Auto merge of rust-lang#134424 - 1c3t3a:null-checks, r=saethlin

Insert null checks for pointer dereferences when debug assertions are enabled

Similar to how the alignment is already checked, this adds a check
for null pointer dereferences in debug mode. It is implemented similarly
to the alignment check as a `MirPass`.

This inserts checks in the same places as the `CheckAlignment` pass and additionally
also inserts checks for `Borrows`, so code like
```rust
let ptr: *const u32 = std::ptr::null();
let val: &u32 = unsafe { &*ptr };
```
will have a check inserted on dereference. This is done because null references
are UB. The alignment check doesn't cover these places, because in `&(*ptr).field`,
the exact requirement is that the final reference must be aligned. This is something to
consider further enhancements of the alignment check.

For now this is implemented as a separate `MirPass`, to make it easy to disable
this check if necessary.

This is related to a 2025H1 project goal for better UB checks in debug
mode: rust-lang/rust-project-goals#177.

r? `@saethlin`

compiler/rustc_codegen_cranelift/src/base.rs

@@ -417,6 +417,16 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                             Some(source_info.span),
                         );
                     }
+                    AssertKind::NullPointerDereference => {
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            rustc_hir::LangItem::PanicNullPointerDereference,
+                            &[location],
+                            Some(source_info.span),
+                        )
+                    }
                     _ => {
                         let location = fx.get_caller_location(source_info).load_scalar(fx);
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -713,6 +713,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 // and `#[track_caller]` adds an implicit third argument.
                 (LangItem::PanicMisalignedPointerDereference, vec![required, found, location])
             }
+            AssertKind::NullPointerDereference => {
+                // It's `fn panic_null_pointer_dereference()`,
+                // `#[track_caller]` adds an implicit argument.
+                (LangItem::PanicNullPointerDereference, vec![location])
+            }
             _ => {
                 // It's `pub fn panic_...()` and `#[track_caller]` adds an implicit argument.
                 (msg.panic_function(), vec![location])

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -508,6 +508,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                     found: eval_to_int(found)?,
                 }
             }
+            NullPointerDereference => NullPointerDereference,
         };
         Err(ConstEvalErrKind::AssertFailure(err)).into()
     }

compiler/rustc_hir/src/lang_items.rs

@@ -316,6 +316,7 @@ language_item_table! {
     PanicAsyncFnResumedPanic, sym::panic_const_async_fn_resumed_panic, panic_const_async_fn_resumed_panic, Target::Fn, GenericRequirement::None;
     PanicAsyncGenFnResumedPanic, sym::panic_const_async_gen_fn_resumed_panic, panic_const_async_gen_fn_resumed_panic, Target::Fn, GenericRequirement::None;
     PanicGenFnNonePanic, sym::panic_const_gen_fn_none_panic, panic_const_gen_fn_none_panic, Target::Fn, GenericRequirement::None;
+    PanicNullPointerDereference, sym::panic_null_pointer_dereference, panic_null_pointer_dereference, Target::Fn, GenericRequirement::None;
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 

compiler/rustc_middle/messages.ftl

@@ -17,6 +17,9 @@ middle_assert_gen_resume_after_panic = `gen` fn or block cannot be further itera
 middle_assert_misaligned_ptr_deref =
     misaligned pointer dereference: address must be a multiple of {$required} but is {$found}
 
+middle_assert_null_ptr_deref =
+    null pointer dereference occurred
+
 middle_assert_op_overflow =
     attempt to compute `{$left} {$op} {$right}`, which would overflow
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -1076,6 +1076,7 @@ pub enum AssertKind<O> {
     ResumedAfterReturn(CoroutineKind),
     ResumedAfterPanic(CoroutineKind),
     MisalignedPointerDereference { required: O, found: O },
+    NullPointerDereference,
 }
 
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]

compiler/rustc_middle/src/mir/terminator.rs

@@ -206,6 +206,7 @@ impl<O> AssertKind<O> {
             ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
                 LangItem::PanicGenFnNonePanic
             }
+            NullPointerDereference => LangItem::PanicNullPointerDereference,
 
             BoundsCheck { .. } | MisalignedPointerDereference { .. } => {
                 bug!("Unexpected AssertKind")
@@ -271,6 +272,7 @@ impl<O> AssertKind<O> {
                     "\"misaligned pointer dereference: address must be a multiple of {{}} but is {{}}\", {required:?}, {found:?}"
                 )
             }
+            NullPointerDereference => write!(f, "\"null pointer dereference occured\""),
             ResumedAfterReturn(CoroutineKind::Coroutine(_)) => {
                 write!(f, "\"coroutine resumed after completion\"")
             }
@@ -341,7 +343,7 @@ impl<O> AssertKind<O> {
             ResumedAfterPanic(CoroutineKind::Coroutine(_)) => {
                 middle_assert_coroutine_resume_after_panic
             }
-
+            NullPointerDereference => middle_assert_null_ptr_deref,
             MisalignedPointerDereference { .. } => middle_assert_misaligned_ptr_deref,
         }
     }
@@ -374,7 +376,7 @@ impl<O> AssertKind<O> {
                 add!("left", format!("{left:#?}"));
                 add!("right", format!("{right:#?}"));
             }
-            ResumedAfterReturn(_) | ResumedAfterPanic(_) => {}
+            ResumedAfterReturn(_) | ResumedAfterPanic(_) | NullPointerDereference => {}
             MisalignedPointerDereference { required, found } => {
                 add!("required", format!("{required:#?}"));
                 add!("found", format!("{found:#?}"));

compiler/rustc_middle/src/mir/visit.rs

@@ -636,7 +636,7 @@ macro_rules! make_mir_visitor {
                     OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) => {
                         self.visit_operand(op, location);
                     }
-                    ResumedAfterReturn(_) | ResumedAfterPanic(_) => {
+                    ResumedAfterReturn(_) | ResumedAfterPanic(_) | NullPointerDereference => {
                         // Nothing to visit
                     }
                     MisalignedPointerDereference { required, found } => {

compiler/rustc_mir_transform/src/check_alignment.rs

@@ -1,11 +1,10 @@
-use rustc_hir::lang_items::LangItem;
 use rustc_index::IndexVec;
 use rustc_middle::mir::interpret::Scalar;
-use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_session::Session;
-use tracing::{debug, trace};
+
+use crate::check_pointers::{BorrowCheckMode, PointerCheck, check_pointers};
 
 pub(super) struct CheckAlignment;
 
@@ -19,166 +18,53 @@ impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-        // This pass emits new panics. If for whatever reason we do not have a panic
-        // implementation, running this pass may cause otherwise-valid code to not compile.
-        if tcx.lang_items().get(LangItem::PanicImpl).is_none() {
-            return;
-        }
-
-        let typing_env = body.typing_env(tcx);
-        let basic_blocks = body.basic_blocks.as_mut();
-        let local_decls = &mut body.local_decls;
-
-        // This pass inserts new blocks. Each insertion changes the Location for all
-        // statements/blocks after. Iterating or visiting the MIR in order would require updating
-        // our current location after every insertion. By iterating backwards, we dodge this issue:
-        // The only Locations that an insertion changes have already been handled.
-        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 { tcx, local_decls, typing_env, pointers: Vec::new() };
-                finder.visit_statement(statement, location);
-
-                for (local, ty) in finder.pointers {
-                    debug!("Inserting alignment check for {:?}", ty);
-                    let new_block = split_block(basic_blocks, location);
-                    insert_alignment_check(
-                        tcx,
-                        local_decls,
-                        &mut basic_blocks[block],
-                        local,
-                        ty,
-                        source_info,
-                        new_block,
-                    );
-                }
-            }
-        }
+        // Skip trivially aligned place types.
+        let excluded_pointees = [tcx.types.bool, tcx.types.i8, tcx.types.u8];
+
+        // We have to exclude borrows here: in `&x.field`, the exact
+        // requirement is that the final reference must be aligned, but
+        // `check_pointers` would check that `x` is aligned, which would be wrong.
+        check_pointers(
+            tcx,
+            body,
+            &excluded_pointees,
+            insert_alignment_check,
+            BorrowCheckMode::ExcludeBorrows,
+        );
     }
 
     fn is_required(&self) -> bool {
         true
     }
 }
 
-struct PointerFinder<'a, 'tcx> {
-    tcx: TyCtxt<'tcx>,
-    local_decls: &'a mut LocalDecls<'tcx>,
-    typing_env: ty::TypingEnv<'tcx>,
-    pointers: Vec<(Place<'tcx>, Ty<'tcx>)>,
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for PointerFinder<'a, 'tcx> {
-    fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
-        // We want to only check reads and writes to Places, so we specifically exclude
-        // Borrow and RawBorrow.
-        match context {
-            PlaceContext::MutatingUse(
-                MutatingUseContext::Store
-                | MutatingUseContext::AsmOutput
-                | MutatingUseContext::Call
-                | MutatingUseContext::Yield
-                | MutatingUseContext::Drop,
-            ) => {}
-            PlaceContext::NonMutatingUse(
-                NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
-            ) => {}
-            _ => {
-                return;
-            }
-        }
-
-        if !place.is_indirect() {
-            return;
-        }
-
-        // Since Deref projections must come first and only once, the pointer for an indirect place
-        // is the Local that the Place is based on.
-        let pointer = Place::from(place.local);
-        let pointer_ty = self.local_decls[place.local].ty;
-
-        // We only want to check places based on unsafe pointers
-        if !pointer_ty.is_unsafe_ptr() {
-            trace!("Indirect, but not based on an unsafe ptr, not checking {:?}", place);
-            return;
-        }
-
-        let pointee_ty =
-            pointer_ty.builtin_deref(true).expect("no builtin_deref for an unsafe pointer");
-        // Ideally we'd support this in the future, but for now we are limited to sized types.
-        if !pointee_ty.is_sized(self.tcx, self.typing_env) {
-            debug!("Unsafe pointer, but pointee is not known to be sized: {:?}", pointer_ty);
-            return;
-        }
-
-        // Try to detect types we are sure have an alignment of 1 and skip the check
-        // We don't need to look for str and slices, we already rejected unsized types above
-        let element_ty = match pointee_ty.kind() {
-            ty::Array(ty, _) => *ty,
-            _ => pointee_ty,
-        };
-        if [self.tcx.types.bool, self.tcx.types.i8, self.tcx.types.u8].contains(&element_ty) {
-            debug!("Trivially aligned place type: {:?}", pointee_ty);
-            return;
-        }
-
-        // Ensure that this place is based on an aligned pointer.
-        self.pointers.push((pointer, pointee_ty));
-
-        self.super_place(place, context, location);
-    }
-}
-
-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)
-}
-
+/// Inserts the actual alignment check's logic. Returns a
+/// [AssertKind::MisalignedPointerDereference] on failure.
 fn insert_alignment_check<'tcx>(
     tcx: TyCtxt<'tcx>,
-    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
-    block_data: &mut BasicBlockData<'tcx>,
     pointer: Place<'tcx>,
     pointee_ty: Ty<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
+    stmts: &mut Vec<Statement<'tcx>>,
     source_info: SourceInfo,
-    new_block: BasicBlock,
-) {
-    // Cast the pointer to a *const ()
+) -> PointerCheck<'tcx> {
+    // Cast the pointer to a *const ().
     let const_raw_ptr = Ty::new_imm_ptr(tcx, tcx.types.unit);
     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
+    stmts
         .push(Statement { source_info, kind: StatementKind::Assign(Box::new((thin_ptr, rvalue))) });
 
-    // Transmute the pointer to a usize (equivalent to `ptr.addr()`)
+    // 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))) });
+    stmts.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 {
+    stmts.push(Statement {
         source_info,
         kind: StatementKind::Assign(Box::new((alignment, rvalue))),
     });
@@ -191,7 +77,7 @@ fn insert_alignment_check<'tcx>(
         user_ty: None,
         const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(1, &tcx)), tcx.types.usize),
     }));
-    block_data.statements.push(Statement {
+    stmts.push(Statement {
         source_info,
         kind: StatementKind::Assign(Box::new((
             alignment_mask,
@@ -202,7 +88,7 @@ fn insert_alignment_check<'tcx>(
     // 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 {
+    stmts.push(Statement {
         source_info,
         kind: StatementKind::Assign(Box::new((
             alignment_bits,
@@ -220,29 +106,21 @@ fn insert_alignment_check<'tcx>(
         user_ty: None,
         const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(0, &tcx)), tcx.types.usize),
     }));
-    block_data.statements.push(Statement {
+    stmts.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: Box::new(AssertKind::MisalignedPointerDereference {
-                required: Operand::Copy(alignment),
-                found: Operand::Copy(addr),
-            }),
-            // This calls panic_misaligned_pointer_dereference, which is #[rustc_nounwind].
-            // We never want to insert an unwind into unsafe code, because unwinding could
-            // make a failing UB check turn into much worse UB when we start unwinding.
-            unwind: UnwindAction::Unreachable,
-        },
-    });
+    // Emit a check that asserts on the alignment and otherwise triggers a
+    // AssertKind::MisalignedPointerDereference.
+    PointerCheck {
+        cond: Operand::Copy(is_ok),
+        assert_kind: Box::new(AssertKind::MisalignedPointerDereference {
+            required: Operand::Copy(alignment),
+            found: Operand::Copy(addr),
+        }),
+    }
 }