Skip to content

Enforce minimum alignment on entries to fix tagged pointer unsoundness #74

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
ibraheemdev merged 3 commits into from
Jun 24, 2025
Merged

Enforce minimum alignment on entries to fix tagged pointer unsoundness #74

Show file tree
Hide file tree
Changes from 2 commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
df3664c
Enforce minimum alignment on entries to fix tagged pointer unsoundness
drakedevel Jun 22, 2025
fdb0aad
Fix Rust 1.72 compatibility
drakedevel Jun 23, 2025
2851f09
Move assertion const to Unpack trait
drakedevel Jun 24, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
5 changes: 3 additions & 2 deletions src/raw/mod.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -118,8 +118,9 @@ pub enum RawInsertResult<'g, K, V> {
},
}

// An entry in the hash-table.
#[repr(C)]
// An entry in the hash-table. We force a minimum of 8-byte alignment because
// we store entry flags in the low 3 bits of pointers to this type.
#[repr(C, align(8))]
pub struct Entry<K, V> {
/// The key for this entry.
pub key: K,
Expand Down
12 changes: 12 additions & 0 deletions src/raw/utils/tagged.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
use std::mem::align_of;
use std::sync::atomic::{AtomicPtr, Ordering};

// Polyfill for the unstable strict-provenance APIs.
Expand All @@ -17,6 +18,16 @@ pub trait Unpack {
const MASK: usize;
}

// This function does nothing, but will fail to compile if T doesn't have an alignment
// that guarantees all valid pointers have zero in the bits excluded by T::MASK.
const fn static_assert_align_of<T: Unpack>() {
struct Dummy<T>(T);
impl<T: Unpack> Dummy<T> {
const ASSERT: () = assert!(align_of::<T>() > !T::MASK);
Copy link
Copy Markdown
Owner

@ibraheemdev ibraheemdev Jun 24, 2025 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Just a bit, can we put this const in the Unpack trait to avoid the dummy struct?

Copy link
Copy Markdown
Contributor Author

@drakedevel drakedevel Jun 24, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Sure, just pushed.

I didn't do that originally since a trait can only supply a "default" implementation, so an impl could in theory override it with e.g. const ASSERT_ALIGNMENT: () = (); and defeat the check. But I don't think that's going to happen by accident!

}
Dummy::<T>::ASSERT
}

unsafe impl<T> StrictProvenance<T> for *mut T {
#[inline(always)]
fn addr(self) -> usize {
Expand All @@ -33,6 +44,7 @@ unsafe impl<T> StrictProvenance<T> for *mut T {
where
T: Unpack,
{
static_assert_align_of::<T>();
Tagged {
raw: self,
ptr: self.map_addr(|addr| addr & T::MASK),
Expand Down