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Diffstat (limited to 'compiler/rustc_const_eval/src/util/check_validity_requirement.rs')
-rw-r--r-- | compiler/rustc_const_eval/src/util/check_validity_requirement.rs | 164 |
1 files changed, 164 insertions, 0 deletions
diff --git a/compiler/rustc_const_eval/src/util/check_validity_requirement.rs b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs new file mode 100644 index 000000000..23fcd22c5 --- /dev/null +++ b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs @@ -0,0 +1,164 @@ +use rustc_middle::ty::layout::{LayoutCx, LayoutError, LayoutOf, TyAndLayout, ValidityRequirement}; +use rustc_middle::ty::{ParamEnv, ParamEnvAnd, Ty, TyCtxt}; +use rustc_session::Limit; +use rustc_target::abi::{Abi, FieldsShape, Scalar, Variants}; + +use crate::const_eval::{CheckAlignment, 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 check_validity_requirement<'tcx>( + tcx: TyCtxt<'tcx>, + kind: ValidityRequirement, + param_env_and_ty: ParamEnvAnd<'tcx, Ty<'tcx>>, +) -> Result<bool, LayoutError<'tcx>> { + let layout = tcx.layout_of(param_env_and_ty)?; + + // There is nothing strict or lax about inhabitedness. + if kind == ValidityRequirement::Inhabited { + return Ok(!layout.abi.is_uninhabited()); + } + + if kind == ValidityRequirement::Uninit || tcx.sess.opts.unstable_opts.strict_init_checks { + might_permit_raw_init_strict(layout, tcx, kind) + } else { + let layout_cx = LayoutCx { tcx, param_env: param_env_and_ty.param_env }; + might_permit_raw_init_lax(layout, &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: ValidityRequirement, +) -> Result<bool, LayoutError<'tcx>> { + let machine = CompileTimeInterpreter::new( + Limit::new(0), + /*can_access_statics:*/ false, + CheckAlignment::Error, + ); + + 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 == ValidityRequirement::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. + Ok(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: ValidityRequirement, +) -> Result<bool, LayoutError<'tcx>> { + let scalar_allows_raw_init = move |s: Scalar| -> bool { + match init_kind { + ValidityRequirement::Inhabited => { + bug!("ValidityRequirement::Inhabited should have been handled above") + } + ValidityRequirement::Zero => { + // The range must contain 0. + s.valid_range(cx).contains(0) + } + ValidityRequirement::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) + } + ValidityRequirement::Uninit => { + bug!("ValidityRequirement::Uninit should have been handled above") + } + } + }; + + // 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 Ok(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)?; + // 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 Ok(false); + } + if pointee.size.bytes() > 0 { + // A 'fake' integer pointer is not sufficiently dereferenceable. + return Ok(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 Ok(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. + } + } + + Ok(true) +} |