cli: Generate driver structs automatically using bindgen

Added a build script that automatically generates
rust bindings for the C structures used by the NE driver.
Currently, rust does not allow to implement a destructor
and to derive the `Copy` trait for a structure. By default,
bindgen derives `Copy` for the generated structures.
Although the behavior can be altered by using the
Builder's `no_copy()` method, this will also prevent
deriving `Clone` (which in turn makes the binding
unusable).

According to these sources (https://users.rust-lang.org/t/how-to-derive-custom-traits-for-struct-definitions-generated-by-bindgen/39710,
rust-lang/rust-bindgen#1089),
adding custom derives for the generated bindings is not
currently supported.

Update: as a workaround (for `ne_user_memory_region`),
the bindgen generated struct has been kept and the CLI's
MemoryRegion now contains a method for converting between
its own format and the driver struct format. This is used
for converting to the driver struct representation, when
issuing a `NE_SET_USER_MEMORY_REGION` ioctl.

The conversion between the two mentioned types is done
by implementing the `From` trait for UserMemoryRegion (
which also automatically provides an implementation for
`.into()`, for constructing a UserMemoryRegion instance
from a MemoryRegion one).

Signed-off-by: Gabriel Bercaru <bercarug@amazon.com>

Cargo.toml

@@ -22,6 +22,9 @@ procfs = "0.7"
 eif_loader = { path = "./eif_loader" }
 enclave_build = { path = "./enclave_build"}
 
+[build-dependencies]
+bindgen = { version=">=0.54" }
+
 [dev-dependencies]
 log = "0.4"
 num-derive = "0.2"

build.rs

@@ -1,5 +1,11 @@
+extern crate bindgen;
+
+use std::path::PathBuf;
 use std::process::Command;
 
+const HEADER_PATH: &str = "include/uapi/linux/nitro_enclaves.h";
+const OUT_FILE: &str = "/driver_structs.rs";
+
 fn main() {
     // Get latest commit SHA.
     let output = Command::new("git")
@@ -21,4 +27,24 @@ fn main() {
     };
 
     println!("cargo:rustc-env=COMMIT_ID={}", output_str.trim());
+    println!("cargo:rerun-if-changed={}", HEADER_PATH);
+
+    let bindings = bindgen::Builder::default()
+        .header(HEADER_PATH)
+        .whitelist_type("ne_image_load_info")
+        .whitelist_type("ne_user_memory_region")
+        .whitelist_type("ne_enclave_start_info")
+        .clang_arg(r"-fretain-comments-from-system-headers")
+        .clang_arg(r"-fparse-all-comments")
+        .parse_callbacks(Box::new(bindgen::CargoCallbacks))
+        .generate_comments(true)
+        .generate()
+        .expect("Unable to generate bindings");
+
+    let mut path_str = std::env::var("OUT_DIR").unwrap();
+    path_str.push_str(&OUT_FILE);
+    let out_path = PathBuf::from(path_str);
+    bindings
+        .write_to_file(out_path)
+        .expect("Could not write bindings");
 }