Merge pull request #277 from Liamolucko/fix-peripheral-ub

extras: Fix UB in `Peripheral`

4 files changed, 181 insertions, 33 deletions

diff --git a/embassy-extras/src/peripheral.rs b/embassy-extras/src/peripheral.rs
index 68972c543..92512a0f6 100644
--- a/embassy-extras/src/peripheral.rs
+++ b/embassy-extras/src/peripheral.rs
@@ -2,9 +2,19 @@ use core::cell::UnsafeCell;
 use core::marker::{PhantomData, PhantomPinned};
 use core::pin::Pin;
 
+use cortex_m::peripheral::scb::VectActive;
+use cortex_m::peripheral::{NVIC, SCB};
 use embassy::interrupt::{Interrupt, InterruptExt};
 
-pub trait PeripheralState {
+/// A type which can be used as state with `PeripheralMutex`.
+///
+/// It needs to be `Send` because `&mut` references are sent back and forth between the 'thread' which owns the `PeripheralMutex` and the interrupt,
+/// and `&mut T` is only `Send` where `T: Send`.
+///
+/// It also requires `'static` to be used safely with `PeripheralMutex::register_interrupt`,
+/// because although `Pin` guarantees that the memory of the state won't be invalidated,
+/// it doesn't guarantee that the lifetime will last.
+pub trait PeripheralState: Send {
     type Interrupt: Interrupt;
     fn on_interrupt(&mut self);
 }
@@ -19,8 +29,51 @@ pub struct PeripheralMutex<S: PeripheralState> {
     _pinned: PhantomPinned,
 }
 
+/// Whether `irq` can be preempted by the current interrupt.
+pub(crate) fn can_be_preempted(irq: &impl Interrupt) -> bool {
+    match SCB::vect_active() {
+        // Thread mode can't preempt anything.
+        VectActive::ThreadMode => false,
+        // Exceptions don't always preempt interrupts,
+        // but there isn't much of a good reason to be keeping a `PeripheralMutex` in an exception anyway.
+        VectActive::Exception(_) => true,
+        VectActive::Interrupt { irqn } => {
+            #[derive(Clone, Copy)]
+            struct NrWrap(u16);
+            unsafe impl cortex_m::interrupt::InterruptNumber for NrWrap {
+                fn number(self) -> u16 {
+                    self.0
+                }
+            }
+            NVIC::get_priority(NrWrap(irqn.into())) < irq.get_priority().into()
+        }
+    }
+}
+
+impl<S: PeripheralState + 'static> PeripheralMutex<S> {
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// This requires this `PeripheralMutex`'s `PeripheralState` to live for `'static`,
+    /// because `Pin` only guarantees that it's memory won't be repurposed,
+    /// not that it's lifetime will last.
+    ///
+    /// To use non-`'static` `PeripheralState`, use the unsafe `register_interrupt_unchecked`.
+    ///
+    /// Note: `'static` doesn't mean it _has_ to live for the entire program, like an `&'static T`;
+    /// it just means it _can_ live for the entire program - for example, `u8` lives for `'static`.
+    pub fn register_interrupt(self: Pin<&mut Self>) {
+        // SAFETY: `S: 'static`, so there's no way it's lifetime can expire.
+        unsafe { self.register_interrupt_unchecked() }
+    }
+}
+
 impl<S: PeripheralState> PeripheralMutex<S> {
+    /// Create a new `PeripheralMutex` wrapping `irq`, with the initial state `state`.
     pub fn new(state: S, irq: S::Interrupt) -> Self {
+        if can_be_preempted(&irq) {
+            panic!("`PeripheralMutex` cannot be created in an interrupt with higher priority than the interrupt it wraps");
+        }
+
         Self {
             irq,
             irq_setup_done: false,
@@ -31,8 +84,18 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         }
     }
 
-    pub fn register_interrupt(self: Pin<&mut Self>) {
-        let this = unsafe { self.get_unchecked_mut() };
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// # Safety
+    /// The lifetime of any data in `PeripheralState` that is accessed by the interrupt handler
+    /// must not end without `Drop` being called on this `PeripheralMutex`.
+    ///
+    /// This can be accomplished by either not accessing any data with a lifetime in `on_interrupt`,
+    /// or making sure that nothing like `mem::forget` is used on the `PeripheralMutex`.
+
+    // TODO: this name isn't the best.
+    pub unsafe fn register_interrupt_unchecked(self: Pin<&mut Self>) {
+        let this = self.get_unchecked_mut();
         if this.irq_setup_done {
             return;
         }
@@ -40,7 +103,9 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         this.irq.disable();
         this.irq.set_handler(|p| {
             // Safety: it's OK to get a &mut to the state, since
-            // - We're in the IRQ, no one else can't preempt us
+            // - We checked that the thread owning the `PeripheralMutex` can't preempt us in `new`.
+            //   Interrupts' priorities can only be changed with raw embassy `Interrupts`,
+            //   which can't safely store a `PeripheralMutex` across invocations.
             // - We can't have preempted a with() call because the irq is disabled during it.
             let state = unsafe { &mut *(p as *mut S) };
             state.on_interrupt();
@@ -52,19 +117,39 @@ impl<S: PeripheralState> PeripheralMutex<S> {
         this.irq_setup_done = true;
     }
 
-    pub fn with<R>(self: Pin<&mut Self>, f: impl FnOnce(&mut S, &mut S::Interrupt) -> R) -> R {
+    pub fn with<R>(self: Pin<&mut Self>, f: impl FnOnce(&mut S) -> R) -> R {
         let this = unsafe { self.get_unchecked_mut() };
 
         this.irq.disable();
 
         // Safety: it's OK to get a &mut to the state, since the irq is disabled.
         let state = unsafe { &mut *this.state.get() };
-        let r = f(state, &mut this.irq);
+        let r = f(state);
 
         this.irq.enable();
 
         r
     }
+
+    /// Returns whether the wrapped interrupt is currently in a pending state.
+    pub fn is_pending(&self) -> bool {
+        self.irq.is_pending()
+    }
+
+    /// Forces the wrapped interrupt into a pending state.
+    pub fn pend(&self) {
+        self.irq.pend()
+    }
+
+    /// Forces the wrapped interrupt out of a pending state.
+    pub fn unpend(&self) {
+        self.irq.unpend()
+    }
+
+    /// Gets the priority of the wrapped interrupt.
+    pub fn priority(&self) -> <S::Interrupt as Interrupt>::Priority {
+        self.irq.get_priority()
+    }
 }
 
 impl<S: PeripheralState> Drop for PeripheralMutex<S> {
diff --git a/embassy-extras/src/peripheral_shared.rs b/embassy-extras/src/peripheral_shared.rs
index c62113396..71d746341 100644
--- a/embassy-extras/src/peripheral_shared.rs
+++ b/embassy-extras/src/peripheral_shared.rs
@@ -1,16 +1,24 @@
-use core::cell::UnsafeCell;
 use core::marker::{PhantomData, PhantomPinned};
 use core::pin::Pin;
 
 use embassy::interrupt::{Interrupt, InterruptExt};
 
-pub trait PeripheralState {
+use crate::peripheral::can_be_preempted;
+
+/// A type which can be used as state with `Peripheral`.
+///
+/// It needs to be `Sync` because references are shared between the 'thread' which owns the `Peripheral` and the interrupt.
+///
+/// It also requires `'static` to be used safely with `Peripheral::register_interrupt`,
+/// because although `Pin` guarantees that the memory of the state won't be invalidated,
+/// it doesn't guarantee that the lifetime will last.
+pub trait PeripheralState: Sync {
     type Interrupt: Interrupt;
     fn on_interrupt(&self);
 }
 
 pub struct Peripheral<S: PeripheralState> {
-    state: UnsafeCell<S>,
+    state: S,
 
     irq_setup_done: bool,
     irq: S::Interrupt,
@@ -19,26 +27,58 @@ pub struct Peripheral<S: PeripheralState> {
     _pinned: PhantomPinned,
 }
 
+impl<S: PeripheralState + 'static> Peripheral<S> {
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// This requires this `Peripheral`'s `PeripheralState` to live for `'static`,
+    /// because `Pin` only guarantees that it's memory won't be repurposed,
+    /// not that it's lifetime will last.
+    ///
+    /// To use non-`'static` `PeripheralState`, use the unsafe `register_interrupt_unchecked`.
+    ///
+    /// Note: `'static` doesn't mean it _has_ to live for the entire program, like an `&'static T`;
+    /// it just means it _can_ live for the entire program - for example, `u8` lives for `'static`.
+    pub fn register_interrupt(self: Pin<&mut Self>) {
+        // SAFETY: `S: 'static`, so there's no way it's lifetime can expire.
+        unsafe { self.register_interrupt_unchecked() }
+    }
+}
+
 impl<S: PeripheralState> Peripheral<S> {
     pub fn new(irq: S::Interrupt, state: S) -> Self {
+        if can_be_preempted(&irq) {
+            panic!("`Peripheral` cannot be created in an interrupt with higher priority than the interrupt it wraps");
+        }
+
         Self {
             irq,
             irq_setup_done: false,
 
-            state: UnsafeCell::new(state),
+            state,
             _not_send: PhantomData,
             _pinned: PhantomPinned,
         }
     }
 
-    pub fn register_interrupt(self: Pin<&mut Self>) {
-        let this = unsafe { self.get_unchecked_mut() };
+    /// Registers `on_interrupt` as the wrapped interrupt's interrupt handler and enables it.
+    ///
+    /// # Safety
+    /// The lifetime of any data in `PeripheralState` that is accessed by the interrupt handler
+    /// must not end without `Drop` being called on this `Peripheral`.
+    ///
+    /// This can be accomplished by either not accessing any data with a lifetime in `on_interrupt`,
+    /// or making sure that nothing like `mem::forget` is used on the `Peripheral`.
+    pub unsafe fn register_interrupt_unchecked(self: Pin<&mut Self>) {
+        let this = self.get_unchecked_mut();
         if this.irq_setup_done {
             return;
         }
 
         this.irq.disable();
         this.irq.set_handler(|p| {
+            // The state can't have been dropped, otherwise the interrupt would have been disabled.
+            // We checked in `new` that the thread owning the `Peripheral` can't preempt the interrupt,
+            // so someone can't have preempted us before this point and dropped the `Peripheral`.
             let state = unsafe { &*(p as *const S) };
             state.on_interrupt();
         });
@@ -50,8 +90,27 @@ impl<S: PeripheralState> Peripheral<S> {
     }
 
     pub fn state(self: Pin<&mut Self>) -> &S {
-        let this = unsafe { self.get_unchecked_mut() };
-        unsafe { &*this.state.get() }
+        &self.into_ref().get_ref().state
+    }
+
+    /// Returns whether the wrapped interrupt is currently in a pending state.
+    pub fn is_pending(&self) -> bool {
+        self.irq.is_pending()
+    }
+
+    /// Forces the wrapped interrupt into a pending state.
+    pub fn pend(&self) {
+        self.irq.pend()
+    }
+
+    /// Forces the wrapped interrupt out of a pending state.
+    pub fn unpend(&self) {
+        self.irq.unpend()
+    }
+
+    /// Gets the priority of the wrapped interrupt.
+    pub fn priority(&self) -> <S::Interrupt as Interrupt>::Priority {
+        self.irq.get_priority()
     }
 }
 
diff --git a/embassy-extras/src/usb/mod.rs b/embassy-extras/src/usb/mod.rs
index 182cd87d0..1fb501d7f 100644
--- a/embassy-extras/src/usb/mod.rs
+++ b/embassy-extras/src/usb/mod.rs
@@ -14,7 +14,7 @@ use embassy::interrupt::Interrupt;
 use usb_serial::{ReadInterface, UsbSerial, WriteInterface};
 
 /// Marker trait to mark an interrupt to be used with the [`Usb`] abstraction.
-pub unsafe trait USBInterrupt: Interrupt {}
+pub unsafe trait USBInterrupt: Interrupt + Send {}
 
 pub(crate) struct State<'bus, B, T, I>
 where
@@ -55,13 +55,17 @@ where
         }
     }
 
-    pub fn start(self: Pin<&mut Self>) {
-        let this = unsafe { self.get_unchecked_mut() };
+    /// # Safety
+    /// The `UsbDevice` passed to `Self::new` must not be dropped without calling `Drop` on this `Usb` first.
+    pub unsafe fn start(self: Pin<&mut Self>) {
+        let this = self.get_unchecked_mut();
         let mut mutex = this.inner.borrow_mut();
-        let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
+        let mutex = Pin::new_unchecked(&mut *mutex);
 
         // Use inner to register the irq
-        mutex.register_interrupt();
+        // SAFETY: the safety contract of this function makes sure the `UsbDevice` won't be invalidated
+        // without the `PeripheralMutex` being dropped.
+        mutex.register_interrupt_unchecked();
     }
 }
 
@@ -137,7 +141,7 @@ where
     }
 }
 
-pub trait ClassSet<B: UsbBus> {
+pub trait ClassSet<B: UsbBus>: Send {
     fn poll_all(&mut self, device: &mut UsbDevice<'_, B>) -> bool;
 }
 
@@ -173,8 +177,8 @@ pub struct Index1;
 
 impl<B, C1> ClassSet<B> for ClassSet1<B, C1>
 where
-    B: UsbBus,
-    C1: UsbClass<B>,
+    B: UsbBus + Send,
+    C1: UsbClass<B> + Send,
 {
     fn poll_all(&mut self, device: &mut UsbDevice<'_, B>) -> bool {
         device.poll(&mut [&mut self.class])
@@ -183,9 +187,9 @@ where
 
 impl<B, C1, C2> ClassSet<B> for ClassSet2<B, C1, C2>
 where
-    B: UsbBus,
-    C1: UsbClass<B>,
-    C2: UsbClass<B>,
+    B: UsbBus + Send,
+    C1: UsbClass<B> + Send,
+    C2: UsbClass<B> + Send,
 {
     fn poll_all(&mut self, device: &mut UsbDevice<'_, B>) -> bool {
         device.poll(&mut [&mut self.class1, &mut self.class2])
@@ -194,8 +198,8 @@ where
 
 impl<B, C1> IntoClassSet<B, ClassSet1<B, C1>> for C1
 where
-    B: UsbBus,
-    C1: UsbClass<B>,
+    B: UsbBus + Send,
+    C1: UsbClass<B> + Send,
 {
     fn into_class_set(self) -> ClassSet1<B, C1> {
         ClassSet1 {
@@ -207,9 +211,9 @@ where
 
 impl<B, C1, C2> IntoClassSet<B, ClassSet2<B, C1, C2>> for (C1, C2)
 where
-    B: UsbBus,
-    C1: UsbClass<B>,
-    C2: UsbClass<B>,
+    B: UsbBus + Send,
+    C1: UsbClass<B> + Send,
+    C2: UsbClass<B> + Send,
 {
     fn into_class_set(self) -> ClassSet2<B, C1, C2> {
         ClassSet2 {
diff --git a/embassy-extras/src/usb/usb_serial.rs b/embassy-extras/src/usb/usb_serial.rs
index 9cbfb2da4..a229b2000 100644
--- a/embassy-extras/src/usb/usb_serial.rs
+++ b/embassy-extras/src/usb/usb_serial.rs
@@ -55,7 +55,7 @@ where
         let this = self.get_mut();
         let mut mutex = this.inner.borrow_mut();
         let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
-        mutex.with(|state, _irq| {
+        mutex.with(|state| {
             let serial = state.classes.get_serial();
             let serial = Pin::new(serial);
 
@@ -77,7 +77,7 @@ where
         let this = self.get_mut();
         let mut mutex = this.inner.borrow_mut();
         let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
-        mutex.with(|state, _irq| {
+        mutex.with(|state| {
             let serial = state.classes.get_serial();
             let serial = Pin::new(serial);
 
@@ -101,7 +101,7 @@ where
         let this = self.get_mut();
         let mut mutex = this.inner.borrow_mut();
         let mutex = unsafe { Pin::new_unchecked(&mut *mutex) };
-        mutex.with(|state, _irq| {
+        mutex.with(|state| {
             let serial = state.classes.get_serial();
             let serial = Pin::new(serial);