types.unit);
40    let rvalue = Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(pointer), const_raw_ptr);
41    let thin_ptr = local_decls.push(LocalDecl::with_source_info(const_raw_ptr, source_info)).into();
42    stmts.push(Statement::new(source_info, StatementKind::Assign(Box::new((thin_ptr, rvalue)))));
43
44    // Transmute the pointer to a usize (equivalent to `ptr.addr()`).
45    let rvalue = Rvalue::Cast(CastKind::Transmute, Operand::Copy(thin_ptr), tcx.types.usize);
46    let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
47    stmts.push(Statement::new(source_info, StatementKind::Assign(Box::new((addr, rvalue)))));
48
49    let zero = Operand::Constant(Box::new(ConstOperand {
50        span: source_info.span,
51        user_ty: None,
52        const_: Const::Val(ConstValue::from_target_usize(0, &tcx), tcx.types.usize),
53    }));
54
55    let (pointee_should_be_checked, assert_kind) = match context {
56        // Borrows pointing to "null" are UB even if the pointee is a ZST.
57        PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
58        | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
59            // Pointer should be checked unconditionally.
60            (
61                Operand::Constant(Box::new(ConstOperand {
62                    span: source_info.span,
63                    user_ty: None,
64                    const_: Const::from_bool(tcx, true),
65                })),
66                AssertKind::NullReferenceConstructed,
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), AssertKind::NullPointerDereference)
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    // the chosen AssertKind.
123    PointerCheck { cond: Operand::Copy(is_ok), assert_kind: Box::new(assert_kind) }
124}
rustc_mir_transform/check_null.rs

rustc_mir_transform/
check_null.rs
