uefi: implement DevicePathHeader::from_ffi_ptr

Allows for making a `DevicePathHeader` from a byte splice,
used primarily to verify whether the byte splice can also
contain `DevicePathHeader.length` bytes.

Author: andre-braga

uefi/CHANGELOG.md

@@ -11,6 +11,7 @@
 - Added `table::{set_system_table, system_table_boot, system_table_runtime}`.
   This provides an initial API for global tables that do not require passing
   around a reference.
+- Implemented `DevicePathHeader::from_ffi_ptr`.
 
 ## Changed
 - `SystemTable::exit_boot_services` is now `unsafe`. See that method's

uefi/src/proto/device_path/mod.rs

@@ -119,6 +119,20 @@ pub struct DevicePathHeader {
     pub length: u16,
 }
 
+impl DevicePathHeader {
+    /// Create a [`DevicePathHeader`] reference from an opaque pointer.
+    ///
+    /// # Safety
+    ///
+    /// The input pointer must point to valid data. That data must
+    /// remain valid for the lifetime `'a`, and cannot be mutated during
+    /// that lifetime.
+    #[must_use]
+    pub unsafe fn from_ffi_ptr<'a>(ptr: *const FfiDevicePath) -> &'a DevicePathHeader {
+        &*ptr.cast::<DevicePathHeader>()
+    }
+}
+
 /// A single node within a [`DevicePath`].
 ///
 /// Each node starts with a [`DevicePathHeader`]. The rest of the data