change might_permit_raw_init to fully detect LLVM UB, but not more than that

compiler/rustc_const_eval/src/lib.rs

@@ -32,7 +32,6 @@ extern crate rustc_middle;
 pub mod const_eval;
 mod errors;
 pub mod interpret;
-mod might_permit_raw_init;
 pub mod transform;
 pub mod util;
 
@@ -61,7 +60,6 @@ pub fn provide(providers: &mut Providers) {
         const_eval::deref_mir_constant(tcx, param_env, value)
     };
     providers.permits_uninit_init =
-        |tcx, ty| might_permit_raw_init::might_permit_raw_init(tcx, ty, InitKind::Uninit);
-    providers.permits_zero_init =
-        |tcx, ty| might_permit_raw_init::might_permit_raw_init(tcx, ty, InitKind::Zero);
+        |tcx, ty| util::might_permit_raw_init(tcx, ty, InitKind::UninitMitigated0x01Fill);
+    providers.permits_zero_init = |tcx, ty| util::might_permit_raw_init(tcx, ty, InitKind::Zero);
 }

compiler/rustc_const_eval/src/might_permit_raw_init.rs

@@ -1,44 +0,0 @@
-use crate::const_eval::CompileTimeInterpreter;
-use crate::interpret::{InterpCx, MemoryKind, OpTy};
-use rustc_middle::ty::layout::LayoutCx;
-use rustc_middle::ty::{layout::TyAndLayout, ParamEnv, TyCtxt};
-use rustc_session::Limit;
-use rustc_target::abi::InitKind;
-
-pub fn might_permit_raw_init<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    ty: TyAndLayout<'tcx>,
-    kind: InitKind,
-) -> bool {
-    let strict = tcx.sess.opts.unstable_opts.strict_init_checks;
-
-    if strict {
-        let machine = CompileTimeInterpreter::new(
-            Limit::new(0),
-            /*can_access_statics:*/ false,
-            /*check_alignment:*/ true,
-        );
-
-        let mut cx = InterpCx::new(tcx, rustc_span::DUMMY_SP, ParamEnv::reveal_all(), machine);
-
-        let allocated = cx
-            .allocate(ty, MemoryKind::Machine(crate::const_eval::MemoryKind::Heap))
-            .expect("OOM: failed to allocate for uninit check");
-
-        if kind == InitKind::Zero {
-            cx.write_bytes_ptr(
-                allocated.ptr,
-                std::iter::repeat(0_u8).take(ty.layout.size().bytes_usize()),
-            )
-            .expect("failed to write bytes for zero valid check");
-        }
-
-        let ot: OpTy<'_, _> = allocated.into();
-
-        // Assume that if it failed, it's a validation failure.
-        cx.validate_operand(&ot).is_ok()
-    } else {
-        let layout_cx = LayoutCx { tcx, param_env: ParamEnv::reveal_all() };
-        ty.might_permit_raw_init(&layout_cx, kind)
-    }
-}

compiler/rustc_const_eval/src/util/might_permit_raw_init.rs

@@ -0,0 +1,151 @@
+use rustc_middle::ty::layout::{LayoutCx, LayoutOf, TyAndLayout};
+use rustc_middle::ty::{ParamEnv, TyCtxt};
+use rustc_session::Limit;
+use rustc_target::abi::{Abi, FieldsShape, InitKind, Scalar, Variants};
+
+use crate::const_eval::CompileTimeInterpreter;
+use crate::interpret::{InterpCx, MemoryKind, OpTy};
+
+/// Determines if this type permits "raw" initialization by just transmuting some memory into an
+/// instance of `T`.
+///
+/// `init_kind` indicates if the memory is zero-initialized or left uninitialized. We assume
+/// uninitialized memory is mitigated by filling it with 0x01, which reduces the chance of causing
+/// LLVM UB.
+///
+/// By default we check whether that operation would cause *LLVM UB*, i.e., whether the LLVM IR we
+/// generate has UB or not. This is a mitigation strategy, which is why we are okay with accepting
+/// Rust UB as long as there is no risk of miscompilations. The `strict_init_checks` can be set to
+/// do a full check against Rust UB instead (in which case we will also ignore the 0x01-filling and
+/// to the full uninit check).
+pub fn might_permit_raw_init<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: TyAndLayout<'tcx>,
+    kind: InitKind,
+) -> bool {
+    if tcx.sess.opts.unstable_opts.strict_init_checks {
+        might_permit_raw_init_strict(ty, tcx, kind)
+    } else {
+        let layout_cx = LayoutCx { tcx, param_env: ParamEnv::reveal_all() };
+        might_permit_raw_init_lax(ty, &layout_cx, kind)
+    }
+}
+
+/// Implements the 'strict' version of the `might_permit_raw_init` checks; see that function for
+/// details.
+fn might_permit_raw_init_strict<'tcx>(
+    ty: TyAndLayout<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    kind: InitKind,
+) -> bool {
+    let machine = CompileTimeInterpreter::new(
+        Limit::new(0),
+        /*can_access_statics:*/ false,
+        /*check_alignment:*/ true,
+    );
+
+    let mut cx = InterpCx::new(tcx, rustc_span::DUMMY_SP, ParamEnv::reveal_all(), machine);
+
+    let allocated = cx
+        .allocate(ty, MemoryKind::Machine(crate::const_eval::MemoryKind::Heap))
+        .expect("OOM: failed to allocate for uninit check");
+
+    if kind == InitKind::Zero {
+        cx.write_bytes_ptr(
+            allocated.ptr,
+            std::iter::repeat(0_u8).take(ty.layout.size().bytes_usize()),
+        )
+        .expect("failed to write bytes for zero valid check");
+    }
+
+    let ot: OpTy<'_, _> = allocated.into();
+
+    // Assume that if it failed, it's a validation failure.
+    // This does *not* actually check that references are dereferenceable, but since all types that
+    // require dereferenceability also require non-null, we don't actually get any false negatives
+    // due to this.
+    cx.validate_operand(&ot).is_ok()
+}
+
+/// Implements the 'lax' (default) version of the `might_permit_raw_init` checks; see that function for
+/// details.
+fn might_permit_raw_init_lax<'tcx>(
+    this: TyAndLayout<'tcx>,
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    init_kind: InitKind,
+) -> bool {
+    let scalar_allows_raw_init = move |s: Scalar| -> bool {
+        match init_kind {
+            InitKind::Zero => {
+                // The range must contain 0.
+                s.valid_range(cx).contains(0)
+            }
+            InitKind::UninitMitigated0x01Fill => {
+                // The range must include an 0x01-filled buffer.
+                let mut val: u128 = 0x01;
+                for _ in 1..s.size(cx).bytes() {
+                    // For sizes >1, repeat the 0x01.
+                    val = (val << 8) | 0x01;
+                }
+                s.valid_range(cx).contains(val)
+            }
+        }
+    };
+
+    // Check the ABI.
+    let valid = match this.abi {
+        Abi::Uninhabited => false, // definitely UB
+        Abi::Scalar(s) => scalar_allows_raw_init(s),
+        Abi::ScalarPair(s1, s2) => scalar_allows_raw_init(s1) && scalar_allows_raw_init(s2),
+        Abi::Vector { element: s, count } => count == 0 || scalar_allows_raw_init(s),
+        Abi::Aggregate { .. } => true, // Fields are checked below.
+    };
+    if !valid {
+        // This is definitely not okay.
+        return false;
+    }
+
+    // Special magic check for references and boxes (i.e., special pointer types).
+    if let Some(pointee) = this.ty.builtin_deref(false) {
+        let pointee = cx.layout_of(pointee.ty).expect("need to be able to compute layouts");
+        // We need to ensure that the LLVM attributes `aligned` and `dereferenceable(size)` are satisfied.
+        if pointee.align.abi.bytes() > 1 {
+            // 0x01-filling is not aligned.
+            return false;
+        }
+        if pointee.size.bytes() > 0 {
+            // A 'fake' integer pointer is not sufficiently dereferenceable.
+            return false;
+        }
+    }
+
+    // If we have not found an error yet, we need to recursively descend into fields.
+    match &this.fields {
+        FieldsShape::Primitive | FieldsShape::Union { .. } => {}
+        FieldsShape::Array { .. } => {
+            // Arrays never have scalar layout in LLVM, so if the array is not actually
+            // accessed, there is no LLVM UB -- therefore we can skip this.
+        }
+        FieldsShape::Arbitrary { offsets, .. } => {
+            for idx in 0..offsets.len() {
+                if !might_permit_raw_init_lax(this.field(cx, idx), cx, init_kind) {
+                    // We found a field that is unhappy with this kind of initialization.
+                    return false;
+                }
+            }
+        }
+    }
+
+    match &this.variants {
+        Variants::Single { .. } => {
+            // All fields of this single variant have already been checked above, there is nothing
+            // else to do.
+        }
+        Variants::Multiple { .. } => {
+            // We cannot tell LLVM anything about the details of this multi-variant layout, so
+            // invalid values "hidden" inside the variant cannot cause LLVM trouble.
+        }
+    }
+
+    true
+}

compiler/rustc_const_eval/src/util/mod.rs

@@ -3,8 +3,10 @@ mod alignment;
 mod call_kind;
 pub mod collect_writes;
 mod find_self_call;
+mod might_permit_raw_init;
 
 pub use self::aggregate::expand_aggregate;
 pub use self::alignment::is_disaligned;
 pub use self::call_kind::{call_kind, CallDesugaringKind, CallKind};
 pub use self::find_self_call::find_self_call;
+pub use self::might_permit_raw_init::might_permit_raw_init;