core: always wrap kernel-install too during scriptlet runs

It's confusing right now how specifically for the kernel, one has to use
this obscure `rpm-ostree cliwrap install-to-root /` command to make it
work. Let's just always enable it: in the client-side layering case, we
don't run kernel scriptlets anyway so the wrapper is unused, and in the
container case, this will allow users to not have to enable cliwrap and
have it leak into their derived image.

I guess in theory, this should also allow us to *stop* ignoring kernel
scriptlets and rely on this instead, though let's leave that for a
separate investigation.

Closes: coreos#4949

ci/test-container.sh

@@ -42,8 +42,6 @@ fi
 rm "${origindir}/clienterror.yaml"
 rpm-ostree ex rebuild
 
-rpm-ostree cliwrap install-to-root /
-
 # Test a critical path package
 yum -y install cowsay && yum clean all
 cowsay "It worked"

rust/src/core.rs

@@ -44,6 +44,8 @@ const USERADD_PATH: &str = "usr/sbin/useradd";
 const USERADD_WRAPPER: &[u8] = include_bytes!("../../src/libpriv/useradd-wrapper.sh");
 const USERMOD_PATH: &str = "usr/sbin/usermod";
 const USERMOD_WRAPPER: &[u8] = include_bytes!("../../src/libpriv/usermod-wrapper.sh");
+const KERNEL_INSTALL_PATH: &str = "usr/bin/kernel-install";
+const KERNEL_INSTALL_WRAPPER: &[u8] = include_bytes!("../../src/libpriv/kernel-install-wrapper.sh");
 
 const RPMOSTREE_CORE_STAGED_RPMS_DIR: &str = "rpm-ostree/staged-rpms";
 
@@ -148,6 +150,7 @@ impl FilesystemScriptPrep {
         (SYSTEMCTL_PATH, SYSTEMCTL_WRAPPER),
         (USERADD_PATH, USERADD_WRAPPER),
         (USERMOD_PATH, USERMOD_WRAPPER),
+        (KERNEL_INSTALL_PATH, KERNEL_INSTALL_WRAPPER),
     ];
 
     fn saved_name(name: &str) -> String {
@@ -441,7 +444,7 @@ mod test {
         // Replaced usermod.
         {
             let original_usermod = "original usermod";
-            d.atomic_write_with_perms(super::USERMOD_PATH, original_usermod, mode)?;
+            d.atomic_write_with_perms(super::USERMOD_PATH, original_usermod, mode.clone())?;
             let contents = d.read_to_string(super::USERMOD_PATH)?;
             assert_eq!(contents, original_usermod);
             let mut g = super::prepare_filesystem_script_prep(d.as_raw_fd())?;
@@ -451,6 +454,19 @@ mod test {
             let contents = d.read_to_string(super::USERMOD_PATH)?;
             assert_eq!(contents, original_usermod);
         }
+        // Replaced kernel-install.
+        {
+            let original_kernel_install = "original kernel_install";
+            d.atomic_write_with_perms(super::KERNEL_INSTALL_PATH, original_kernel_install, mode)?;
+            let contents = d.read_to_string(super::KERNEL_INSTALL_PATH)?;
+            assert_eq!(contents, original_kernel_install);
+            let mut g = super::prepare_filesystem_script_prep(d.as_raw_fd())?;
+            let contents = d.read_to_string(super::KERNEL_INSTALL_PATH)?;
+            assert_eq!(contents.as_bytes(), super::KERNEL_INSTALL_WRAPPER);
+            g.undo()?;
+            let contents = d.read_to_string(super::KERNEL_INSTALL_PATH)?;
+            assert_eq!(contents, original_kernel_install);
+        }
         Ok(())
     }
 

src/libpriv/kernel-install-wrapper.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/bash
+# Used in the container layering path to make kernel replacements Just Work
+# without having to enable cliwrap first. If cliwrap is enabled, then this will
+# technically override the cliwrap wrapper, but the script is exactly the same.
+# This wrapper is technically also installed when doing client-side layering,
+# but we already ignore kernel scriptlets there anyway.
+# See also https://github.com/coreos/rpm-ostree/issues/4949
+
+exec /usr/bin/rpm-ostree cliwrap kernel-install "$@"