refactor(cpu-template-helper): Build mock kernels with build.rs

The mock kernel images were added in commit 8377e7f
("chore(cpu-template-helper): Add mock kernels") and were generated from
C code with Makefile. During its review cycle, the kernel generation
became cheap enough to run as part of Firecracker compilation process.
Replaces Makefile with build.rs and generates at the Firecracker
compilation time.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

.gitignore

@@ -11,3 +11,4 @@ test_results/*
 *.core
 *.profraw
 .DS_Store
+*.bin

src/cpu-template-helper/build.rs

@@ -0,0 +1,65 @@
+// Copyright 2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+use std::io::Write;
+
+const MOCK_KERNEL_PATH: &str = "src/utils/mock_kernel/kernel.bin";
+
+// Kernel header for aarch64 that comes from the kernel doc Documentation/arm64/booting.txt.
+#[derive(Default)]
+#[repr(C, packed)]
+struct KernelHeader {
+    code0: u32,       // Executable code
+    code1: u32,       // Executable code
+    text_offset: u64, // Image load offset,
+    image_size: u64,  // Effective Image size, little endian
+    flags: u64,       // kernel flags, little endian
+    res2: u64,        // reserved
+    res3: u64,        // reserved
+    res4: u64,        // reserved
+    magic: u32,       // Magic number, little endian, "ARM\x64"
+    res5: u32,        // reserved (used for PE COFF offset)
+}
+
+fn main() {
+    if cfg!(target_arch = "x86_64") {
+        println!("cargo:rerun-if-changed=src/utils/mock_kernel/main.c");
+        let status = std::process::Command::new("gcc")
+            .args([
+                // Do not use the standard system startup files or libraries when linking.
+                "-nostdlib",
+                // Prevents linking with the shared libraries.
+                "-static",
+                // Do not generate unwind tables.
+                "-fno-asynchronous-unwind-tables",
+                // Remove all symbol table and relocation information.
+                "-s",
+                "-o",
+                MOCK_KERNEL_PATH,
+                "src/utils/mock_kernel/main.c",
+            ])
+            .status()
+            .expect("Failed to execute gcc command");
+        if !status.success() {
+            panic!("Failed to compile mock kernel");
+        }
+    } else if cfg!(target_arch = "aarch64") {
+        let header = KernelHeader {
+            magic: 0x644D5241,
+            ..std::default::Default::default()
+        };
+        // SAFETY: This is safe as long as `header` is valid as `KernelHeader`.
+        let header_bytes = unsafe {
+            std::slice::from_raw_parts(
+                (&header as *const KernelHeader).cast::<u8>(),
+                std::mem::size_of::<KernelHeader>(),
+            )
+        };
+
+        let mut file = std::fs::File::create(MOCK_KERNEL_PATH).expect("Failed to create a file");
+        file.write_all(header_bytes)
+            .expect("Failed to write kernel header to a file");
+    } else {
+        panic!("Unsupported arch");
+    }
+}

src/cpu-template-helper/src/utils/mock_kernel/Makefile

@@ -75,12 +75,9 @@ pub enum UtilsError {
 // Utility function to prepare scratch kernel image and rootfs and build mock Firecracker config.
 fn build_mock_config() -> Result<(TempFile, TempFile, String), UtilsError> {
     let kernel = TempFile::new()?;
-    kernel.as_file().write_all(
-        #[cfg(target_arch = "aarch64")]
-        include_bytes!("mock_kernel/aarch64.bin"),
-        #[cfg(target_arch = "x86_64")]
-        include_bytes!("mock_kernel/x86_64.bin"),
-    )?;
+    kernel
+        .as_file()
+        .write_all(include_bytes!("mock_kernel/kernel.bin"))?;
     let rootfs = TempFile::new()?;
     let config = format!(
         r#"{{