Rollup merge of #136601 - compiler-errors:borrow-null-zst, r=saethlin

Detect (non-raw) borrows of null ZST pointers in CheckNull

Fixes #136568. Ensures that we check that borrows of derefs are non-null in the `CheckNull` pass **even if** it's a ZST pointee.

I'm actually surprised that this is UB in Miri, but if it's certainly UB, then this PR modifies the null check to be stricter. I couldn't find anywhere in https://doc.rust-lang.org/reference/behavior-considered-undefined.html that discusses this case specifically, but I didn't read it too closely, or perhaps it's just missing a bullet point.

On the contrary, if this is actually erroneous UB in Miri, then I'm happy to close this (and perhaps fix the null check in Miri to exclude ZSTs?)

On the double contrary, if this is still an "open question", I'm also happy to close this and wait for a decision to be made.

r? ``@saethlin`` cc ``@RalfJung`` (perhaps you feel strongly about this change)

compiler/rustc_middle/src/mir/terminator.rs

@@ -272,7 +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\""),
+            NullPointerDereference => write!(f, "\"null pointer dereference occurred\""),
             ResumedAfterReturn(CoroutineKind::Coroutine(_)) => {
                 write!(f, "\"coroutine resumed after completion\"")
             }

compiler/rustc_mir_transform/src/check_alignment.rs

@@ -1,5 +1,6 @@
 use rustc_index::IndexVec;
 use rustc_middle::mir::interpret::Scalar;
+use rustc_middle::mir::visit::PlaceContext;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_session::Session;
@@ -44,6 +45,7 @@ fn insert_alignment_check<'tcx>(
     tcx: TyCtxt<'tcx>,
     pointer: Place<'tcx>,
     pointee_ty: Ty<'tcx>,
+    _context: PlaceContext,
     local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
     stmts: &mut Vec<Statement<'tcx>>,
     source_info: SourceInfo,

compiler/rustc_mir_transform/src/check_null.rs

@@ -1,5 +1,5 @@
 use rustc_index::IndexVec;
-use rustc_middle::mir::interpret::Scalar;
+use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_session::Session;
@@ -26,6 +26,7 @@ fn insert_null_check<'tcx>(
     tcx: TyCtxt<'tcx>,
     pointer: Place<'tcx>,
     pointee_ty: Ty<'tcx>,
+    context: PlaceContext,
     local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
     stmts: &mut Vec<Statement<'tcx>>,
     source_info: SourceInfo,
@@ -42,30 +43,51 @@ fn insert_null_check<'tcx>(
     let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
     stmts.push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) });
 
-    // Get size of the pointee (zero-sized reads and writes are allowed).
-    let rvalue = Rvalue::NullaryOp(NullOp::SizeOf, pointee_ty);
-    let sizeof_pointee =
-        local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
-    stmts.push(Statement {
-        source_info,
-        kind: StatementKind::Assign(Box::new((sizeof_pointee, rvalue))),
-    });
-
-    // Check that the pointee is not a ZST.
     let zero = Operand::Constant(Box::new(ConstOperand {
         span: source_info.span,
         user_ty: None,
-        const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(0, &tcx)), tcx.types.usize),
+        const_: Const::Val(ConstValue::from_target_usize(0, &tcx), tcx.types.usize),
     }));
-    let is_pointee_no_zst =
-        local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
-    stmts.push(Statement {
-        source_info,
-        kind: StatementKind::Assign(Box::new((
-            is_pointee_no_zst,
-            Rvalue::BinaryOp(BinOp::Ne, Box::new((Operand::Copy(sizeof_pointee), zero.clone()))),
-        ))),
-    });
+
+    let pointee_should_be_checked = match context {
+        // Borrows pointing to "null" are UB even if the pointee is a ZST.
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
+        | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
+            // Pointer should be checked unconditionally.
+            Operand::Constant(Box::new(ConstOperand {
+                span: source_info.span,
+                user_ty: None,
+                const_: Const::Val(ConstValue::from_bool(true), tcx.types.bool),
+            }))
+        }
+        // Other usages of null pointers only are UB if the pointee is not a ZST.
+        _ => {
+            let rvalue = Rvalue::NullaryOp(NullOp::SizeOf, pointee_ty);
+            let sizeof_pointee =
+                local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
+            stmts.push(Statement {
+                source_info,
+                kind: StatementKind::Assign(Box::new((sizeof_pointee, rvalue))),
+            });
+
+            // Check that the pointee is not a ZST.
+            let is_pointee_not_zst =
+                local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+            stmts.push(Statement {
+                source_info,
+                kind: StatementKind::Assign(Box::new((
+                    is_pointee_not_zst,
+                    Rvalue::BinaryOp(
+                        BinOp::Ne,
+                        Box::new((Operand::Copy(sizeof_pointee), zero.clone())),
+                    ),
+                ))),
+            });
+
+            // Pointer needs to be checked only if pointee is not a ZST.
+            Operand::Copy(is_pointee_not_zst)
+        }
+    };
 
     // Check whether the pointer is null.
     let is_null = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
@@ -77,7 +99,8 @@ fn insert_null_check<'tcx>(
         ))),
     });
 
-    // We want to throw an exception if the pointer is null and doesn't point to a ZST.
+    // We want to throw an exception if the pointer is null and the pointee is not unconditionally
+    // allowed (which for all non-borrow place uses, is when the pointee is ZST).
     let should_throw_exception =
         local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
     stmts.push(Statement {
@@ -86,7 +109,7 @@ fn insert_null_check<'tcx>(
             should_throw_exception,
             Rvalue::BinaryOp(
                 BinOp::BitAnd,
-                Box::new((Operand::Copy(is_null), Operand::Copy(is_pointee_no_zst))),
+                Box::new((Operand::Copy(is_null), pointee_should_be_checked)),
             ),
         ))),
     });

compiler/rustc_mir_transform/src/check_pointers.rs

@@ -40,17 +40,18 @@ pub(crate) enum BorrowCheckMode {
 ///   success and fail the check otherwise.
 /// This utility will insert a terminator block that asserts on the condition
 /// and panics on failure.
-pub(crate) fn check_pointers<'a, 'tcx, F>(
+pub(crate) fn check_pointers<'tcx, F>(
     tcx: TyCtxt<'tcx>,
     body: &mut Body<'tcx>,
-    excluded_pointees: &'a [Ty<'tcx>],
+    excluded_pointees: &[Ty<'tcx>],
     on_finding: F,
     borrow_check_mode: BorrowCheckMode,
 ) where
     F: Fn(
         /* tcx: */ TyCtxt<'tcx>,
         /* pointer: */ Place<'tcx>,
         /* pointee_ty: */ Ty<'tcx>,
+        /* context: */ PlaceContext,
         /* local_decls: */ &mut IndexVec<Local, LocalDecl<'tcx>>,
         /* stmts: */ &mut Vec<Statement<'tcx>>,
         /* source_info: */ SourceInfo,
@@ -86,7 +87,7 @@ pub(crate) fn check_pointers<'a, 'tcx, F>(
             );
             finder.visit_statement(statement, location);
 
-            for (local, ty) in finder.into_found_pointers() {
+            for (local, ty, context) in finder.into_found_pointers() {
                 debug!("Inserting check for {:?}", ty);
                 let new_block = split_block(basic_blocks, location);
 
@@ -98,6 +99,7 @@ pub(crate) fn check_pointers<'a, 'tcx, F>(
                     tcx,
                     local,
                     ty,
+                    context,
                     local_decls,
                     &mut block_data.statements,
                     source_info,
@@ -125,7 +127,7 @@ struct PointerFinder<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     local_decls: &'a mut LocalDecls<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
-    pointers: Vec<(Place<'tcx>, Ty<'tcx>)>,
+    pointers: Vec<(Place<'tcx>, Ty<'tcx>, PlaceContext)>,
     excluded_pointees: &'a [Ty<'tcx>],
     borrow_check_mode: BorrowCheckMode,
 }
@@ -148,7 +150,7 @@ impl<'a, 'tcx> PointerFinder<'a, 'tcx> {
         }
     }
 
-    fn into_found_pointers(self) -> Vec<(Place<'tcx>, Ty<'tcx>)> {
+    fn into_found_pointers(self) -> Vec<(Place<'tcx>, Ty<'tcx>, PlaceContext)> {
         self.pointers
     }
 
@@ -211,7 +213,7 @@ impl<'a, 'tcx> Visitor<'tcx> for PointerFinder<'a, 'tcx> {
             return;
         }
 
-        self.pointers.push((pointer, pointee_ty));
+        self.pointers.push((pointer, pointee_ty, context));
 
         self.super_place(place, context, location);
     }

compiler/stable_mir/src/mir/body.rs

@@ -307,7 +307,7 @@ impl AssertMessage {
             AssertMessage::MisalignedPointerDereference { .. } => {
                 Ok("misaligned pointer dereference")
             }
-            AssertMessage::NullPointerDereference => Ok("null pointer dereference occured"),
+            AssertMessage::NullPointerDereference => Ok("null pointer dereference occurred"),
         }
     }
 }

compiler/stable_mir/src/mir/pretty.rs

@@ -299,7 +299,7 @@ fn pretty_assert_message<W: Write>(writer: &mut W, msg: &AssertMessage) -> io::R
             )
         }
         AssertMessage::NullPointerDereference => {
-            write!(writer, "\"null pointer dereference occured.\"")
+            write!(writer, "\"null pointer dereference occurred\"")
         }
         AssertMessage::ResumedAfterReturn(_) | AssertMessage::ResumedAfterPanic(_) => {
             write!(writer, "{}", msg.description().unwrap())

library/core/src/panicking.rs

@@ -302,7 +302,7 @@ fn panic_null_pointer_dereference() -> ! {
     }
 
     panic_nounwind_fmt(
-        format_args!("null pointer dereference occured"),
+        format_args!("null pointer dereference occurred"),
         /* force_no_backtrace */ false,
     )
 }

tests/ui/mir/null/borrowed_mut_null.rs

@@ -1,6 +1,6 @@
 //@ run-fail
 //@ compile-flags: -C debug-assertions
-//@ error-pattern: null pointer dereference occured
+//@ error-pattern: null pointer dereference occurred
 
 fn main() {
     let ptr: *mut u32 = std::ptr::null_mut();

tests/ui/mir/null/borrowed_null.rs

@@ -1,6 +1,6 @@
 //@ run-fail
 //@ compile-flags: -C debug-assertions
-//@ error-pattern: null pointer dereference occured
+//@ error-pattern: null pointer dereference occurred
 
 fn main() {
     let ptr: *const u32 = std::ptr::null();

tests/ui/mir/null/borrowed_null_zst.rs

@@ -0,0 +1,8 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: null pointer dereference occurred
+
+fn main() {
+    let ptr: *const () = std::ptr::null();
+    let _ptr: &() = unsafe { &*ptr };
+}

tests/ui/mir/null/null_lhs.rs

@@ -1,6 +1,6 @@
 //@ run-fail
 //@ compile-flags: -C debug-assertions
-//@ error-pattern: null pointer dereference occured
+//@ error-pattern: null pointer dereference occurred
 
 fn main() {
     let ptr: *mut u32 = std::ptr::null_mut();

tests/ui/mir/null/null_rhs.rs

@@ -1,6 +1,6 @@
 //@ run-fail
 //@ compile-flags: -C debug-assertions
-//@ error-pattern: null pointer dereference occured
+//@ error-pattern: null pointer dereference occurred
 
 fn main() {
     let ptr: *mut u32 = std::ptr::null_mut();

tests/ui/mir/null/place_without_read.rs

@@ -6,5 +6,6 @@ fn main() {
     let ptr: *const u16 = std::ptr::null();
     unsafe {
         let _ = *ptr;
+        let _ = &raw const *ptr;
     }
 }

tests/ui/mir/null/place_without_read_zst.rs

@@ -0,0 +1,11 @@
+// Make sure that we don't insert a check for places that do not read.
+//@ run-pass
+//@ compile-flags: -C debug-assertions
+
+fn main() {
+    let ptr: *const () = std::ptr::null();
+    unsafe {
+        let _ = *ptr;
+        let _ = &raw const *ptr;
+    }
+}

tests/ui/mir/null/two_pointers.rs

@@ -1,6 +1,6 @@
 //@ run-fail
 //@ compile-flags: -C debug-assertions
-//@ error-pattern: null pointer dereference occured
+//@ error-pattern: null pointer dereference occurred
 
 fn main() {
     let ptr = std::ptr::null();