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Rollup merge of #101061 - RalfJung:panic-on-uninit, r=oli-obk
panic-on-uninit: adjust checks to 0x01-filling Now that `mem::uninitiailized` actually fills memory with `0x01` (#99182), we can make it panic in a few less cases without risking a lot more UB -- which hopefully slightly improves compatibility with some old code, and which might increase the chance that we can check inside arrays in the future. We detect almost all of these with our lint, so authors of such code should still be warned -- but if this happens deep inside a dependency, the panic can be quite interruptive, so it might be better not to do it when there is no risk of LLVM UB. Therefore, adjust the `might_permit_raw_init` logic to care primarily about LLVM UB. To my knowledge, it actually covers all cases of LLVM UB now. Fixes #66151 Cc ``@5225225``
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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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