Make BiLock strict-provenance compatible (#2716)

Author: taiki-e

futures-util/src/lock/bilock.rs

@@ -3,11 +3,11 @@
 use alloc::boxed::Box;
 use alloc::sync::Arc;
 use core::cell::UnsafeCell;
-use core::fmt;
 use core::ops::{Deref, DerefMut};
 use core::pin::Pin;
-use core::sync::atomic::AtomicUsize;
+use core::sync::atomic::AtomicPtr;
 use core::sync::atomic::Ordering::SeqCst;
+use core::{fmt, ptr};
 #[cfg(feature = "bilock")]
 use futures_core::future::Future;
 use futures_core::task::{Context, Poll, Waker};
@@ -41,7 +41,7 @@ pub struct BiLock<T> {
 
 #[derive(Debug)]
 struct Inner<T> {
-    state: AtomicUsize,
+    state: AtomicPtr<Waker>,
     value: Option<UnsafeCell<T>>,
 }
 
@@ -61,7 +61,10 @@ impl<T> BiLock<T> {
     /// Similarly, reuniting the lock and extracting the inner value is only
     /// possible when `T` is `Unpin`.
     pub fn new(t: T) -> (Self, Self) {
-        let arc = Arc::new(Inner { state: AtomicUsize::new(0), value: Some(UnsafeCell::new(t)) });
+        let arc = Arc::new(Inner {
+            state: AtomicPtr::new(ptr::null_mut()),
+            value: Some(UnsafeCell::new(t)),
+        });
 
         (Self { arc: arc.clone() }, Self { arc })
     }
@@ -87,7 +90,8 @@ impl<T> BiLock<T> {
     pub fn poll_lock(&self, cx: &mut Context<'_>) -> Poll<BiLockGuard<'_, T>> {
         let mut waker = None;
         loop {
-            match self.arc.state.swap(1, SeqCst) {
+            let n = self.arc.state.swap(invalid_ptr(1), SeqCst);
+            match n as usize {
                 // Woohoo, we grabbed the lock!
                 0 => return Poll::Ready(BiLockGuard { bilock: self }),
 
@@ -96,27 +100,27 @@ impl<T> BiLock<T> {
 
                 // A task was previously blocked on this lock, likely our task,
                 // so we need to update that task.
-                n => unsafe {
-                    let mut prev = Box::from_raw(n as *mut Waker);
+                _ => unsafe {
+                    let mut prev = Box::from_raw(n);
                     *prev = cx.waker().clone();
                     waker = Some(prev);
                 },
             }
 
             // type ascription for safety's sake!
             let me: Box<Waker> = waker.take().unwrap_or_else(|| Box::new(cx.waker().clone()));
-            let me = Box::into_raw(me) as usize;
+            let me = Box::into_raw(me);
 
-            match self.arc.state.compare_exchange(1, me, SeqCst, SeqCst) {
+            match self.arc.state.compare_exchange(invalid_ptr(1), me, SeqCst, SeqCst) {
                 // The lock is still locked, but we've now parked ourselves, so
                 // just report that we're scheduled to receive a notification.
                 Ok(_) => return Poll::Pending,
 
                 // Oops, looks like the lock was unlocked after our swap above
                 // and before the compare_exchange. Deallocate what we just
                 // allocated and go through the loop again.
-                Err(0) => unsafe {
-                    waker = Some(Box::from_raw(me as *mut Waker));
+                Err(n) if n.is_null() => unsafe {
+                    waker = Some(Box::from_raw(me));
                 },
 
                 // The top of this loop set the previous state to 1, so if we
@@ -125,7 +129,7 @@ impl<T> BiLock<T> {
                 // but we're trying to acquire the lock and there's only one
                 // other reference of the lock, so it should be impossible for
                 // that task to ever block itself.
-                Err(n) => panic!("invalid state: {}", n),
+                Err(n) => panic!("invalid state: {}", n as usize),
             }
         }
     }
@@ -164,7 +168,8 @@ impl<T> BiLock<T> {
     }
 
     fn unlock(&self) {
-        match self.arc.state.swap(0, SeqCst) {
+        let n = self.arc.state.swap(ptr::null_mut(), SeqCst);
+        match n as usize {
             // we've locked the lock, shouldn't be possible for us to see an
             // unlocked lock.
             0 => panic!("invalid unlocked state"),
@@ -174,8 +179,8 @@ impl<T> BiLock<T> {
 
             // Another task has parked themselves on this lock, let's wake them
             // up as its now their turn.
-            n => unsafe {
-                Box::from_raw(n as *mut Waker).wake();
+            _ => unsafe {
+                Box::from_raw(n).wake();
             },
         }
     }
@@ -189,7 +194,7 @@ impl<T: Unpin> Inner<T> {
 
 impl<T> Drop for Inner<T> {
     fn drop(&mut self) {
-        assert_eq!(self.state.load(SeqCst), 0);
+        assert!(self.state.load(SeqCst).is_null());
     }
 }
 
@@ -277,3 +282,12 @@ impl<'a, T> Future for BiLockAcquire<'a, T> {
         self.bilock.poll_lock(cx)
     }
 }
+
+// Based on core::ptr::invalid_mut. Equivalent to `addr as *mut T`, but is strict-provenance compatible.
+#[allow(clippy::useless_transmute)]
+#[inline]
+fn invalid_ptr<T>(addr: usize) -> *mut T {
+    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
+    // pointer).
+    unsafe { core::mem::transmute(addr) }
+}