Add VFS.GetDynamicCharDevMajor().

Updates #14

PiperOrigin-RevId: 529803365

pkg/sentry/vfs/device.go

@@ -105,6 +105,35 @@ func (vfs *VirtualFilesystem) OpenDeviceSpecialFile(ctx context.Context, mnt *Mo
 	return rd.dev.Open(ctx, mnt, d, *opts)
 }
 
+// GetDynamicCharDevMajor allocates and returns an unused major device number
+// for a character device or set of character devices.
+func (vfs *VirtualFilesystem) GetDynamicCharDevMajor() (uint32, error) {
+	vfs.dynCharDevMajorMu.Lock()
+	defer vfs.dynCharDevMajorMu.Unlock()
+	// Compare Linux's fs/char_dev.c:find_dynamic_major().
+	for major := uint32(254); major >= 234; major-- {
+		if _, ok := vfs.dynCharDevMajorUsed[major]; !ok {
+			vfs.dynCharDevMajorUsed[major] = struct{}{}
+			return major, nil
+		}
+	}
+	for major := uint32(511); major >= 384; major-- {
+		if _, ok := vfs.dynCharDevMajorUsed[major]; !ok {
+			vfs.dynCharDevMajorUsed[major] = struct{}{}
+			return major, nil
+		}
+	}
+	return 0, linuxerr.EBUSY
+}
+
+// PutDynamicCharDevMajor deallocates a major device number returned by a
+// previous call to GetDynamicCharDevMajor.
+func (vfs *VirtualFilesystem) PutDynamicCharDevMajor(major uint32) {
+	vfs.dynCharDevMajorMu.Lock()
+	defer vfs.dynCharDevMajorMu.Unlock()
+	delete(vfs.dynCharDevMajorUsed, major)
+}
+
 // GetAnonBlockDevMinor allocates and returns an unused minor device number for
 // an "anonymous" block device with major number UNNAMED_MAJOR.
 func (vfs *VirtualFilesystem) GetAnonBlockDevMinor() (uint32, error) {

pkg/sentry/vfs/vfs.go

@@ -116,6 +116,11 @@ type VirtualFilesystem struct {
 	devicesMu sync.RWMutex `state:"nosave"`
 	devices   map[devTuple]*registeredDevice
 
+	// dynCharDevMajorUsed contains all allocated dynamic character device
+	// major numbers. dynCharDevMajor is protected by dynCharDevMajorMu.
+	dynCharDevMajorMu   sync.Mutex `state:"nosave"`
+	dynCharDevMajorUsed map[uint32]struct{}
+
 	// anonBlockDevMinor contains all allocated anonymous block device minor
 	// numbers. anonBlockDevMinorNext is a lower bound for the smallest
 	// unallocated anonymous block device number. anonBlockDevMinorNext and
@@ -149,6 +154,7 @@ func (vfs *VirtualFilesystem) Init(ctx context.Context) error {
 	}
 	vfs.mountpoints = make(map[*Dentry]map[*Mount]struct{})
 	vfs.devices = make(map[devTuple]*registeredDevice)
+	vfs.dynCharDevMajorUsed = make(map[uint32]struct{})
 	vfs.anonBlockDevMinorNext = 1
 	vfs.anonBlockDevMinor = make(map[uint32]struct{})
 	vfs.fsTypes = make(map[string]*registeredFilesystemType)