Add anonymous device number allocation to VFS2.

Note that in VFS2, filesystem device numbers are per-vfs.FilesystemImpl rather
than global, avoiding the need for a "registry" type to handle save/restore.
(This is more consistent with Linux anyway: compare e.g.
mm/shmem.c:shmem_mount() => fs/super.c:mount_nodev() => (indirectly)
set_anon_super().)

PiperOrigin-RevId: 291425193

pkg/sentry/vfs/device.go

@@ -98,3 +98,32 @@ func (vfs *VirtualFilesystem) OpenDeviceSpecialFile(ctx context.Context, mnt *Mo
 	}
 	return rd.dev.Open(ctx, mnt, d, *opts)
 }
+
+// GetAnonBlockDevMinor allocates and returns an unused minor device number for
+// an "anonymous" block device with major number 0.
+func (vfs *VirtualFilesystem) GetAnonBlockDevMinor() (uint32, error) {
+	vfs.anonBlockDevMinorMu.Lock()
+	defer vfs.anonBlockDevMinorMu.Unlock()
+	minor := vfs.anonBlockDevMinorNext
+	const maxDevMinor = (1 << 20) - 1
+	for minor < maxDevMinor {
+		if _, ok := vfs.anonBlockDevMinor[minor]; !ok {
+			vfs.anonBlockDevMinor[minor] = struct{}{}
+			vfs.anonBlockDevMinorNext = minor + 1
+			return minor, nil
+		}
+		minor++
+	}
+	return 0, syserror.EMFILE
+}
+
+// PutAnonBlockDevMinor deallocates a minor device number returned by a
+// previous call to GetAnonBlockDevMinor.
+func (vfs *VirtualFilesystem) PutAnonBlockDevMinor(minor uint32) {
+	vfs.anonBlockDevMinorMu.Lock()
+	defer vfs.anonBlockDevMinorMu.Unlock()
+	delete(vfs.anonBlockDevMinor, minor)
+	if minor < vfs.anonBlockDevMinorNext {
+		vfs.anonBlockDevMinorNext = minor
+	}
+}

pkg/sentry/vfs/vfs.go

@@ -80,6 +80,14 @@ type VirtualFilesystem struct {
 	devicesMu sync.RWMutex
 	devices   map[devTuple]*registeredDevice
 
+	// anonBlockDevMinor contains all allocated anonymous block device minor
+	// numbers. anonBlockDevMinorNext is a lower bound for the smallest
+	// unallocated anonymous block device number. anonBlockDevMinorNext and
+	// anonBlockDevMinor are protected by anonBlockDevMinorMu.
+	anonBlockDevMinorMu   sync.Mutex
+	anonBlockDevMinorNext uint32
+	anonBlockDevMinor     map[uint32]struct{}
+
 	// fsTypes contains all registered FilesystemTypes. fsTypes is protected by
 	// fsTypesMu.
 	fsTypesMu sync.RWMutex
@@ -94,10 +102,12 @@ type VirtualFilesystem struct {
 // New returns a new VirtualFilesystem with no mounts or FilesystemTypes.
 func New() *VirtualFilesystem {
 	vfs := &VirtualFilesystem{
-		mountpoints: make(map[*Dentry]map[*Mount]struct{}),
-		devices:     make(map[devTuple]*registeredDevice),
-		fsTypes:     make(map[string]*registeredFilesystemType),
-		filesystems: make(map[*Filesystem]struct{}),
+		mountpoints:           make(map[*Dentry]map[*Mount]struct{}),
+		devices:               make(map[devTuple]*registeredDevice),
+		anonBlockDevMinorNext: 1,
+		anonBlockDevMinor:     make(map[uint32]struct{}),
+		fsTypes:               make(map[string]*registeredFilesystemType),
+		filesystems:           make(map[*Filesystem]struct{}),
 	}
 	vfs.mounts.Init()
 	return vfs