Support multiple CDROMS in metadata

This checks if any CDROM in the system has metadata. Useful if
you boot off CDROM and also have a metadata CDROM.

Also switch the metadata container to read only, as it is only
writing to bind mounted in directories, and /tmp.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/metadata/Dockerfile

@@ -6,10 +6,13 @@ ENV GOPATH=/go PATH=$PATH:/go/bin
 COPY .  /go/src/metadata/
 RUN go-compile.sh /go/src/metadata
 
+RUN mkdir -p out/tmp out/var out/dev out/etc
+
 FROM scratch
 ENTRYPOINT []
 CMD []
 WORKDIR /
 COPY --from=mirror /go/bin/metadata /usr/bin/metadata
+COPY --from=mirror /out/ /
 CMD ["/usr/bin/metadata"]
-LABEL org.mobyproject.config='{"binds": ["/dev:/dev", "/var:/var", "/etc/resolv.conf:/etc/resolv.conf"], "capabilities": ["CAP_SYS_ADMIN"]}'
+LABEL org.mobyproject.config='{"binds": ["/dev:/dev", "/var:/var", "/etc/resolv.conf:/etc/resolv.conf"], "tmpfs": ["/tmp"], "readonly": true, "capabilities": ["CAP_SYS_ADMIN"]}'

pkg/metadata/main.go

@@ -14,17 +14,11 @@ const (
 	// ConfigPath is where the data is extracted to
 	ConfigPath = "/var/config"
 
-	// MountPoint is where the CDROM is mounted
-	MountPoint = "/cdrom"
-
 	// Hostname is the filename in configPath where the hostname is stored
 	Hostname = "hostname"
 
 	// SSH is the path where sshd configuration from the provider is stored
 	SSH = "ssh"
-
-	// TODO(rneugeba): Need to check this is the same everywhere
-	cdromDev = "/dev/sr0"
 )
 
 // Provider is a generic interface for metadata/userdata providers.
@@ -48,7 +42,7 @@ var cdromProviders []Provider
 
 func init() {
 	netProviders = []Provider{NewGCP(), NewVultr(), NewAWS()}
-	cdromProviders = []Provider{NewCDROM()}
+	cdromProviders = ListCDROMs()
 }
 
 func main() {
@@ -69,19 +63,8 @@ func main() {
 		}
 	}
 	if !found {
-		log.Printf("Trying CDROM")
-		if err := os.MkdirAll(MountPoint, 0755); err != nil {
-			log.Printf("CDROM: Failed to create %s: %s", MountPoint, err)
-			goto ErrorOut
-		}
-		if err := mountCDROM(cdromDev, MountPoint); err != nil {
-			log.Printf("Failed to mount cdrom: %s", err)
-			goto ErrorOut
-		}
-		defer syscall.Unmount(MountPoint, 0)
-		// Don't worry about removing MountPoint. We are in a container
-
 		for _, p = range cdromProviders {
+			log.Printf("Trying %s", p.String())
 			if p.Probe() {
 				log.Printf("%s: Probe succeeded", p)
 				userdata, err = p.Extract()
@@ -91,7 +74,6 @@ func main() {
 		}
 	}
 
-ErrorOut:
 	if !found {
 		log.Printf("No metadata/userdata found. Bye")
 		return
@@ -202,9 +184,3 @@ func processUserData(data []byte) error {
 
 	return nil
 }
-
-// mountCDROM mounts a CDROM/DVD device under mountPoint
-func mountCDROM(device, mountPoint string) error {
-	// We may need to poll a little for device ready
-	return syscall.Mount(device, mountPoint, "iso9660", syscall.MS_RDONLY, "")
-}

pkg/metadata/provider_cdrom.go

@@ -3,39 +3,73 @@ package main
 import (
 	"fmt"
 	"io/ioutil"
-	"os"
 	"path"
+	"path/filepath"
+	"syscall"
 )
 
 const (
 	configFile = "config"
+	cdromDevs  = "/dev/sr[0-9]*"
 )
 
 // ProviderCDROM is the type implementing the Provider interface for CDROMs
 // It looks for a file called 'configFile' in the root
 type ProviderCDROM struct {
+	device     string
+	mountPoint string
+	err        error
+	data       []byte
+}
+
+// ListCDROMs lists all the cdroms in the system
+func ListCDROMs() []Provider {
+	cdroms, err := filepath.Glob(cdromDevs)
+	if err != nil {
+		// Glob can only error on invalid pattern
+		panic(fmt.Sprintf("Invalid glob pattern: %s", cdromDevs))
+	}
+	providers := []Provider{}
+	for _, device := range cdroms {
+		providers = append(providers, NewCDROM(device))
+	}
+	return providers
 }
 
 // NewCDROM returns a new ProviderCDROM
-func NewCDROM() *ProviderCDROM {
-	return &ProviderCDROM{}
+func NewCDROM(device string) *ProviderCDROM {
+	mountPoint, err := ioutil.TempDir("", "cd")
+	p := ProviderCDROM{device, mountPoint, err, []byte{}}
+	if err == nil {
+		if p.err = p.mount(); p.err == nil {
+			p.data, p.err = ioutil.ReadFile(path.Join(p.mountPoint, configFile))
+			p.unmount()
+		}
+	}
+	return &p
 }
 
 func (p *ProviderCDROM) String() string {
-	return "CDROM"
+	return "CDROM " + p.device
 }
 
 // Probe checks if the CD has the right file
 func (p *ProviderCDROM) Probe() bool {
-	_, err := os.Stat(path.Join(MountPoint, configFile))
-	return (!os.IsNotExist(err))
+	return len(p.data) != 0
 }
 
 // Extract gets both the CDROM specific and generic userdata
 func (p *ProviderCDROM) Extract() ([]byte, error) {
-	data, err := ioutil.ReadFile(path.Join(MountPoint, configFile))
-	if err != nil {
-		return nil, fmt.Errorf("CDROM: Error reading file: %s", err)
-	}
-	return data, nil
+	return p.data, p.err
+}
+
+// mount mounts a CDROM/DVD device under mountPoint
+func (p *ProviderCDROM) mount() error {
+	// We may need to poll a little for device ready
+	return syscall.Mount(p.device, p.mountPoint, "iso9660", syscall.MS_RDONLY, "")
+}
+
+// unmount removes the mount
+func (p *ProviderCDROM) unmount() {
+	_ = syscall.Unmount(p.mountPoint, 0)
 }