core-initrd,snap-bootstrap: look at env to mount directly to /sysroot

Previously we looked at the model and mounted base/rootfs directly if
it was Ubuntu 24+. Now, we check instead an environment variable that
is set if the initramfs is 24+.

This is done so now the kernels that contain the 24+ initramfs can be
booted in older Ubuntu releases with older bases. This situation is
not something we really support, as the systemd of the initramfs will
not match the one in the system, but it is something that could happen
while remodeling and it seems safer to allow this at least
temporarily.

cmd/snap-bootstrap/cmd_initramfs_mounts.go

@@ -94,6 +94,7 @@ func (c *cmdInitramfsMounts) Execute([]string) error {
 
 var (
 	osutilIsMounted = osutil.IsMounted
+	osGetenv        = os.Getenv
 
 	snapTypeToMountDir = map[snap.Type]string{
 		snap.TypeBase:   "base",
@@ -325,8 +326,8 @@ func doInstall(mst *initramfsMountsState, model *asserts.Model, sysSnaps map[sna
 		return err
 	}
 	var baseSnap *snap.Info
-	if is24plusSystem(model) {
-		// On UC24 the base is not mounted yet, peek into the file
+	if createSysrootMount() {
+		// On UC24+ the base is not mounted yet, peek into the file
 		baseSnap, err = readSnapInfoFromPath(sysSnaps[snap.TypeKernel].Path)
 	} else {
 		baseSnap, err = readSnapInfo(sysSnaps, snap.TypeBase)
@@ -1874,13 +1875,15 @@ func mountNonDataPartitionMatchingKernelDisk(dir, fallbacklabel string, opts *sy
 	return doSystemdMount(partSrc, dir, opts)
 }
 
-func is24plusSystem(model *asserts.Model) bool {
-	switch model.Base() {
-	case "core20", "core22", "core22-desktop":
-		return false
-	default:
-		return true
-	}
+func createSysrootMount() bool {
+	// This env var is set by snap-initramfs-mounts.service for 24+ initramfs. We
+	// prefer this to checking the model so 24+ kernels can run with models using
+	// older bases. Although this situation is not really supported as the
+	// initramfs systemd bits would not match those in the base, we allow it as
+	// it has been something done in the past and updates could break those
+	// systems.
+	isCore24plus := osGetenv("CORE24_PLUS_INITRAMFS")
+	return isCore24plus == "1" || isCore24plus == "true"
 }
 
 func generateMountsCommonInstallRecoverStart(mst *initramfsMountsState) (model *asserts.Model, sysSnaps map[snap.Type]*seed.Snap, err error) {
@@ -1943,7 +1946,7 @@ func generateMountsCommonInstallRecoverStart(mst *initramfsMountsState) (model *
 
 	for _, essentialSnap := range essSnaps {
 		systemSnaps[essentialSnap.EssentialType] = essentialSnap
-		if essentialSnap.EssentialType == snap.TypeBase && is24plusSystem(model) {
+		if essentialSnap.EssentialType == snap.TypeBase && createSysrootMount() {
 			// Create unit to mount directly to /sysroot. We restrict
 			// this to UC24+ for the moment, until we backport necessary
 			// changes to the UC20/22 initramfs. Note that a transient
@@ -2441,7 +2444,7 @@ func generateMountsModeRun(mst *initramfsMountsState) error {
 	//            code actually belongs in the bootloader implementation itself
 
 	typesToMount := typs
-	if is24plusSystem(model) {
+	if createSysrootMount() {
 		// Create unit for sysroot (mounts either base or rootfs). We
 		// restrict this to UC24+ for the moment, until we backport necessary
 		// changes to the UC20/22 initramfs. Note that a transient unit is
@@ -2517,7 +2520,7 @@ func generateMountsModeRun(mst *initramfsMountsState) error {
 		}
 	}
 
-	if is24plusSystem(model) {
+	if createSysrootMount() {
 		// Do a daemon reload so systemd knows about the new sysroot mount unit
 		// (populate-writable.service depends on sysroot.mount, we need to make
 		// sure systemd knows this unit before snap-initramfs-mounts.service

cmd/snap-bootstrap/cmd_initramfs_mounts_test.go

@@ -346,6 +346,9 @@ func (s *baseInitramfsMountsSuite) SetUpTest(c *C) {
 
 	s.tmpDir = c.MkDir()
 
+	restore = main.MockOsGetenv(func(envVar string) string { return "" })
+	s.AddCleanup(restore)
+
 	// mock /run/mnt
 	dirs.SetRootDir(s.tmpDir)
 	restore = func() { dirs.SetRootDir("") }
@@ -811,6 +814,12 @@ func (s *initramfsMountsSuite) testInitramfsMountsInstallModeWithCompsHappy(c *C
 	default:
 		c.Skip("Unknown EFI arch")
 	}
+	defer main.MockOsGetenv(func(envVar string) string {
+		if envVar == "CORE24_PLUS_INITRAMFS" {
+			return "1"
+		}
+		return ""
+	})()
 
 	var systemctlArgs [][]string
 	systemctlNumCalls := 0
@@ -3607,6 +3616,12 @@ func (s *initramfsMountsSuite) TestInitramfsMountsRecoverModeHappy(c *C) {
 		return nil, nil
 	})
 	defer systemctlMock()
+	defer main.MockOsGetenv(func(envVar string) string {
+		if envVar == "CORE24_PLUS_INITRAMFS" {
+			return "1"
+		}
+		return ""
+	})()
 
 	// setup a bootloader for setting the bootenv after we are done
 	bloader := bootloadertest.Mock("mock", c.MkDir())

cmd/snap-bootstrap/export_test.go

@@ -249,3 +249,11 @@ func MockBuildInstallObserver(f func(model *asserts.Model, gadgetDir string, use
 		installBuildInstallObserver = old
 	}
 }
+
+func MockOsGetenv(mock func(string) string) (restore func()) {
+	old := osGetenv
+	osGetenv = mock
+	return func() {
+		osGetenv = old
+	}
+}

core-initrd/latest/factory/usr/lib/systemd/system/snap-initramfs-mounts.service

@@ -13,6 +13,7 @@ After=dbus.socket
 [Service]
 Type=oneshot
 RemainAfterExit=true
+Environment="CORE24_PLUS_INITRAMFS=1"
 ExecStart=/usr/lib/snapd/snap-bootstrap initramfs-mounts
 StandardOutput=journal+console
 StandardError=journal+console