xtask: copy kernel config into the initramfs image

When preparing the VM initramfs detect the `config-*` file that ships alongside
the vmlinuz/modules in each kernel archive and install it under `/boot` (both
as `/boot/config` and `/boot/config-<version>`). This makes the running
kernel’s configuration available inside the guest for the integration tests.

test/integration-test/src/tests/feature_probe.rs

@@ -14,7 +14,7 @@ use crate::utils::kernel_assert;
 #[test_log::test]
 fn probe_supported_programs() {
     let current = aya::util::KernelVersion::current().unwrap();
-    let kernel_config = kernel_config().unwrap_or_default();
+    let kernel_config = kernel_config().unwrap();
 
     macro_rules! is_supported {
         ($prog_type:expr) => {

xtask/src/run.rs

@@ -318,6 +318,7 @@ pub(crate) fn run(opts: Options) -> Result<()> {
                 }
 
                 let mut kernel_images = Vec::new();
+                let mut configs = Vec::new();
                 for entry in WalkDir::new(&archive_dir) {
                     let entry = entry.with_context(|| {
                         format!("failed to read entry in {}", archive_dir.display())
@@ -345,6 +346,10 @@ pub(crate) fn run(opts: Options) -> Result<()> {
                                         .to_os_string();
                                 kernel_images.push((path, kernel_version))
                             }
+                            // "config-"
+                            [b'c', b'o', b'n', b'f', b'i', b'g', b'-', ..] => {
+                                configs.push(path);
+                            }
                             _ => {}
                         }
                     }
@@ -358,6 +363,10 @@ pub(crate) fn run(opts: Options) -> Result<()> {
                         kernel_images
                     ),
                 };
+                let config = match configs.as_slice() {
+                    [config] => config,
+                    configs => bail!("multiple configs in {}: {:?}", archive.display(), configs),
+                };
 
                 let mut modules_dirs = Vec::new();
                 for entry in WalkDir::new(&archive_dir) {
@@ -487,9 +496,15 @@ pub(crate) fn run(opts: Options) -> Result<()> {
 
                 write_dir(Path::new("/bin"));
                 write_dir(Path::new("/sbin"));
+                write_dir(Path::new("/boot"));
                 write_dir(Path::new("/lib"));
                 write_dir(Path::new("/lib/modules"));
 
+                write_file(Path::new("/boot/config"), config, "644 0 0");
+                if let Some(name) = config.file_name() {
+                    write_file(&Path::new("/boot").join(name), config, "644 0 0");
+                }
+
                 test_distro.iter().for_each(|(name, path)| {
                     if name == "init" {
                         write_file(Path::new("/init"), path, "755 0 0");