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Investigate unsound mapped RwLock write guard downgrading

Abierto
  • ¿Algo no está bien?
#362 3 comentarios 0 reacciones 0 asignados Ver en GitHub

Los mantenedores suelen responder en 1 día

@orthur2 ya está trabajando en esto.

Desde el 11/10/2026.

  • #365 de @orthur2 — abierto

Evaluación

Dificultad
5/5
Tiempo estimado
Más de una semana
Aptitud para principiantes
25/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
rust
Área
backend, testing-qa

Línea de trabajo

Start by reviewing MappedRwLockWriteGuard::downgrade and OwnedMappedRwLockWriteGuard::downgrade, along with the mapped-guard history in #65, #61, #62, and the closed proposal in #358. Run the safe reproducer under Miri and compare it with downgrading the original write guard before projecting a shared reference. Done means verifying the soundness concern and reaching an agreed API contract and solution.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Summary

@orthur2 @jeffoodchain Could you help verify the soundness issue below and discuss the appropriate API contract and solution?

MappedRwLockWriteGuard::downgrade and OwnedMappedRwLockWriteGuard::downgrade retain a pointer obtained from an arbitrary mutable projection while allowing other readers of the original value. A safe-code reproducer triggers undefined behavior under Miri. This issue tracks verification and the solution; the attempted fix in #358 has been closed pending discussion.

Reproducer

The following entirely safe program reproduces the problem on commit 44fc2d8:

use asyncband::rwlock::{RwLock, RwLockWriteGuard};
use std::sync::Mutex;

fn main() {
    let lock = RwLock::new(Mutex::new(0u8));
    let write = RwLockWriteGuard::map(lock.try_write().unwrap(), |inner| {
        inner.get_mut().unwrap()
    });
    let read = write.downgrade();
    let shared = &*read;
    let outer = lock.try_read().unwrap();
    *outer.lock().unwrap() = 1;
    println!("{shared}");
}

Miri reports that the later mutable access through outer.lock() invalidates the shared reference. The corresponding owned mapped guard reproducer also fails.

Soundness concern

The mutable projection &mut T -> &mut U can reach interior-mutable storage using exclusive access, as Mutex::get_mut does here. After downgrading, another reader of the original T can mutate that storage through its interior-mutability API while the mapped guard still exposes &U.

Both Mutex<u8> and u8 are Send + Sync, so adding those bounds alone does not address this example. The expected behavior is that safe guard operations cannot permit mutation that invalidates a live shared reference.

History and possible solutions

  • #65 introduced both mapped downgrade methods; they first shipped in mea 0.4.2 on 2025-08-30 and remain present in v0.7.3. This is not new in 0.7.3.
  • #62 tracks the original write-lock downgrade request; #61 contains the preceding mapped-guard work and follow-up discussion.
  • #358 contains the reproduction analysis and a proposed removal, but is closed pending agreement on the solution.

Keeping all exclusive permits appears sound but continues blocking readers, changing the downgrade contract. Downgrading the original write guard before applying a shared projection is another option: RwLockReadGuard::map(write.downgrade(), |root| &root.field) retains the lock throughout. A convenience API accepting a projection from the original &T could express the same boundary, but cannot simply reuse an arbitrary mutable projection.

Could you confirm whether the reproducer and reasoning establish that these APIs are unsound, and help decide whether to remove mapped downgrading or support it under a different contract?

Lenguaje dominante
Rust
Estrellas
277
Forks
44
Merge medio
1 d 9 h
PR fusionados (30 d)
39

Preparar el entorno

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Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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