uefi: Add TryFrom u8 slice to DevicePathHeader

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 +119,19 @@ pub struct DevicePathHeader {
     pub length: u16,
 }
 
+impl<'a> TryFrom<&[u8]> for &'a DevicePathHeader {
+    type Error = ByteConversionError;
+
+    fn try_from(bytes: &[u8]) -> Result<Self, Self::Error> {
+        if mem::size_of::<DevicePathHeader>() <= bytes.len() {
+            unsafe {
+                return Ok(&*bytes.as_ptr().cast::<DevicePathHeader>());
+            }
+        }
+        Err(ByteConversionError::InvalidLength)
+    }
+}
+
 /// A single node within a [`DevicePath`].
 ///
 /// Each node starts with a [`DevicePathHeader`]. The rest of the data
@@ -729,6 +742,15 @@ impl DeviceSubType {
     pub const END_ENTIRE: DeviceSubType = DeviceSubType(0xff);
 }
 
+/// Error returned when attempting to convert from a `&[u8]` to a
+/// [`DevicePath`] type using:
+/// - [`DevicePathHeader::try_from`](struct.DevicePathHeader.html#method.try_from)
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ByteConversionError {
+    /// The length of the given slice is not valid for its [`DevicePath`] type.
+    InvalidLength,
+}
+
 /// Error returned when converting from a [`DevicePathNode`] to a more
 /// specific node type.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]