Lint on reference casting to bigger underlying allocation

Author: Urgau

compiler/rustc_lint/messages.ftl

@@ -319,6 +319,11 @@ lint_invalid_nan_comparisons_lt_le_gt_ge = incorrect NaN comparison, NaN is not
 lint_invalid_reference_casting_assign_to_ref = assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
     .label = casting happend here
 
+lint_invalid_reference_casting_bigger_layout = casting references to a bigger memory layout than the backing allocation is undefined behavior, even if the reference is unused
+    .label = casting happend here
+    .alloc = backing allocation comes from here
+    .layout = casting from `{$from_ty}` ({$from_size} bytes) to `{$to_ty}` ({$to_size} bytes)
+
 lint_invalid_reference_casting_borrow_as_mut = casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
     .label = casting happend here
 

compiler/rustc_lint/src/lints.rs

@@ -716,7 +716,7 @@ pub enum InvalidFromUtf8Diag {
 
 // reference_casting.rs
 #[derive(LintDiagnostic)]
-pub enum InvalidReferenceCastingDiag {
+pub enum InvalidReferenceCastingDiag<'tcx> {
     #[diag(lint_invalid_reference_casting_borrow_as_mut)]
     #[note(lint_invalid_reference_casting_note_book)]
     BorrowAsMut {
@@ -733,6 +733,18 @@ pub enum InvalidReferenceCastingDiag {
         #[note(lint_invalid_reference_casting_note_ty_has_interior_mutability)]
         ty_has_interior_mutability: Option<()>,
     },
+    #[diag(lint_invalid_reference_casting_bigger_layout)]
+    #[note(lint_layout)]
+    BiggerLayout {
+        #[label]
+        orig_cast: Option<Span>,
+        #[label(lint_alloc)]
+        alloc: Span,
+        from_ty: Ty<'tcx>,
+        from_size: u64,
+        to_ty: Ty<'tcx>,
+        to_size: u64,
+    },
 }
 
 // hidden_unicode_codepoints.rs

compiler/rustc_lint/src/reference_casting.rs

@@ -1,6 +1,7 @@
 use rustc_ast::Mutability;
 use rustc_hir::{Expr, ExprKind, UnOp};
-use rustc_middle::ty::{self, TypeAndMut};
+use rustc_middle::ty::layout::LayoutOf as _;
+use rustc_middle::ty::{self, layout::TyAndLayout, TypeAndMut};
 use rustc_span::sym;
 
 use crate::{lints::InvalidReferenceCastingDiag, LateContext, LateLintPass, LintContext};
@@ -38,13 +39,12 @@ declare_lint_pass!(InvalidReferenceCasting => [INVALID_REFERENCE_CASTING]);
 impl<'tcx> LateLintPass<'tcx> for InvalidReferenceCasting {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
         if let Some((e, pat)) = borrow_or_assign(cx, expr) {
-            if matches!(pat, PatternKind::Borrow { mutbl: Mutability::Mut } | PatternKind::Assign) {
-                let init = cx.expr_or_init(e);
+            let init = cx.expr_or_init(e);
+            let orig_cast = if init.span != e.span { Some(init.span) } else { None };
 
-                let Some(ty_has_interior_mutability) = is_cast_from_ref_to_mut_ptr(cx, init) else {
-                    return;
-                };
-                let orig_cast = if init.span != e.span { Some(init.span) } else { None };
+            if matches!(pat, PatternKind::Borrow { mutbl: Mutability::Mut } | PatternKind::Assign)
+                && let Some(ty_has_interior_mutability) = is_cast_from_ref_to_mut_ptr(cx, init)
+            {
                 let ty_has_interior_mutability = ty_has_interior_mutability.then_some(());
 
                 cx.emit_span_lint(
@@ -63,6 +63,23 @@ impl<'tcx> LateLintPass<'tcx> for InvalidReferenceCasting {
                     },
                 );
             }
+
+            if let Some((from_ty_layout, to_ty_layout, e_alloc)) =
+                is_cast_to_bigger_memory_layout(cx, init)
+            {
+                cx.emit_span_lint(
+                    INVALID_REFERENCE_CASTING,
+                    expr.span,
+                    InvalidReferenceCastingDiag::BiggerLayout {
+                        orig_cast,
+                        alloc: e_alloc.span,
+                        from_ty: from_ty_layout.ty,
+                        from_size: from_ty_layout.layout.size().bytes(),
+                        to_ty: to_ty_layout.ty,
+                        to_size: to_ty_layout.layout.size().bytes(),
+                    },
+                );
+            }
         }
     }
 }
@@ -151,6 +168,48 @@ fn is_cast_from_ref_to_mut_ptr<'tcx>(
     }
 }
 
+fn is_cast_to_bigger_memory_layout<'tcx>(
+    cx: &LateContext<'tcx>,
+    orig_expr: &'tcx Expr<'tcx>,
+) -> Option<(TyAndLayout<'tcx>, TyAndLayout<'tcx>, Expr<'tcx>)> {
+    let end_ty = cx.typeck_results().node_type(orig_expr.hir_id);
+
+    let ty::RawPtr(TypeAndMut { ty: inner_end_ty, mutbl: _ }) = end_ty.kind() else {
+        return None;
+    };
+
+    let (e, _) = peel_casts(cx, orig_expr);
+    let start_ty = cx.typeck_results().node_type(e.hir_id);
+
+    let ty::Ref(_, inner_start_ty, _) = start_ty.kind() else {
+        return None;
+    };
+
+    // try to find the underlying allocation
+    let e_alloc = cx.expr_or_init(e);
+    let e_alloc =
+        if let ExprKind::AddrOf(_, _, inner_expr) = e_alloc.kind { inner_expr } else { e_alloc };
+    let alloc_ty = cx.typeck_results().node_type(e_alloc.hir_id);
+
+    // if we do not find it we bail out, as this may not be UB
+    // see https://github.com/rust-lang/unsafe-code-guidelines/issues/256
+    if alloc_ty.is_any_ptr() {
+        return None;
+    }
+
+    let from_layout = cx.layout_of(*inner_start_ty).ok()?;
+    let alloc_layout = cx.layout_of(alloc_ty).ok()?;
+    let to_layout = cx.layout_of(*inner_end_ty).ok()?;
+
+    if to_layout.layout.size() > from_layout.layout.size()
+        && to_layout.layout.size() > alloc_layout.layout.size()
+    {
+        Some((from_layout, to_layout, *e_alloc))
+    } else {
+        None
+    }
+}
+
 fn peel_casts<'tcx>(cx: &LateContext<'tcx>, mut e: &'tcx Expr<'tcx>) -> (&'tcx Expr<'tcx>, bool) {
     let mut gone_trough_unsafe_cell_raw_get = false;
 

tests/ui/cast/cast-rfc0401.rs

@@ -81,14 +81,14 @@ fn main()
     assert_eq!(u as *const u16, p as *const u16);
 
     // ptr-ptr-cast (Length vtables)
-    let mut l : [u8; 2] = [0,1];
-    let w: *mut [u16; 2] = &mut l as *mut [u8; 2] as *mut _;
-    let w: *mut [u16] = unsafe {&mut *w};
-    let w_u8 : *const [u8] = w as *const [u8];
-    assert_eq!(unsafe{&*w_u8}, &l);
+    let mut l : [u16; 2] = [0,1];
+    let w: *mut [u8; 2] = &mut l as *mut [u16; 2] as *mut _;
+    let w: *mut [u8] = unsafe {&mut *w};
+    let w_u16 : *const [u16] = w as *const [u16];
+    assert_eq!(unsafe{&*w_u16}, &l);
 
     let s: *mut str = w as *mut str;
-    let l_via_str = unsafe{&*(s as *const [u8])};
+    let l_via_str = unsafe{&*(s as *const [u16])};
     assert_eq!(&l, l_via_str);
 
     // ptr-ptr-cast (Length vtables, check length is preserved)

tests/ui/lint/reference_casting.rs

@@ -30,7 +30,7 @@ unsafe fn ref_to_mut() {
     //~^ ERROR casting `&T` to `&mut T` is undefined behavior
     let _num = &mut *(num as *const i32).cast::<i32>().cast_mut().cast_const().cast_mut();
     //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-    let _num = &mut *(std::ptr::from_ref(static_u8()) as *mut i32);
+    let _num = &mut *(std::ptr::from_ref(static_u8()) as *mut i8);
     //~^ ERROR casting `&T` to `&mut T` is undefined behavior
     let _num = &mut *std::mem::transmute::<_, *mut i32>(num);
     //~^ ERROR casting `&T` to `&mut T` is undefined behavior
@@ -141,6 +141,109 @@ unsafe fn assign_to_ref() {
     }
 }
 
+#[repr(align(16))]
+struct I64(i64);
+
+#[repr(C)]
+struct Mat3<T> {
+    a: Vec3<T>,
+    b: Vec3<T>,
+    c: Vec3<T>,
+}
+
+#[repr(C)]
+struct Vec3<T>(T, T, T);
+
+unsafe fn bigger_layout() {
+    {
+        let num = &mut 3i32;
+
+        let _num = &*(num as *const i32 as *const i64);
+        //~^ ERROR casting references to a bigger memory layout
+        let _num = &mut *(num as *mut i32 as *mut i64);
+        //~^ ERROR casting references to a bigger memory layout
+        let _num = &mut *(num as *mut i32 as *mut I64);
+        //~^ ERROR casting references to a bigger memory layout
+        std::ptr::write(num as *mut i32 as *mut i64, 2);
+        //~^ ERROR casting references to a bigger memory layout
+
+        let _num = &mut *(num as *mut i32);
+    }
+
+    {
+        let num = &mut [0i32; 3];
+
+        let _num = &mut *(num as *mut _ as *mut [i64; 2]);
+        //~^ ERROR casting references to a bigger memory layout
+        std::ptr::write_unaligned(num as *mut _ as *mut [i32; 4], [0, 0, 1, 1]);
+        //~^ ERROR casting references to a bigger memory layout
+
+        let _num = &mut *(num as *mut _ as *mut [u32; 3]);
+        let _num = &mut *(num as *mut _ as *mut [u32; 2]);
+    }
+
+    {
+        let num = &mut [0i32; 3] as &mut [i32];
+
+        let _num = &mut *(num as *mut _ as *mut i128);
+        //~^ ERROR casting references to a bigger memory layout
+        let _num = &mut *(num as *mut _ as *mut [i64; 4]);
+        //~^ ERROR casting references to a bigger memory layout
+
+        let _num = &mut *(num as *mut _ as *mut [u32]);
+        let _num = &mut *(num as *mut _ as *mut [i16]);
+    }
+
+    {
+        let mat3 = Mat3 { a: Vec3(0i32, 0, 0), b: Vec3(0, 0, 0), c: Vec3(0, 0, 0) };
+
+        let _num = &mut *(&mat3 as *const _ as *mut [[i64; 3]; 3]);
+        //~^ ERROR casting `&T` to `&mut T`
+        //~^^ ERROR casting references to a bigger memory layout
+        let _num = &*(&mat3 as *const _ as *mut [[i64; 3]; 3]);
+        //~^ ERROR casting references to a bigger memory layout
+
+        let _num = &*(&mat3 as *const _ as *mut [[i32; 3]; 3]);
+    }
+
+    {
+        let mut l: [u8; 2] = [0,1];
+        let w: *mut [u16; 2] = &mut l as *mut [u8; 2] as *mut _;
+        let w: *mut [u16] = unsafe {&mut *w};
+        //~^ ERROR casting references to a bigger memory layout
+    }
+
+    {
+        fn foo() -> [i32; 1] { todo!() }
+
+        let num = foo();
+        let _num = &*(&num as *const i32 as *const i64);
+        //~^ ERROR casting references to a bigger memory layout
+        let _num = &*(&foo() as *const i32 as *const i64);
+        //~^ ERROR casting references to a bigger memory layout
+    }
+
+    {
+        fn bar(_a: &[i32; 2]) -> &[i32; 1] { todo!() }
+
+        let num = bar(&[0, 0]);
+        let _num = &*(num as *const i32 as *const i64);
+        let _num = &*(bar(&[0, 0]) as *const i32 as *const i64);
+    }
+
+    {
+        fn foi<T>() -> T { todo!() }
+
+        let num = foi::<i32>();
+        let _num = &*(&num as *const i32 as *const i64);
+        //~^ ERROR casting references to a bigger memory layout
+    }
+
+    unsafe fn from_ref(this: &i32) -> &i64 {
+        &*(this as *const i32 as *const i64)
+    }
+}
+
 const RAW_PTR: *mut u8 = 1 as *mut u8;
 unsafe fn no_warn() {
     let num = &3i32;