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uefi: Change IMAGE_HANDLE to an atomic pointer #976

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merged 1 commit into from Oct 27, 2023
Merged

uefi: Change IMAGE_HANDLE to an atomic pointer #976

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42 changes: 11 additions & 31 deletions uefi/src/table/boot.rs
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Expand Up @@ -13,6 +13,7 @@ use core::ffi::c_void;
use core::mem::{self, MaybeUninit};
use core::ops::{Deref, DerefMut};
use core::ptr::NonNull;
use core::sync::atomic::{AtomicPtr, Ordering};
use core::{ptr, slice};

#[cfg(feature = "alloc")]
Expand All @@ -22,19 +23,9 @@ pub use uefi_raw::table::boot::{
EventType, InterfaceType, MemoryAttribute, MemoryDescriptor, MemoryType, Tpl,
};

// TODO: this similar to `SyncUnsafeCell`. Once that is stabilized we
// can use it instead.
struct GlobalImageHandle {
handle: UnsafeCell<Option<Handle>>,
}

// Safety: reads and writes are managed via `set_image_handle` and
// `BootServices::image_handle`.
unsafe impl Sync for GlobalImageHandle {}

static IMAGE_HANDLE: GlobalImageHandle = GlobalImageHandle {
handle: UnsafeCell::new(None),
};
/// Global image handle. This is only set by `BootServices::set_image_handle`,
/// and it is only read by `BootServices::image_handle`.
static IMAGE_HANDLE: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());

/// Size in bytes of a UEFI page.
///
Expand Down Expand Up @@ -95,22 +86,10 @@ pub struct BootServices(uefi_raw::table::boot::BootServices);
impl BootServices {
/// Get the [`Handle`] of the currently-executing image.
pub fn image_handle(&self) -> Handle {
// Safety:
//
// `IMAGE_HANDLE` is only set by `set_image_handle`, see that
// documentation for more details.
//
// Additionally, `image_handle` takes a `&self` which ensures it
// can only be called while boot services are active. (After
// exiting boot services, the image handle should not be
// considered valid.)
unsafe {
IMAGE_HANDLE
.handle
.get()
.read()
.expect("set_image_handle has not been called")
}
let ptr = IMAGE_HANDLE.load(Ordering::Acquire);
// Safety: the image handle must be valid. We know it is, because it was
// set by `set_image_handle`, which has that same safety requirement.
unsafe { Handle::from_ptr(ptr) }.expect("set_image_handle has not been called")
}

/// Update the global image [`Handle`].
Expand All @@ -123,14 +102,15 @@ impl BootServices {
///
/// # Safety
///
/// This function should only be called as described above. The
/// This function should only be called as described above,
/// and the `image_handle` must be a valid image [`Handle`]. Then
/// safety guarantees of [`BootServices::open_protocol_exclusive`]
/// rely on the global image handle being correct.
pub unsafe fn set_image_handle(&self, image_handle: Handle) {
// As with `image_handle`, `&self` isn't actually used, but it
// enforces that this function is only called while boot
// services are active.
IMAGE_HANDLE.handle.get().write(Some(image_handle));
IMAGE_HANDLE.store(image_handle.as_ptr(), Ordering::Release);
}

/// Raises a task's priority level and returns its previous level.
Expand Down