Allow volatile access to non-Rust memory, including address 0

library/core/src/ptr/mod.rs

@@ -28,7 +28,8 @@
 //!   undefined behavior to perform two concurrent accesses to the same location from different
 //!   threads unless both accesses only read from memory. Notice that this explicitly
 //!   includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
-//!   be used for inter-thread synchronization.
+//!   be used for inter-thread synchronization, regardless of whether it is acting on
+//!   Rust memory or not.
 //! * The result of casting a reference to a pointer is valid for as long as the
 //!   underlying object is live and no reference (just raw pointers) is used to
 //!   access the same memory. That is, reference and pointer accesses cannot be
@@ -112,6 +113,10 @@
 //! fully contiguous (i.e., has no "holes"), there is no guarantee that this
 //! will not change in the future.
 //!
+//! Allocated objects must behave like "normal" memory: in particular, reads must not have
+//! side-effects, and writes must become visible to other threads using the usual synchronization
+//! primitives.
+//!
 //! For any allocated object with `base` address, `size`, and a set of
 //! `addresses`, the following are guaranteed:
 //! - For all addresses `a` in `addresses`, `a` is in the range `base .. (base +
@@ -1744,54 +1749,63 @@ pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
     }
 }
 
-/// Performs a volatile read of the value from `src` without moving it. This
-/// leaves the memory in `src` unchanged.
-///
-/// Volatile operations are intended to act on I/O memory, and are guaranteed
-/// to not be elided or reordered by the compiler across other volatile
-/// operations.
-///
-/// # Notes
-///
-/// Rust does not currently have a rigorously and formally defined memory model,
-/// so the precise semantics of what "volatile" means here is subject to change
-/// over time. That being said, the semantics will almost always end up pretty
-/// similar to [C11's definition of volatile][c11].
-///
-/// The compiler shouldn't change the relative order or number of volatile
-/// memory operations. However, volatile memory operations on zero-sized types
-/// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
-/// and may be ignored.
+/// Performs a volatile read of the value from `src` without moving it.
+///
+/// Rust does not currently have a rigorously and formally defined memory model, so the precise
+/// semantics of what "volatile" means here is subject to change over time. That being said, the
+/// semantics will almost always end up pretty similar to [C11's definition of volatile][c11].
+///
+/// Volatile operations are intended to act on I/O memory. As such, they are considered externally
+/// observable events (just like syscalls), and are guaranteed to not be elided or reordered by the
+/// compiler across other externally observable events. With this in mind, there are two cases of
+/// usage that need to be distinguished:
+///
+/// - When a volatile operation is used for memory inside an [allocation], it behaves exactly like
+///   [`read`], except for the additional guarantee that it won't be elided or reordered (see
+///   above). This implies that the operation will actually access memory and not e.g. be lowered to
+///   a register access or stack pop. Other than that, all the usual rules for memory accesses
+///   apply. In particular, just like in C, whether an operation is volatile has no bearing
+///   whatsoever on questions involving concurrent access from multiple threads. Volatile accesses
+///   behave exactly like non-atomic accesses in that regard.
+///
+/// - Volatile operations, however, may also be used access memory that is _outside_ of any Rust
+///   allocation. The main use-case is CPU and peripheral registers that must be accessed via an I/O
+///   memory mapping, most commonly at fixed addresses reserved by the hardware. These often have
+///   special semantics associated to their manipulation, and cannot be used as general purpose
+///   memory. Here, any address value is possible, including 0 and [`usize::MAX`], so long as the
+///   semantics of such a read are well-defined by the target hardware. The access must not trap. It
+///   can (and usually will) cause side-effects, but those must not affect Rust-allocated memory in
+///   in any way. In this use-case, the pointer does *not* have to be [valid] for reads.
+///
+/// Note that volatile memory operations on zero-sized types (e.g., if a zero-sized type is passed
+/// to `read_volatile`) are noops and may be ignored.
 ///
 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
+/// [allocation]: crate::ptr#allocated-object
 ///
 /// # Safety
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `src` must be [valid] for reads.
+/// * `src` must be either [valid] for reads, or it must point to memory outside of all Rust
+///   allocations and reading from that memory must:
+///   - not trap, and
+///   - not cause any memory inside a Rust allocation to be modified.
 ///
 /// * `src` must be properly aligned.
 ///
-/// * `src` must point to a properly initialized value of type `T`.
+/// * Reading from `src` must produce a properly initialized value of type `T`.
 ///
-/// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
-/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
-/// value and the value at `*src` can [violate memory safety][read-ownership].
-/// However, storing non-[`Copy`] types in volatile memory is almost certainly
-/// incorrect.
+/// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of whether `T` is
+/// [`Copy`]. If `T` is not [`Copy`], using both the returned value and the value at `*src` can
+/// [violate memory safety][read-ownership]. However, modeling volatile memory with non-[`Copy`]
+/// types is almost certainly incorrect.
 ///
 /// Note that even if `T` has size `0`, the pointer must be properly aligned.
 ///
 /// [valid]: self#safety
 /// [read-ownership]: read#ownership-of-the-returned-value
 ///
-/// Just like in C, whether an operation is volatile has no bearing whatsoever
-/// on questions involving concurrent access from multiple threads. Volatile
-/// accesses behave exactly like non-atomic accesses in that regard. In particular,
-/// a race between a `read_volatile` and any write operation to the same location
-/// is undefined behavior.
-///
 /// # Examples
 ///
 /// Basic usage:
@@ -1813,63 +1827,75 @@ pub unsafe fn read_volatile<T>(src: *const T) -> T {
     unsafe {
         ub_checks::assert_unsafe_precondition!(
             check_language_ub,
-            "ptr::read_volatile requires that the pointer argument is aligned and non-null",
+            "ptr::read_volatile requires that the pointer argument is aligned",
             (
                 addr: *const () = src as *const (),
                 align: usize = align_of::<T>(),
-                is_zst: bool = T::IS_ZST,
-            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
+            ) => ub_checks::maybe_is_aligned(addr, align)
         );
         intrinsics::volatile_load(src)
     }
 }
 
-/// Performs a volatile write of a memory location with the given value without
-/// reading or dropping the old value.
-///
-/// Volatile operations are intended to act on I/O memory, and are guaranteed
-/// to not be elided or reordered by the compiler across other volatile
-/// operations.
-///
-/// `write_volatile` does not drop the contents of `dst`. This is safe, but it
-/// could leak allocations or resources, so care should be taken not to overwrite
-/// an object that should be dropped.
-///
-/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
-/// location pointed to by `dst`.
-///
-/// # Notes
-///
-/// Rust does not currently have a rigorously and formally defined memory model,
-/// so the precise semantics of what "volatile" means here is subject to change
-/// over time. That being said, the semantics will almost always end up pretty
-/// similar to [C11's definition of volatile][c11].
-///
-/// The compiler shouldn't change the relative order or number of volatile
-/// memory operations. However, volatile memory operations on zero-sized types
-/// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
-/// and may be ignored.
+/// Performs a volatile write of a memory location with the given value without reading or dropping
+/// the old value.
+///
+/// Rust does not currently have a rigorously and formally defined memory model, so the precise
+/// semantics of what "volatile" means here is subject to change over time. That being said, the
+/// semantics will almost always end up pretty similar to [C11's definition of volatile][c11].
+///
+/// Volatile operations are intended to act on I/O memory. As such, they are considered externally
+/// observable events (just like syscalls), and are guaranteed to not be elided or reordered by the
+/// compiler across other externally observable events. With this in mind, there are two cases of
+/// usage that need to be distinguished:
+///
+/// - When a volatile operation is used for memory inside an [allocation], it behaves exactly like
+///   [`write()`], except for the additional guarantee that it won't be elided or reordered (see
+///   above). This implies that the operation will actually access memory and not e.g. be lowered to
+///   a register access or stack pop. Other than that, all the usual rules for memory accesses
+///   apply. In particular, just like in C, whether an operation is volatile has no bearing
+///   whatsoever on questions involving concurrent access from multiple threads. Volatile accesses
+///   behave exactly like non-atomic accesses in that regard.
+///
+/// - Volatile operations, however, may also be used access memory that is _outside_ of any Rust
+///   allocation. The main use-case is CPU and peripheral registers that must be accessed via an I/O
+///   memory mapping, most commonly at fixed addresses reserved by the hardware. These often have
+///   special semantics associated to their manipulation, and cannot be used as general purpose
+///   memory. Here, any address value is possible, including 0 and [`usize::MAX`], so long as the
+///   semantics of such a write are well-defined by the target hardware. The access must not trap.
+///   It can (and usually will) cause side-effects, but those must not affect Rust-allocated memory
+///   in any way. In this use-case, the pointer does *not* have to be [valid] for writes.
+///
+/// Note that volatile memory operations on zero-sized types (e.g., if a zero-sized type is passed
+/// to `write_volatile`) are noops and may be ignored.
+///
+/// `write_volatile` does not drop the contents of `dst`. This is safe, but it could leak
+/// allocations or resources, so care should be taken not to overwrite an object that should be
+/// dropped when operating on Rust memory.
+///
+/// Additionally, it does not drop `src`. Semantically, `src` is moved into the location pointed to
+/// by `dst`.
 ///
 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
+/// [allocation]: crate::ptr#allocated-object
 ///
 /// # Safety
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `dst` must be [valid] for writes.
+/// * `dst` must be either [valid] for writes, or it must point to memory outside of all Rust
+///   allocations and writing to that memory must:
+///   - not trap, and
+///   - not cause any memory inside a Rust allocation to be modified.
 ///
 /// * `dst` must be properly aligned.
 ///
+/// * `src` must be a properly initialized value of type `T`.
+///
 /// Note that even if `T` has size `0`, the pointer must be properly aligned.
 ///
 /// [valid]: self#safety
 ///
-/// Just like in C, whether an operation is volatile has no bearing whatsoever
-/// on questions involving concurrent access from multiple threads. Volatile
-/// accesses behave exactly like non-atomic accesses in that regard. In particular,
-/// a race between a `write_volatile` and any other operation (reading or writing)
-/// on the same location is undefined behavior.
-///
 /// # Examples
 ///
 /// Basic usage:
@@ -1893,12 +1919,11 @@ pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
     unsafe {
         ub_checks::assert_unsafe_precondition!(
             check_language_ub,
-            "ptr::write_volatile requires that the pointer argument is aligned and non-null",
+            "ptr::write_volatile requires that the pointer argument is aligned",
             (
                 addr: *mut () = dst as *mut (),
                 align: usize = align_of::<T>(),
-                is_zst: bool = T::IS_ZST,
-            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
+            ) => ub_checks::maybe_is_aligned(addr, align)
         );
         intrinsics::volatile_store(dst, src);
     }

library/core/src/ub_checks.rs

@@ -129,6 +129,25 @@ pub(crate) const fn maybe_is_aligned_and_not_null(
     )
 }
 
+/// Checks whether `ptr` is properly aligned with respect to the given alignment.
+///
+/// In `const` this is approximate and can fail spuriously. It is primarily intended
+/// for `assert_unsafe_precondition!` with `check_language_ub`, in which case the
+/// check is anyway not executed in `const`.
+#[inline]
+#[rustc_allow_const_fn_unstable(const_eval_select)]
+pub(crate) const fn maybe_is_aligned(ptr: *const (), align: usize) -> bool {
+    // This is just for safety checks so we can const_eval_select.
+    const_eval_select!(
+        @capture { ptr: *const (), align: usize } -> bool:
+        if const {
+            true
+        } else {
+            ptr.is_aligned_to(align)
+        }
+    )
+}
+
 #[inline]
 pub(crate) const fn is_valid_allocation_size(size: usize, len: usize) -> bool {
     let max_len = if size == 0 { usize::MAX } else { isize::MAX as usize / size };

tests/ui/precondition-checks/read_volatile.rs

@@ -1,7 +1,7 @@
 //@ run-fail
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::read_volatile requires
-//@ revisions: null misaligned
+//@ revisions: misaligned
 
 #![allow(invalid_null_arguments)]
 
@@ -11,8 +11,6 @@ fn main() {
     let src = [0u16; 2];
     let src = src.as_ptr();
     unsafe {
-        #[cfg(null)]
-        ptr::read_volatile(ptr::null::<u8>());
         #[cfg(misaligned)]
         ptr::read_volatile(src.byte_add(1));
     }

tests/ui/precondition-checks/write_volatile.rs

@@ -1,18 +1,14 @@
 //@ run-fail
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::write_volatile requires
-//@ revisions: null misaligned
-
-#![allow(invalid_null_arguments)]
+//@ revisions: misaligned
 
 use std::ptr;
 
 fn main() {
     let mut dst = [0u16; 2];
     let mut dst = dst.as_mut_ptr();
     unsafe {
-        #[cfg(null)]
-        ptr::write_volatile(ptr::null_mut::<u8>(), 1u8);
         #[cfg(misaligned)]
         ptr::write_volatile(dst.byte_add(1), 1u16);
     }