[PATCH 3/5] rust: sync: support using bool with READ_ONCE

[Alice Ryhl]

Normally it is undefined behavior for a bool to take any value other
than 0 or 1. However, in the case of READ_ONCE(some_bool) is used, this
UB seems dangerous and unnecessary. I can easily imagine some Rust code
that looks like this:

	if READ_ONCE(&raw const (*my_c_struct).my_bool_field) {
	    ...
	}

And by making an analogy to what the equivalent C code is, anyone
writing this probably just meant to treat any non-zero value as true.

For WRITE_ONCE no special logic is required.

Signed-off-by: Alice Ryhl <aliceryhl at google.com>
---
 rust/kernel/sync/rwonce.rs | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/rust/kernel/sync/rwonce.rs b/rust/kernel/sync/rwonce.rs
index a1660e43c9ef94011812d1816713cf031a73de1d..73477f53131926996614df573b2d50fff98e624f 100644
--- a/rust/kernel/sync/rwonce.rs
+++ b/rust/kernel/sync/rwonce.rs
@@ -163,6 +163,7 @@ unsafe fn write_once(ptr: *mut Self, val: Self) {
 // sizes, so picking the wrong helper should lead to a build error.
 
 impl_rw_once_type! {
+    bool, read_once_bool, write_once_1;
     u8,   read_once_1, write_once_1;
     i8,   read_once_1, write_once_1;
     u16,  read_once_2, write_once_2;
@@ -186,3 +187,21 @@ unsafe fn write_once(ptr: *mut Self, val: Self) {
     usize, read_once_8, write_once_8;
     isize, read_once_8, write_once_8;
 }
+
+/// Read an integer as a boolean once.
+///
+/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`.
+///
+/// # Safety
+///
+/// It must be safe to `READ_ONCE` the `ptr` with type `u8`.
+#[inline(always)]
+#[track_caller]
+unsafe fn read_once_bool(ptr: *const bool) -> bool {
+    // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside
+    // of `read_once_1`.
+    //
+    // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`.
+    let byte = unsafe { read_once_1(ptr.cast::<u8>()) };
+    byte != 0u8
+}

-- 
2.52.0.351.gbe84eed79e-goog