x86_64: load initrd at max address

This matches behavior of other bootloaders (grub2, iPXE), and the kernel
seems to be relying on this; decompression of the initrd fails if the
initrd is loaded right after the kernel as before, but succeeds if
loaded at the maximum address.

BUG=None
TEST=Boot Debian kernel + initrd on workstation

Change-Id: If7712efb05f55ef413a419dfe276ed3f68c335b7
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1616989
Tested-by: kokoro <noreply+kokoro@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

arch/src/lib.rs

@@ -271,6 +271,7 @@ pub fn add_serial_devices(
 /// Errors for image loading.
 #[derive(Debug)]
 pub enum LoadImageError {
+    BadAlignment(u64),
     Seek(io::Error),
     ImageSizeTooLarge(u64),
     ReadToMemory(GuestMemoryError),
@@ -281,6 +282,7 @@ impl Display for LoadImageError {
         use self::LoadImageError::*;
 
         match self {
+            BadAlignment(a) => write!(f, "Alignment not a power of two: {}", a),
             Seek(e) => write!(f, "Seek failed: {}", e),
             ImageSizeTooLarge(size) => write!(f, "Image size too large: {}", size),
             ReadToMemory(e) => write!(f, "Reading image into memory failed: {}", e),
@@ -326,3 +328,54 @@ where
 
     Ok(size)
 }
+
+/// Load an image from a file into guest memory at the highest possible address.
+///
+/// # Arguments
+///
+/// * `guest_mem` - The memory to be used by the guest.
+/// * `image` - The file containing the image to be loaded.
+/// * `min_guest_addr` - The minimum address of the start of the image.
+/// * `max_guest_addr` - The address to load the last byte of the image.
+/// * `align` - The minimum alignment of the start address of the image in bytes
+///   (must be a power of two).
+///
+/// The guest address and size in bytes of the loaded image are returned.
+pub fn load_image_high<F>(
+    guest_mem: &GuestMemory,
+    image: &mut F,
+    min_guest_addr: GuestAddress,
+    max_guest_addr: GuestAddress,
+    align: u64,
+) -> Result<(GuestAddress, usize), LoadImageError>
+where
+    F: Read + Seek,
+{
+    if !align.is_power_of_two() {
+        return Err(LoadImageError::BadAlignment(align));
+    }
+
+    let max_size = max_guest_addr.offset_from(min_guest_addr) & !(align - 1);
+    let size = image.seek(SeekFrom::End(0)).map_err(LoadImageError::Seek)?;
+
+    if size > usize::max_value() as u64 || size > max_size {
+        return Err(LoadImageError::ImageSizeTooLarge(size));
+    }
+
+    image
+        .seek(SeekFrom::Start(0))
+        .map_err(LoadImageError::Seek)?;
+
+    // Load image at the maximum aligned address allowed.
+    // The subtraction cannot underflow because of the size checks above.
+    let guest_addr = GuestAddress((max_guest_addr.offset() - size) & !(align - 1));
+
+    // This is safe due to the bounds check above.
+    let size = size as usize;
+
+    guest_mem
+        .read_to_memory(guest_addr, image, size)
+        .map_err(LoadImageError::ReadToMemory)?;
+
+    Ok((guest_addr, size))
+}

x86_64/src/lib.rs

@@ -458,16 +458,26 @@ impl X8664arch {
 
         let initrd = match initrd_file {
             Some(mut initrd_file) => {
-                let initrd_start = free_addr;
-                let initrd_max_size = mem_size - initrd_start;
-                let initrd_size = arch::load_image(
+                let mut initrd_addr_max = u64::from(params.hdr.initrd_addr_max);
+                // Default initrd_addr_max for old kernels (see Documentation/x86/boot.txt).
+                if initrd_addr_max == 0 {
+                    initrd_addr_max = 0x37FFFFFF;
+                }
+
+                let mem_max = mem.end_addr().offset() - 1;
+                if initrd_addr_max > mem_max {
+                    initrd_addr_max = mem_max;
+                }
+
+                let (initrd_start, initrd_size) = arch::load_image_high(
                     mem,
                     &mut initrd_file,
-                    GuestAddress(initrd_start),
-                    initrd_max_size,
+                    GuestAddress(free_addr),
+                    GuestAddress(initrd_addr_max),
+                    sys_util::pagesize() as u64,
                 )
                 .map_err(Error::LoadInitrd)?;
-                Some((GuestAddress(initrd_start), initrd_size))
+                Some((initrd_start, initrd_size))
             }
             None => None,
         };