Create a fat filesystem image

disk_image/Cargo.toml

@@ -7,3 +7,4 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+fatfs = "0.3.5"

disk_image/src/main.rs

@@ -1,3 +1,36 @@
+use std::{fs, io, path::Path};
+
 fn main() {
     println!("Hello, world!");
 }
+
+fn create_fat_filesystem(fat_path: &Path, efi_file: &Path) {
+    // retrieve size of `.efi` file and round it up
+    let efi_size = fs::metadata(&efi_file).unwrap().len();
+    let mb = 1024 * 1024; // size of a megabyte
+                          // round it to next megabyte
+    let efi_size_rounded = ((efi_size - 1) / mb + 1) * mb;
+
+    // create new filesystem image file at the given path and set its length
+    let fat_file = fs::OpenOptions::new()
+        .read(true)
+        .write(true)
+        .create(true)
+        .truncate(true)
+        .open(&fat_path)
+        .unwrap();
+    fat_file.set_len(efi_size_rounded).unwrap();
+
+    // create new FAT file system and open it
+    let format_options = fatfs::FormatVolumeOptions::new();
+    fatfs::format_volume(&fat_file, format_options).unwrap();
+    let filesystem = fatfs::FileSystem::new(&fat_file, fatfs::FsOptions::new()).unwrap();
+
+    // copy EFI file to FAT filesystem
+    let root_dir = filesystem.root_dir();
+    root_dir.create_dir("efi").unwrap();
+    root_dir.create_dir("efi/boot").unwrap();
+    let mut bootx64 = root_dir.create_file("efi/boot/bootx64.efi").unwrap();
+    bootx64.truncate().unwrap();
+    io::copy(&mut fs::File::open(&efi_file).unwrap(), &mut bootx64).unwrap();
+}