Allow sending and importing a host event fd.

PiperOrigin-RevId: 724046820

pkg/sentry/fsimpl/eventfd/eventfd.go

@@ -82,6 +82,25 @@ func New(ctx context.Context, vfsObj *vfs.VirtualFilesystem, initVal uint64, sem
 	return &efd.vfsfd, nil
 }
 
+// NewFromHost creates a new event fd from the given host fd.
+func NewFromHost(ctx context.Context, vfsObj *vfs.VirtualFilesystem, hostfd int, flags uint32) (*vfs.FileDescription, error) {
+	semMode := flags&linux.EFD_SEMAPHORE != 0
+	fileFlags := uint32(linux.O_RDWR)
+	if flags&linux.EFD_NONBLOCK != 0 {
+		fileFlags |= linux.O_NONBLOCK
+	}
+	fd, err := New(ctx, vfsObj, 0, semMode, flags)
+	if err != nil {
+		return nil, err
+	}
+	efd := fd.Impl().(*EventFileDescription)
+	efd.hostfd = hostfd
+	if err := fdnotifier.AddFD(int32(hostfd), &efd.queue); err != nil {
+		return nil, err
+	}
+	return fd, nil
+}
+
 // HostFD returns the host eventfd associated with this event.
 func (efd *EventFileDescription) HostFD() (int, error) {
 	efd.mu.Lock()

pkg/sentry/fsimpl/host/BUILD

@@ -42,6 +42,7 @@ go_library(
         "//pkg/refs",
         "//pkg/safemem",
         "//pkg/sentry/arch",
+        "//pkg/sentry/fsimpl/eventfd",
         "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/fsimpl/lock",
         "//pkg/sentry/fsutil",

pkg/sentry/fsimpl/host/host.go

@@ -30,6 +30,7 @@ import (
 	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/eventfd"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/hostfd"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -281,6 +282,11 @@ func NewFD(ctx context.Context, mnt *vfs.Mount, hostFD int, opts *NewFDOptions)
 	}
 
 	fileType := linux.FileMode(stat.Mode).FileType()
+	if fileType == 0 && isHostEventFdDevice(stat.Dev) {
+		// This is an event fd. No inode needed.
+		vfsObj := mnt.Filesystem().VirtualFilesystem()
+		return eventfd.NewFromHost(ctx, vfsObj, hostFD, flags)
+	}
 	i, err := newInode(ctx, fs, hostFD, opts.Savable, opts.RestoreKey, fileType, opts.IsTTY, opts.Readonly)
 	if err != nil {
 		return nil, err
@@ -1027,3 +1033,28 @@ func (f *fileDescription) Ioctl(ctx context.Context, uio usermem.IO, sysno uintp
 
 	return f.FileDescriptionDefaultImpl.Ioctl(ctx, uio, sysno, args)
 }
+
+// hostEventFdDevice is the host device that host event fds are associated
+// with. It is calculated once lazily.
+var hostEventFdDevice uint64
+var hostEventFdDeviceOnce sync.Once
+
+// isHostEventFdDevice initializes hostEventFdDevice and compares it to the
+// given host device id.
+func isHostEventFdDevice(dev uint64) bool {
+	hostEventFdDeviceOnce.Do(func() {
+		efd, _, err := unix.RawSyscall(unix.SYS_EVENTFD2, 0, 0, 0)
+		if err != 0 {
+			log.Warningf("failed to create dummy eventfd: %v. Importing eventfds will fail", error(err))
+			return
+		}
+		defer unix.Close(int(efd))
+		var stat unix.Stat_t
+		if err := unix.Fstat(int(efd), &stat); err != nil {
+			log.Warningf("failed to stat dummy eventfd: %v. Importing eventfds will fail", error(err))
+			return
+		}
+		hostEventFdDevice = stat.Dev
+	})
+	return dev == hostEventFdDevice
+}