rootmap: use full path for 'root=' karg when rootfs is directly on multipath

Issue: coreos/fedora-coreos-tracker#1980

This was first observed on s390x builders under high system load, where `ext.config.multipath.resilient`
would intermittently fail during subsequent boot:
```
[    2.781559] multipathd[321]: sdd [8:48]: path added to devmap 0xcadf6fadb3ee446d
[    2.853163] multipathd[321]: sdb [8:16]: path added to devmap 0x000000000000000b
[    3.012431] systemd[1]: Reached target coreos-multipath-wait.target - CoreOS Wait For Multipathed Boot.
[    3.139605] systemd[1]: Mounting sysroot.mount - /sysroot...
[    3.450666] mount[806]: mount: /sysroot: fsconfig system call failed: /dev/sdd4: Can't open blockdev.
```

It looks like a race condition between multipathd taking ownership of the root device and systemd trying to
mount /sysroot. This might happen because the udev database isn't ready in time.

Signed-off-by: Nikita Dubrovskii <nikita@linux.ibm.com>

Author: nikita-dubrovskii

docs/release-notes.md

@@ -15,6 +15,7 @@ Minor changes:
 Internal changes:
 
 - Add initial TMT tests and a new workflow to execute tests on PRs
+- rootmap: use root full path when on multipath
 
 Packaging changes:
 

src/bin/rdcore/rootmap.rs

@@ -30,6 +30,12 @@ use crate::cmdline::*;
 /// composefs, the mount of / and /sysroot will be distinct.
 const PHYSICAL_ROOT_MOUNT: &str = "sysroot";
 
+fn is_multipath() -> Result<bool> {
+    let cmdline =
+        std::fs::read_to_string("/proc/cmdline").context("reading kernel command line")?;
+    Ok(cmdline.contains("rd.multipath=default"))
+}
+
 pub fn rootmap(config: RootmapConfig) -> Result<()> {
     let root_mount_path = Path::new(&config.root_mount);
     // Get the mount point for the deployment root, which will have e.g. /etc which we might parse
@@ -51,13 +57,22 @@ pub fn rootmap(config: RootmapConfig) -> Result<()> {
         }
     }
 
+    // use full path when the rootfs is directly on multipath;
+    // this prevents race condition between systemd's autogenerated sysroot.mount and multipathd/udev,
+    // see also: https://github.com/coreos/fedora-coreos-tracker/issues/1980
+    let root = if is_multipath()? {
+        // https://github.com/coreos/fedora-coreos-config/blob/testing-devel/overlay.d/05core/usr/lib/udev/rules.d/90-coreos-device-mapper.rules#L25
+        format!(
+            "root=/dev/disk/by-uuid/dm-mpath-{}",
+            physical_mount.get_filesystem_uuid()?
+        )
+    } else {
+        format!("root=UUID={}", physical_mount.get_filesystem_uuid()?)
+    };
     // we push the root kargs last, this has the nice property that the final order of kargs goes
     // from lowest level to highest; see also
     // https://github.com/coreos/fedora-coreos-tracker/issues/465
-    kargs.push(format!(
-        "root=UUID={}",
-        physical_mount.get_filesystem_uuid()?
-    ));
+    kargs.push(root);
 
     // we need this because with root= it's systemd that takes care of mounting via
     // systemd-fstab-generator, and it defaults to read-only otherwise