loaded image: add set_image function

This allows overriding the data in `LoadedImage`. It's useful in the
following scenario:

1. Secure boot is enabled, so images loaded with `LoadImage` must be
   signed with an appropriate key known to the firmware.
2. The bootloader has its own key embedded, and uses that key to
   verify the next stage. This key is not known to the firmware, so
   the next stage's image can't be loaded with `LoadImage`.
3. Since image handles are created by `LoadImage`, which we can't
   call, we have to make use of an existing image handle -- the one
   passed into the bootloader's entry function. By modifying that
   image handle (after appropriately verifying the signature of the
   new data of course), we can repurpose the image handle for the next
   stage.

See [shim] for an example of this scenario in action.

[shim]: https://github.com/rhboot/shim/blob/4d64389c6c941d21548b06423b8131c872e3c3c7/pe.c#L1143

Author: nicholasbishop

src/proto/loaded_image.rs

@@ -27,7 +27,7 @@ pub struct LoadedImage {
     load_options: *const Char16,
 
     // Location where image was loaded
-    image_base: usize,
+    image_base: *const c_void,
     image_size: u64,
     image_code_type: MemoryType,
     image_data_type: MemoryType,
@@ -65,6 +65,18 @@ impl LoadedImage {
         }
     }
 
+    /// Set the image data address and size.
+    ///
+    /// # Safety
+    ///
+    /// This function takes `data` as a raw pointer because the data is not
+    /// owned by `LoadedImage`. The caller must ensure that the memory lives
+    /// long enough.
+    pub unsafe fn set_image(&mut self, data: *const c_void, size: u64) {
+        self.image_base = data;
+        self.image_size = size;
+    }
+
     /// Set the load options for the image. This can be used prior to
     /// calling `BootServices.start_image` to control the command line
     /// passed to the image.
@@ -80,7 +92,7 @@ impl LoadedImage {
     }
 
     /// Returns the base address and the size in bytes of the loaded image.
-    pub fn info(&self) -> (usize, u64) {
+    pub fn info(&self) -> (*const c_void, u64) {
         (self.image_base, self.image_size)
     }
 }