Check if root= is a regular file and if yes create a loopback device

... which allows to mount this file.

Such functionality is useful in case if we have a erofs image inside an ISO.

The erofs image cannot be mounted directly, thus we need to represent it as a device file first.

Implements: #232

docs/manpage.md

@@ -142,6 +142,7 @@ Device reference has one of the following values:
 
  * `/dev/XXX` path to specific device file, it can be either a path to real device/partition like `/dev/sda1`, `/dev/nvme0n1` or path to dm-mapper virtual device like
    `/dev/mapper/root` or `/dev/vg_mesos/lv_mesos_containers`.
+ * If the path does not start with `/dev/` and represents a regular file then booster will assume it is an image file of the root filesystem (e.g. erofs inside of an ISO). Booster will try to setup a loopback device and then mount it as a root filesystem.
  * `UUID=$UUID` or `/dev/disk/by-uuid/$UUID` references device by its filesystem/LUKS UUID. See notes about UUID formatting rules below.
  * `LABEL=$LABEL` or `/dev/disk/by-label/$LABEL` references device by its filesystem/LUKS label.
  * `PARTUUID=$UUID` or `/dev/disk/by-partuuid/$UUID` references device by GPT partition UUID.

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/anatol/tang.go v0.0.0-20220716012326-2fda8b4503da
 	github.com/anatol/vmtest v0.0.0-20230711210602-87511df0d4bc
 	github.com/cavaliergopher/cpio v1.0.1
+	github.com/freddierice/go-losetup/v2 v2.0.1
 	github.com/google/go-tpm v0.9.0
 	github.com/google/renameio/v2 v2.0.0
 	github.com/insomniacslk/dhcp v0.0.0-20230612134759-b20c9ba983df

go.sum

@@ -22,6 +22,8 @@ github.com/decred/dcrd/dcrec/secp256k1/v4 v4.2.0 h1:8UrgZ3GkP4i/CLijOJx79Yu+etly
 github.com/decred/dcrd/dcrec/secp256k1/v4 v4.2.0/go.mod h1:v57UDF4pDQJcEfFUCRop3lJL149eHGSe9Jvczhzjo/0=
 github.com/dgryski/go-camellia v0.0.0-20191119043421-69a8a13fb23d h1:CPqTNIigGweVPT4CYb+OO2E6XyRKFOmvTHwWRLgCAlE=
 github.com/dgryski/go-camellia v0.0.0-20191119043421-69a8a13fb23d/go.mod h1:QX5ZVULjAfZJux/W62Y91HvCh9hyW6enAwcrrv/sLj0=
+github.com/freddierice/go-losetup/v2 v2.0.1 h1:wPDx/Elu9nDV8y/CvIbEDz5Xi5Zo80y4h7MKbi3XaAI=
+github.com/freddierice/go-losetup/v2 v2.0.1/go.mod h1:TEyBrvlOelsPEhfWD5rutNXDmUszBXuFnwT1kIQF4J8=
 github.com/goccy/go-json v0.10.2 h1:CrxCmQqYDkv1z7lO7Wbh2HN93uovUHgrECaO5ZrCXAU=
 github.com/goccy/go-json v0.10.2/go.mod h1:6MelG93GURQebXPDq3khkgXZkazVtN9CRI+MGFi0w8I=
 github.com/google/go-tpm v0.9.0 h1:sQF6YqWMi+SCXpsmS3fd21oPy/vSddwZry4JnmltHVk=

init/blkinfo_test.go

@@ -62,11 +62,6 @@ func shell(script string, env ...string) error {
 	return sh.Run()
 }
 
-func fileExists(file string) bool {
-	_, err := os.Stat(file)
-	return err == nil
-}
-
 func TestBlkInfoFAT(t *testing.T) {
 	checkFs(t, "fat", "fat", "2a341c62", "FATLBL", 10, "mkfs.vfat -F32 -n $LABEL -i $UUID $OUTPUT", nil)
 }

init/main.go

@@ -15,6 +15,7 @@ import (
 	"time"
 	"unsafe"
 
+	"github.com/freddierice/go-losetup/v2"
 	"github.com/yookoala/realpath"
 	"golang.org/x/sys/unix"
 	"gopkg.in/yaml.v3"
@@ -856,6 +857,23 @@ func boost() error {
 		}
 	}
 
+	if cmdRoot.format == refPath {
+		rootFS := cmdRoot.data.(string)
+		if !strings.HasPrefix(rootFS, "/dev/") && fileExists(rootFS) {
+			// there is a special case when a root filesystem is a filesystem image rather than a device (e.g. in case of erofs inside of an ISO)
+			// so if it is just a regular file then we create a loopback device for it and then try to mount
+			dev, err := losetup.Attach(rootFS, 0, rootRw)
+			if err != nil {
+				return err
+			}
+			cmdRoot.data = dev.Path()
+
+			if err := addBlockDevice(dev.Path(), true, nil); err != nil {
+				return err
+			}
+		}
+	}
+
 	if config.MountTimeout != 0 {
 		// TODO: cancellable, timeout context?
 		timeout := waitTimeout(&rootMounted, time.Duration(config.MountTimeout)*time.Second)

init/util.go

@@ -174,3 +174,8 @@ func unwrapExitError(err error) error {
 	}
 	return err
 }
+
+func fileExists(file string) bool {
+	_, err := os.Stat(file)
+	return err == nil
+}