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change might_permit_raw_init to fully detect LLVM UB, but not more th…
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compiler/rustc_const_eval/src/util/might_permit_raw_init.rs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -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}; | ||
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| /// 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) | ||
| } | ||
| } | ||
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| /// 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, | ||
| ); | ||
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| let mut cx = InterpCx::new(tcx, rustc_span::DUMMY_SP, ParamEnv::reveal_all(), machine); | ||
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| 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"); | ||
| } | ||
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| let ot: OpTy<'_, _> = allocated.into(); | ||
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| // 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() | ||
| } | ||
|
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||
| /// 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) | ||
| } | ||
| } | ||
| }; | ||
|
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||
| // 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; | ||
| } | ||
|
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||
| // 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; | ||
| } | ||
| } | ||
|
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||
| // 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; | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
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||
| 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 | ||
| } |
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