Plumb VFS2 imported fds into virtual filesystem.

- When setting up the virtual filesystem, mount a host.filesystem to contain
  all files that need to be imported.
- Make read/preadv syscalls to the host in cases where preadv2 may not be
  supported yet (likewise for writing).
- Make save/restore functions in kernel/kernel.go return early if vfs2 is
  enabled.

PiperOrigin-RevId: 300922353

11 files changed, 246 insertions, 70 deletions

diff --git a/pkg/abi/linux/file.go b/pkg/abi/linux/file.go
index e229ac21c..dbe58acbe 100644
--- a/pkg/abi/linux/file.go
+++ b/pkg/abi/linux/file.go
@@ -266,6 +266,9 @@ type Statx struct {
 	DevMinor       uint32
 }
 
+// SizeOfStatx is the size of a Statx struct.
+var SizeOfStatx = binary.Size(Statx{})
+
 // FileMode represents a mode_t.
 type FileMode uint16
 
diff --git a/pkg/sentry/fs/host/control.go b/pkg/sentry/fs/host/control.go
index 1658979fc..cd84e1337 100644
--- a/pkg/sentry/fs/host/control.go
+++ b/pkg/sentry/fs/host/control.go
@@ -32,6 +32,8 @@ func newSCMRights(fds []int) control.SCMRights {
 }
 
 // Files implements control.SCMRights.Files.
+//
+// TODO(gvisor.dev/issue/2017): Port to VFS2.
 func (c *scmRights) Files(ctx context.Context, max int) (control.RightsFiles, bool) {
 	n := max
 	var trunc bool
diff --git a/pkg/sentry/fsimpl/host/BUILD b/pkg/sentry/fsimpl/host/BUILD
index 731f192b3..5d67f88e3 100644
--- a/pkg/sentry/fsimpl/host/BUILD
+++ b/pkg/sentry/fsimpl/host/BUILD
@@ -9,9 +9,11 @@ go_library(
         "host.go",
         "util.go",
     ],
+    visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",
         "//pkg/context",
+        "//pkg/fd",
         "//pkg/log",
         "//pkg/refs",
         "//pkg/safemem",
diff --git a/pkg/sentry/fsimpl/host/default_file.go b/pkg/sentry/fsimpl/host/default_file.go
index 172cdb161..98682ba5e 100644
--- a/pkg/sentry/fsimpl/host/default_file.go
+++ b/pkg/sentry/fsimpl/host/default_file.go
@@ -21,6 +21,7 @@ import (
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fd"
 	"gvisor.dev/gvisor/pkg/safemem"
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
@@ -64,9 +65,7 @@ func (f *defaultFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts v
 			panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped")
 		}
 
-		f.mu.Lock()
 		n, err := readFromHostFD(ctx, f.inode.hostFD, dst, -1, int(opts.Flags))
-		f.mu.Unlock()
 		if isBlockError(err) {
 			// If we got any data at all, return it as a "completed" partial read
 			// rather than retrying until complete.
@@ -86,16 +85,22 @@ func (f *defaultFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts v
 	return n, err
 }
 
-func readFromHostFD(ctx context.Context, fd int, dst usermem.IOSequence, offset int64, flags int) (int64, error) {
-	if flags&^(linux.RWF_VALID) != 0 {
+func readFromHostFD(ctx context.Context, hostFD int, dst usermem.IOSequence, offset int64, flags int) (int64, error) {
+	// TODO(gvisor.dev/issue/1672): Support select preadv2 flags.
+	if flags != 0 {
 		return 0, syserror.EOPNOTSUPP
 	}
 
-	reader := safemem.FromVecReaderFunc{
-		func(srcs [][]byte) (int64, error) {
-			n, err := unix.Preadv2(fd, srcs, offset, flags)
-			return int64(n), err
-		},
+	var reader safemem.Reader
+	if offset == -1 {
+		reader = safemem.FromIOReader{fd.NewReadWriter(hostFD)}
+	} else {
+		reader = safemem.FromVecReaderFunc{
+			func(srcs [][]byte) (int64, error) {
+				n, err := unix.Preadv(hostFD, srcs, offset)
+				return int64(n), err
+			},
+		}
 	}
 	n, err := dst.CopyOutFrom(ctx, reader)
 	return int64(n), err
@@ -120,9 +125,7 @@ func (f *defaultFileFD) Write(ctx context.Context, src usermem.IOSequence, opts
 			panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped")
 		}
 
-		f.mu.Lock()
 		n, err := writeToHostFD(ctx, f.inode.hostFD, src, -1, int(opts.Flags))
-		f.mu.Unlock()
 		if isBlockError(err) {
 			err = syserror.ErrWouldBlock
 		}
@@ -137,16 +140,22 @@ func (f *defaultFileFD) Write(ctx context.Context, src usermem.IOSequence, opts
 	return n, err
 }
 
-func writeToHostFD(ctx context.Context, fd int, src usermem.IOSequence, offset int64, flags int) (int64, error) {
-	if flags&^(linux.RWF_VALID) != 0 {
+func writeToHostFD(ctx context.Context, hostFD int, src usermem.IOSequence, offset int64, flags int) (int64, error) {
+	// TODO(gvisor.dev/issue/1672): Support select pwritev2 flags.
+	if flags != 0 {
 		return 0, syserror.EOPNOTSUPP
 	}
 
-	writer := safemem.FromVecWriterFunc{
-		func(srcs [][]byte) (int64, error) {
-			n, err := unix.Pwritev2(fd, srcs, offset, flags)
-			return int64(n), err
-		},
+	var writer safemem.Writer
+	if offset == -1 {
+		writer = safemem.FromIOWriter{fd.NewReadWriter(hostFD)}
+	} else {
+		writer = safemem.FromVecWriterFunc{
+			func(srcs [][]byte) (int64, error) {
+				n, err := unix.Pwritev(hostFD, srcs, offset)
+				return int64(n), err
+			},
+		}
 	}
 	n, err := src.CopyInTo(ctx, writer)
 	return int64(n), err
diff --git a/pkg/sentry/fsimpl/host/host.go b/pkg/sentry/fsimpl/host/host.go
index c205e6a0b..0be812d13 100644
--- a/pkg/sentry/fsimpl/host/host.go
+++ b/pkg/sentry/fsimpl/host/host.go
@@ -38,10 +38,19 @@ type filesystem struct {
 	kernfs.Filesystem
 }
 
+// NewMount returns a new disconnected mount in vfsObj that may be passed to ImportFD.
+func NewMount(vfsObj *vfs.VirtualFilesystem) (*vfs.Mount, error) {
+	fs := &filesystem{}
+	fs.Init(vfsObj)
+	vfsfs := fs.VFSFilesystem()
+	// NewDisconnectedMount will take an additional reference on vfsfs.
+	defer vfsfs.DecRef()
+	return vfsObj.NewDisconnectedMount(vfsfs, nil, &vfs.MountOptions{})
+}
+
 // ImportFD sets up and returns a vfs.FileDescription from a donated fd.
 func ImportFD(mnt *vfs.Mount, hostFD int, ownerUID auth.KUID, ownerGID auth.KGID, isTTY bool) (*vfs.FileDescription, error) {
-	// Must be importing to a mount of host.filesystem.
-	fs, ok := mnt.Filesystem().Impl().(*filesystem)
+	fs, ok := mnt.Filesystem().Impl().(*kernfs.Filesystem)
 	if !ok {
 		return nil, fmt.Errorf("can't import host FDs into filesystems of type %T", mnt.Filesystem().Impl())
 	}
@@ -54,8 +63,7 @@ func ImportFD(mnt *vfs.Mount, hostFD int, ownerUID auth.KUID, ownerGID auth.KGID
 
 	fileMode := linux.FileMode(s.Mode)
 	fileType := fileMode.FileType()
-	// Pipes, character devices, and sockets can return EWOULDBLOCK for
-	// operations that would block.
+	// Pipes, character devices, and sockets.
 	isStream := fileType == syscall.S_IFIFO || fileType == syscall.S_IFCHR || fileType == syscall.S_IFSOCK
 
 	i := &inode{
@@ -143,11 +151,109 @@ func (i *inode) Mode() linux.FileMode {
 
 // Stat implements kernfs.Inode.
 func (i *inode) Stat(_ *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error) {
+	if opts.Mask&linux.STATX__RESERVED != 0 {
+		return linux.Statx{}, syserror.EINVAL
+	}
+	if opts.Sync&linux.AT_STATX_SYNC_TYPE == linux.AT_STATX_SYNC_TYPE {
+		return linux.Statx{}, syserror.EINVAL
+	}
+
+	// Limit our host call only to known flags.
+	mask := opts.Mask & linux.STATX_ALL
 	var s unix.Statx_t
-	if err := unix.Statx(i.hostFD, "", int(unix.AT_EMPTY_PATH|opts.Sync), int(opts.Mask), &s); err != nil {
+	err := unix.Statx(i.hostFD, "", int(unix.AT_EMPTY_PATH|opts.Sync), int(mask), &s)
+	// Fallback to fstat(2), if statx(2) is not supported on the host.
+	//
+	// TODO(b/151263641): Remove fallback.
+	if err == syserror.ENOSYS {
+		return i.fstat(opts)
+	} else if err != nil {
+		return linux.Statx{}, err
+	}
+
+	ls := linux.Statx{Mask: mask}
+	// Unconditionally fill blksize, attributes, and device numbers, as indicated
+	// by /include/uapi/linux/stat.h.
+	//
+	// RdevMajor/RdevMinor are left as zero, so as not to expose host device
+	// numbers.
+	//
+	// TODO(gvisor.dev/issue/1672): Use kernfs-specific, internally defined
+	// device numbers. If we use the device number from the host, it may collide
+	// with another sentry-internal device number. We handle device/inode
+	// numbers without relying on the host to prevent collisions.
+	ls.Blksize = s.Blksize
+	ls.Attributes = s.Attributes
+	ls.AttributesMask = s.Attributes_mask
+
+	if mask|linux.STATX_TYPE != 0 {
+		ls.Mode |= s.Mode & linux.S_IFMT
+	}
+	if mask|linux.STATX_MODE != 0 {
+		ls.Mode |= s.Mode &^ linux.S_IFMT
+	}
+	if mask|linux.STATX_NLINK != 0 {
+		ls.Nlink = s.Nlink
+	}
+	if mask|linux.STATX_ATIME != 0 {
+		ls.Atime = unixToLinuxStatxTimestamp(s.Atime)
+	}
+	if mask|linux.STATX_BTIME != 0 {
+		ls.Btime = unixToLinuxStatxTimestamp(s.Btime)
+	}
+	if mask|linux.STATX_CTIME != 0 {
+		ls.Ctime = unixToLinuxStatxTimestamp(s.Ctime)
+	}
+	if mask|linux.STATX_MTIME != 0 {
+		ls.Mtime = unixToLinuxStatxTimestamp(s.Mtime)
+	}
+	if mask|linux.STATX_SIZE != 0 {
+		ls.Size = s.Size
+	}
+	if mask|linux.STATX_BLOCKS != 0 {
+		ls.Blocks = s.Blocks
+	}
+
+	// Use our own internal inode number and file owner.
+	if mask|linux.STATX_INO != 0 {
+		ls.Ino = i.ino
+	}
+	if mask|linux.STATX_UID != 0 {
+		ls.UID = uint32(i.uid)
+	}
+	if mask|linux.STATX_GID != 0 {
+		ls.GID = uint32(i.gid)
+	}
+
+	return ls, nil
+}
+
+// fstat is a best-effort fallback for inode.Stat() if the host does not
+// support statx(2).
+//
+// We ignore the mask and sync flags in opts and simply supply
+// STATX_BASIC_STATS, as fstat(2) itself does not allow the specification
+// of a mask or sync flags. fstat(2) does not provide any metadata
+// equivalent to Statx.Attributes, Statx.AttributesMask, or Statx.Btime, so
+// those fields remain empty.
+func (i *inode) fstat(opts vfs.StatOptions) (linux.Statx, error) {
+	var s unix.Stat_t
+	if err := unix.Fstat(i.hostFD, &s); err != nil {
 		return linux.Statx{}, err
 	}
-	ls := unixToLinuxStatx(s)
+
+	// Note that rdev numbers are left as 0; do not expose host device numbers.
+	ls := linux.Statx{
+		Mask:    linux.STATX_BASIC_STATS,
+		Blksize: uint32(s.Blksize),
+		Nlink:   uint32(s.Nlink),
+		Mode:    uint16(s.Mode),
+		Size:    uint64(s.Size),
+		Blocks:  uint64(s.Blocks),
+		Atime:   timespecToStatxTimestamp(s.Atim),
+		Ctime:   timespecToStatxTimestamp(s.Ctim),
+		Mtime:   timespecToStatxTimestamp(s.Mtim),
+	}
 
 	// Use our own internal inode number and file owner.
 	//
@@ -159,9 +265,6 @@ func (i *inode) Stat(_ *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, erro
 	ls.UID = uint32(i.uid)
 	ls.GID = uint32(i.gid)
 
-	// Update file mode from the host.
-	i.mode = linux.FileMode(ls.Mode)
-
 	return ls, nil
 }
 
@@ -217,7 +320,6 @@ func (i *inode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptio
 }
 
 func (i *inode) open(d *vfs.Dentry, mnt *vfs.Mount) (*vfs.FileDescription, error) {
-
 	fileType := i.mode.FileType()
 	if fileType == syscall.S_IFSOCK {
 		if i.isTTY {
@@ -227,6 +329,8 @@ func (i *inode) open(d *vfs.Dentry, mnt *vfs.Mount) (*vfs.FileDescription, error
 		return nil, errors.New("importing host sockets not supported")
 	}
 
+	// TODO(gvisor.dev/issue/1672): Whitelist specific file types here, so that
+	// we don't allow importing arbitrary file types without proper support.
 	if i.isTTY {
 		// TODO(gvisor.dev/issue/1672): support importing host fd as TTY.
 		return nil, errors.New("importing host fd as TTY not supported")
diff --git a/pkg/sentry/fsimpl/host/util.go b/pkg/sentry/fsimpl/host/util.go
index e1ccacb4d..d519feef5 100644
--- a/pkg/sentry/fsimpl/host/util.go
+++ b/pkg/sentry/fsimpl/host/util.go
@@ -35,34 +35,14 @@ func toTimespec(ts linux.StatxTimestamp, omit bool) unix.Timespec {
 	}
 }
 
-func unixToLinuxStatx(s unix.Statx_t) linux.Statx {
-	return linux.Statx{
-		Mask:           s.Mask,
-		Blksize:        s.Blksize,
-		Attributes:     s.Attributes,
-		Nlink:          s.Nlink,
-		UID:            s.Uid,
-		GID:            s.Gid,
-		Mode:           s.Mode,
-		Ino:            s.Ino,
-		Size:           s.Size,
-		Blocks:         s.Blocks,
-		AttributesMask: s.Attributes_mask,
-		Atime:          unixToLinuxStatxTimestamp(s.Atime),
-		Btime:          unixToLinuxStatxTimestamp(s.Btime),
-		Ctime:          unixToLinuxStatxTimestamp(s.Ctime),
-		Mtime:          unixToLinuxStatxTimestamp(s.Mtime),
-		RdevMajor:      s.Rdev_major,
-		RdevMinor:      s.Rdev_minor,
-		DevMajor:       s.Dev_major,
-		DevMinor:       s.Dev_minor,
-	}
-}
-
 func unixToLinuxStatxTimestamp(ts unix.StatxTimestamp) linux.StatxTimestamp {
 	return linux.StatxTimestamp{Sec: ts.Sec, Nsec: ts.Nsec}
 }
 
+func timespecToStatxTimestamp(ts unix.Timespec) linux.StatxTimestamp {
+	return linux.StatxTimestamp{Sec: int64(ts.Sec), Nsec: uint32(ts.Nsec)}
+}
+
 // wouldBlock returns true for file types that can return EWOULDBLOCK
 // for blocking operations, e.g. pipes, character devices, and sockets.
 func wouldBlock(fileType uint32) bool {
diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 1d627564f..6feda8fa1 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -467,6 +467,11 @@ func (k *Kernel) flushMountSourceRefs() error {
 //
 // Precondition: Must be called with the kernel paused.
 func (ts *TaskSet) forEachFDPaused(f func(*fs.File, *vfs.FileDescription) error) (err error) {
+	// TODO(gvisor.dev/issue/1663): Add save support for VFS2.
+	if VFS2Enabled {
+		return nil
+	}
+
 	ts.mu.RLock()
 	defer ts.mu.RUnlock()
 	for t := range ts.Root.tids {
@@ -484,7 +489,7 @@ func (ts *TaskSet) forEachFDPaused(f func(*fs.File, *vfs.FileDescription) error)
 }
 
 func (ts *TaskSet) flushWritesToFiles(ctx context.Context) error {
-	// TODO(gvisor.dev/issues/1663): Add save support for VFS2.
+	// TODO(gvisor.dev/issue/1663): Add save support for VFS2.
 	return ts.forEachFDPaused(func(file *fs.File, _ *vfs.FileDescription) error {
 		if flags := file.Flags(); !flags.Write {
 			return nil
@@ -533,6 +538,11 @@ func (k *Kernel) invalidateUnsavableMappings(ctx context.Context) error {
 }
 
 func (ts *TaskSet) unregisterEpollWaiters() {
+	// TODO(gvisor.dev/issue/1663): Add save support for VFS2.
+	if VFS2Enabled {
+		return
+	}
+
 	ts.mu.RLock()
 	defer ts.mu.RUnlock()
 	for t := range ts.Root.tids {
@@ -1005,11 +1015,14 @@ func (k *Kernel) pauseTimeLocked() {
 		// This means we'll iterate FDTables shared by multiple tasks repeatedly,
 		// but ktime.Timer.Pause is idempotent so this is harmless.
 		if t.fdTable != nil {
-			t.fdTable.forEach(func(_ int32, file *fs.File, _ *vfs.FileDescription, _ FDFlags) {
-				if tfd, ok := file.FileOperations.(*timerfd.TimerOperations); ok {
-					tfd.PauseTimer()
-				}
-			})
+			// TODO(gvisor.dev/issue/1663): Add save support for VFS2.
+			if !VFS2Enabled {
+				t.fdTable.forEach(func(_ int32, file *fs.File, _ *vfs.FileDescription, _ FDFlags) {
+					if tfd, ok := file.FileOperations.(*timerfd.TimerOperations); ok {
+						tfd.PauseTimer()
+					}
+				})
+			}
 		}
 	}
 	k.timekeeper.PauseUpdates()
@@ -1034,12 +1047,15 @@ func (k *Kernel) resumeTimeLocked() {
 				it.ResumeTimer()
 			}
 		}
-		if t.fdTable != nil {
-			t.fdTable.forEach(func(_ int32, file *fs.File, _ *vfs.FileDescription, _ FDFlags) {
-				if tfd, ok := file.FileOperations.(*timerfd.TimerOperations); ok {
-					tfd.ResumeTimer()
-				}
-			})
+		// TODO(gvisor.dev/issue/1663): Add save support for VFS2.
+		if !VFS2Enabled {
+			if t.fdTable != nil {
+				t.fdTable.forEach(func(_ int32, file *fs.File, _ *vfs.FileDescription, _ FDFlags) {
+					if tfd, ok := file.FileOperations.(*timerfd.TimerOperations); ok {
+						tfd.ResumeTimer()
+					}
+				})
+			}
 		}
 	}
 }
diff --git a/pkg/sentry/syscalls/linux/sys_stat.go b/pkg/sentry/syscalls/linux/sys_stat.go
index 9bd2df104..a11a87cd1 100644
--- a/pkg/sentry/syscalls/linux/sys_stat.go
+++ b/pkg/sentry/syscalls/linux/sys_stat.go
@@ -136,7 +136,10 @@ func Statx(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 	mask := args[3].Uint()
 	statxAddr := args[4].Pointer()
 
-	if mask&linux.STATX__RESERVED > 0 {
+	if mask&linux.STATX__RESERVED != 0 {
+		return 0, nil, syserror.EINVAL
+	}
+	if flags&^(linux.AT_SYMLINK_NOFOLLOW|linux.AT_EMPTY_PATH|linux.AT_STATX_SYNC_TYPE) != 0 {
 		return 0, nil, syserror.EINVAL
 	}
 	if flags&linux.AT_STATX_SYNC_TYPE == linux.AT_STATX_SYNC_TYPE {
diff --git a/pkg/sentry/syscalls/linux/vfs2/stat.go b/pkg/sentry/syscalls/linux/vfs2/stat.go
index a74ea6fd5..97eaedd66 100644
--- a/pkg/sentry/syscalls/linux/vfs2/stat.go
+++ b/pkg/sentry/syscalls/linux/vfs2/stat.go
@@ -150,7 +150,11 @@ func Statx(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscall
 	mask := args[3].Uint()
 	statxAddr := args[4].Pointer()
 
-	if flags&^(linux.AT_EMPTY_PATH|linux.AT_SYMLINK_NOFOLLOW) != 0 {
+	if flags&^(linux.AT_EMPTY_PATH|linux.AT_SYMLINK_NOFOLLOW|linux.AT_STATX_SYNC_TYPE) != 0 {
+		return 0, nil, syserror.EINVAL
+	}
+
+	if mask&linux.STATX__RESERVED != 0 {
 		return 0, nil, syserror.EINVAL
 	}
 
diff --git a/runsc/boot/filter/config.go b/runsc/boot/filter/config.go
index a4627905e..f459d1973 100644
--- a/runsc/boot/filter/config.go
+++ b/runsc/boot/filter/config.go
@@ -284,6 +284,7 @@ var allowedSyscalls = seccomp.SyscallRules{
 		{seccomp.AllowAny{}, seccomp.AllowValue(syscall.SHUT_RDWR)},
 	},
 	syscall.SYS_SIGALTSTACK:     {},
+	unix.SYS_STATX:              {},
 	syscall.SYS_SYNC_FILE_RANGE: {},
 	syscall.SYS_TGKILL: []seccomp.Rule{
 		{
diff --git a/test/syscalls/linux/stat.cc b/test/syscalls/linux/stat.cc
index c951ac3b3..513b9cd1c 100644
--- a/test/syscalls/linux/stat.cc
+++ b/test/syscalls/linux/stat.cc
@@ -607,7 +607,7 @@ int statx(int dirfd, const char* pathname, int flags, unsigned int mask,
 }
 
 TEST_F(StatTest, StatxAbsPath) {
-  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, 0) < 0 &&
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
           errno == ENOSYS);
 
   struct kernel_statx stx;
@@ -617,7 +617,7 @@ TEST_F(StatTest, StatxAbsPath) {
 }
 
 TEST_F(StatTest, StatxRelPathDirFD) {
-  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, 0) < 0 &&
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
           errno == ENOSYS);
 
   struct kernel_statx stx;
@@ -631,7 +631,7 @@ TEST_F(StatTest, StatxRelPathDirFD) {
 }
 
 TEST_F(StatTest, StatxRelPathCwd) {
-  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, 0) < 0 &&
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
           errno == ENOSYS);
 
   ASSERT_THAT(chdir(GetAbsoluteTestTmpdir().c_str()), SyscallSucceeds());
@@ -643,7 +643,7 @@ TEST_F(StatTest, StatxRelPathCwd) {
 }
 
 TEST_F(StatTest, StatxEmptyPath) {
-  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, 0) < 0 &&
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
           errno == ENOSYS);
 
   const auto fd = ASSERT_NO_ERRNO_AND_VALUE(Open(test_file_name_, O_RDONLY));
@@ -653,6 +653,58 @@ TEST_F(StatTest, StatxEmptyPath) {
   EXPECT_TRUE(S_ISREG(stx.stx_mode));
 }
 
+TEST_F(StatTest, StatxDoesNotRejectExtraneousMaskBits) {
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
+          errno == ENOSYS);
+
+  struct kernel_statx stx;
+  // Set all mask bits except for STATX__RESERVED.
+  uint mask = 0xffffffff & ~0x80000000;
+  EXPECT_THAT(statx(-1, test_file_name_.c_str(), 0, mask, &stx),
+              SyscallSucceeds());
+  EXPECT_TRUE(S_ISREG(stx.stx_mode));
+}
+
+TEST_F(StatTest, StatxRejectsReservedMaskBit) {
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
+          errno == ENOSYS);
+
+  struct kernel_statx stx;
+  // Set STATX__RESERVED in the mask.
+  EXPECT_THAT(statx(-1, test_file_name_.c_str(), 0, 0x80000000, &stx),
+              SyscallFailsWithErrno(EINVAL));
+}
+
+TEST_F(StatTest, StatxSymlink) {
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
+          errno == ENOSYS);
+
+  std::string parent_dir = "/tmp";
+  TempPath link = ASSERT_NO_ERRNO_AND_VALUE(
+      TempPath::CreateSymlinkTo(parent_dir, test_file_name_));
+  std::string p = link.path();
+
+  struct kernel_statx stx;
+  EXPECT_THAT(statx(AT_FDCWD, p.c_str(), AT_SYMLINK_NOFOLLOW, STATX_ALL, &stx),
+              SyscallSucceeds());
+  EXPECT_TRUE(S_ISLNK(stx.stx_mode));
+  EXPECT_THAT(statx(AT_FDCWD, p.c_str(), 0, STATX_ALL, &stx),
+              SyscallSucceeds());
+  EXPECT_TRUE(S_ISREG(stx.stx_mode));
+}
+
+TEST_F(StatTest, StatxInvalidFlags) {
+  SKIP_IF(!IsRunningOnGvisor() && statx(-1, nullptr, 0, 0, nullptr) < 0 &&
+          errno == ENOSYS);
+
+  struct kernel_statx stx;
+  EXPECT_THAT(statx(AT_FDCWD, test_file_name_.c_str(), 12345, 0, &stx),
+              SyscallFailsWithErrno(EINVAL));
+  EXPECT_THAT(statx(AT_FDCWD, test_file_name_.c_str(),
+                    0x6000 /* AT_STATX_SYNC_TYPE */, 0, &stx),
+              SyscallFailsWithErrno(EINVAL));
+}
+
 }  // namespace
 
 }  // namespace testing