Skip to content

Commit

Permalink
Auto merge of rust-lang#118324 - RalfJung:ctfe-read-only-pointers, r=…
Browse files Browse the repository at this point in the history
…saethlin

compile-time evaluation: detect writes through immutable pointers

This has two motivations:
- it unblocks rust-lang#116745 (and therefore takes a big step towards `const_mut_refs` stabilization), because we can now detect if the memory that we find in `const` can be interned as "immutable"
- it would detect the UB that was uncovered in rust-lang#117905, which was caused by accidental stabilization of `copy` functions in `const` that can only be called with UB

When UB is detected, we emit a future-compat warn-by-default lint. This is not a breaking change, so completely in line with [the const-UB RFC](https://rust-lang.github.io/rfcs/3016-const-ub.html), meaning we don't need t-lang FCP here. I made the lint immediately show up for dependencies since it is nearly impossible to even trigger this lint without `const_mut_refs` -- the accidentally stabilized `copy` functions are the only way this can happen, so the crates that popped up in rust-lang#117905 are the only causes of such UB (in the code that crater covers), and the three cases of UB that we know about have all been fixed in their respective crates already.

The way this is implemented is by making use of the fact that our interpreter is already generic over the notion of provenance. For CTFE we now use the new `CtfeProvenance` type which is conceptually an `AllocId` plus a boolean `immutable` flag (but packed for a more efficient representation). This means we can mark a pointer as immutable when it is created as a shared reference. The flag will be propagated to all pointers derived from this one. We can then check the immutable flag on each write to reject writes through immutable pointers.

I just hope perf works out.
  • Loading branch information
bors committed Dec 7, 2023
2 parents 3cfff05 + cb32ffd commit 17f0dd5
Show file tree
Hide file tree
Showing 2 changed files with 5 additions and 3 deletions.
3 changes: 2 additions & 1 deletion src/common.rs
Original file line number Diff line number Diff line change
Expand Up @@ -199,7 +199,8 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
}
}
Scalar::Ptr(ptr, _size) => {
let (alloc_id, offset) = ptr.into_parts();
let (prov, offset) = ptr.into_parts(); // we know the `offset` is relative
let alloc_id = prov.alloc_id();
let base_addr =
match self.tcx.global_alloc(alloc_id) {
GlobalAlloc::Memory(alloc) => {
Expand Down
5 changes: 3 additions & 2 deletions src/consts.rs
Original file line number Diff line number Diff line change
Expand Up @@ -285,7 +285,8 @@ pub fn const_alloc_to_gcc<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, alloc: ConstAl
let pointer_size = dl.pointer_size.bytes() as usize;

let mut next_offset = 0;
for &(offset, alloc_id) in alloc.provenance().ptrs().iter() {
for &(offset, prov) in alloc.provenance().ptrs().iter() {
let alloc_id = prov.alloc_id();
let offset = offset.bytes();
assert_eq!(offset as usize as u64, offset);
let offset = offset as usize;
Expand Down Expand Up @@ -313,7 +314,7 @@ pub fn const_alloc_to_gcc<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, alloc: ConstAl

llvals.push(cx.scalar_to_backend(
InterpScalar::from_pointer(
interpret::Pointer::new(alloc_id, Size::from_bytes(ptr_offset)),
interpret::Pointer::new(prov, Size::from_bytes(ptr_offset)),
&cx.tcx,
),
abi::Scalar::Initialized { value: Primitive::Pointer(address_space), valid_range: WrappingRange::full(dl.pointer_size) },
Expand Down

0 comments on commit 17f0dd5

Please sign in to comment.