virtiofs: Fix writable virtiofs mounts

Regression from previous commit; we unconditionally added
`--readonly` if virtiofsd supported it (which it doesn't yet
in the Ubuntu 24.04 but does in Fedora 42+).

We want to support writable mounts.

It turns out the integration tests didn't catch this because
they only looked at the permission bits.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/integration-tests/src/tests/mount_feature.rs

@@ -20,57 +20,48 @@ use tempfile::TempDir;
 
 use crate::{get_test_image, run_bcvk, INTEGRATION_TEST_LABEL};
 
-/// Create a systemd unit that verifies a mount exists and contains expected content
+/// Create a systemd unit that verifies a mount exists and tests writability
 fn create_mount_verify_unit(
     unit_dir: &Utf8Path,
     mount_name: &str,
     expected_file: &str,
-    expected_content: &str,
+    expected_content: Option<&str>,
+    readonly: bool,
 ) -> std::io::Result<()> {
-    let unit_content = format!(
-        r#"[Unit]
-Description=Verify mount {mount_name} and poweroff
-RequiresMountsFor=/run/virtiofs-mnt-{mount_name}
+    let (description, content_check, write_check, unit_prefix) = if readonly {
+        (
+            format!("Verify read-only mount {mount_name} and poweroff"),
+            format!("ExecStart=test -f /run/virtiofs-mnt-{mount_name}/{expected_file}"),
+            format!("ExecStart=/bin/sh -c '! echo test-write > /run/virtiofs-mnt-{mount_name}/write-test.txt 2>/dev/null'"),
+            "verify-ro-mount",
+        )
+    } else {
+        let content = expected_content.expect("expected_content required for writable mounts");
+        (
+            format!("Verify mount {mount_name} and poweroff"),
+            format!("ExecStart=grep -qF \"{content}\" /run/virtiofs-mnt-{mount_name}/{expected_file}"),
+            format!("ExecStart=/bin/sh -c 'echo test-write > /run/virtiofs-mnt-{mount_name}/write-test.txt'"),
+            "verify-mount",
+        )
+    };
 
-[Service]
-Type=oneshot
-ExecStart=grep -qF "{expected_content}" /run/virtiofs-mnt-{mount_name}/{expected_file}
-ExecStart=test -w /run/virtiofs-mnt-{mount_name}/{expected_file}
-ExecStart=echo ok mount verify {mount_name}
-ExecStart=systemctl poweroff
-StandardOutput=journal+console
-StandardError=journal+console
-"#
-    );
-
-    let unit_path = unit_dir.join(format!("verify-mount-{}.service", mount_name));
-    fs::write(&unit_path, unit_content)?;
-    Ok(())
-}
-
-/// Create a systemd unit that tries to write to a mount to verify read-only status
-fn create_ro_mount_verify_unit(
-    unit_dir: &Utf8Path,
-    mount_name: &str,
-    expected_file: &str,
-) -> std::io::Result<()> {
     let unit_content = format!(
         r#"[Unit]
-Description=Verify read-only mount {mount_name} and poweroff
+Description={description}
 RequiresMountsFor=/run/virtiofs-mnt-{mount_name}
 
 [Service]
 Type=oneshot
-ExecStart=test -f /run/virtiofs-mnt-{mount_name}/{expected_file}
-ExecStart=test '!' -w /run/virtiofs-mnt-{mount_name}/{expected_file}
+{content_check}
+{write_check}
 ExecStart=echo ok mount verify {mount_name}
 ExecStart=systemctl poweroff
 StandardOutput=journal+console
 StandardError=journal+console
 "#
     );
 
-    let unit_path = unit_dir.join(format!("verify-ro-mount-{}.service", mount_name));
+    let unit_path = unit_dir.join(format!("{unit_prefix}-{mount_name}.service"));
     fs::write(&unit_path, unit_content)?;
     Ok(())
 }
@@ -90,8 +81,14 @@ pub fn test_mount_feature_bind() {
     fs::create_dir(&system_dir).expect("Failed to create system directory");
 
     // Create verification unit
-    create_mount_verify_unit(&system_dir, "testmount", "test.txt", test_content)
-        .expect("Failed to create verify unit");
+    create_mount_verify_unit(
+        &system_dir,
+        "testmount",
+        "test.txt",
+        Some(test_content),
+        false,
+    )
+    .expect("Failed to create verify unit");
 
     println!("Testing bind mount with temp directory: {}", temp_dir_path);
 
@@ -135,7 +132,7 @@ pub fn test_mount_feature_ro_bind() {
     fs::create_dir(&system_dir).expect("Failed to create system directory");
 
     // Create verification unit for read-only mount
-    create_ro_mount_verify_unit(&system_dir, "romount", "readonly.txt")
+    create_mount_verify_unit(&system_dir, "romount", "readonly.txt", None, true)
         .expect("Failed to create verify unit");
 
     println!(

crates/kit/src/qemu.rs

@@ -898,6 +898,8 @@ pub struct VirtiofsConfig {
     /// Host directory to share
     pub shared_dir: Utf8PathBuf,
     pub debug: bool,
+    /// Mount as read-only
+    pub readonly: bool,
 }
 
 impl Default for VirtiofsConfig {
@@ -906,6 +908,8 @@ impl Default for VirtiofsConfig {
             socket_path: "/run/inner-shared/virtiofs.sock".into(),
             shared_dir: "/run/source-image".into(),
             debug: false,
+            // We don't need to write to this, there's a transient overlay
+            readonly: true,
         }
     }
 }
@@ -977,8 +981,8 @@ pub async fn spawn_virtiofsd_async(config: &VirtiofsConfig) -> Result<tokio::pro
         "--sandbox=none",
     ]);
 
-    // Only add --readonly if supported
-    if supports_readonly {
+    // Only add --readonly if requested and supported
+    if config.readonly && supports_readonly {
         cmd.arg("--readonly");
     }
 

crates/kit/src/run_ephemeral.rs

@@ -816,6 +816,7 @@ pub(crate) async fn run_impl(opts: RunEphemeralOpts) -> Result<()> {
                 socket_path: socket_path.clone().into(),
                 shared_dir: source_path,
                 debug: false,
+                readonly: is_readonly,
             };
             additional_mounts.push((virtiofsd_config, tag.clone()));