Merge branch 'master' into pr_lazy_fpsimd_2

4 files changed, 632 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/host/BUILD b/pkg/sentry/fsimpl/host/BUILD
new file mode 100644
index 000000000..731f192b3
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/BUILD
@@ -0,0 +1,27 @@
+load("//tools:defs.bzl", "go_library")
+
+licenses(["notice"])
+
+go_library(
+    name = "host",
+    srcs = [
+        "default_file.go",
+        "host.go",
+        "util.go",
+    ],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/context",
+        "//pkg/log",
+        "//pkg/refs",
+        "//pkg/safemem",
+        "//pkg/sentry/fsimpl/kernfs",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/memmap",
+        "//pkg/sentry/vfs",
+        "//pkg/sync",
+        "//pkg/syserror",
+        "//pkg/usermem",
+        "@org_golang_x_sys//unix:go_default_library",
+    ],
+)
diff --git a/pkg/sentry/fsimpl/host/default_file.go b/pkg/sentry/fsimpl/host/default_file.go
new file mode 100644
index 000000000..172cdb161
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/default_file.go
@@ -0,0 +1,233 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package host
+
+import (
+	"math"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/safemem"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// defaultFileFD implements FileDescriptionImpl for non-socket, non-TTY files.
+type defaultFileFD struct {
+	fileDescription
+
+	// canMap specifies whether we allow the file to be memory mapped.
+	canMap bool
+
+	// mu protects the fields below.
+	mu sync.Mutex
+
+	// offset specifies the current file offset.
+	offset int64
+}
+
+// TODO(gvisor.dev/issue/1672): Implement Waitable interface.
+
+// PRead implements FileDescriptionImpl.
+func (f *defaultFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if f.inode.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	return readFromHostFD(ctx, f.inode.hostFD, dst, offset, int(opts.Flags))
+}
+
+// Read implements FileDescriptionImpl.
+func (f *defaultFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if f.inode.isStream {
+		// These files can't be memory mapped, assert this.
+		if f.canMap {
+			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.
+			if n != 0 {
+				err = nil
+			} else {
+				err = syserror.ErrWouldBlock
+			}
+		}
+		return n, err
+	}
+	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
+	f.mu.Lock()
+	n, err := readFromHostFD(ctx, f.inode.hostFD, dst, f.offset, int(opts.Flags))
+	f.offset += n
+	f.mu.Unlock()
+	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 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	reader := safemem.FromVecReaderFunc{
+		func(srcs [][]byte) (int64, error) {
+			n, err := unix.Preadv2(fd, srcs, offset, flags)
+			return int64(n), err
+		},
+	}
+	n, err := dst.CopyOutFrom(ctx, reader)
+	return int64(n), err
+}
+
+// PWrite implements FileDescriptionImpl.
+func (f *defaultFileFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if f.inode.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	return writeToHostFD(ctx, f.inode.hostFD, src, offset, int(opts.Flags))
+}
+
+// Write implements FileDescriptionImpl.
+func (f *defaultFileFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if f.inode.isStream {
+		// These files can't be memory mapped, assert this.
+		if f.canMap {
+			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
+		}
+		return n, err
+	}
+	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
+	// TODO(gvisor.dev/issue/1672): Write to end of file and update offset if O_APPEND is set on this file.
+	f.mu.Lock()
+	n, err := writeToHostFD(ctx, f.inode.hostFD, src, f.offset, int(opts.Flags))
+	f.offset += n
+	f.mu.Unlock()
+	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 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	writer := safemem.FromVecWriterFunc{
+		func(srcs [][]byte) (int64, error) {
+			n, err := unix.Pwritev2(fd, srcs, offset, flags)
+			return int64(n), err
+		},
+	}
+	n, err := src.CopyInTo(ctx, writer)
+	return int64(n), err
+}
+
+// Seek implements FileDescriptionImpl.
+//
+// Note that we do not support seeking on directories, since we do not even
+// allow directory fds to be imported at all.
+func (f *defaultFileFD) Seek(_ context.Context, offset int64, whence int32) (int64, error) {
+	// TODO(b/34716638): Some char devices do support seeking, e.g. /dev/null.
+	if f.inode.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	switch whence {
+	case linux.SEEK_SET:
+		if offset < 0 {
+			return f.offset, syserror.EINVAL
+		}
+		f.offset = offset
+
+	case linux.SEEK_CUR:
+		// Check for overflow. Note that underflow cannot occur, since f.offset >= 0.
+		if offset > math.MaxInt64-f.offset {
+			return f.offset, syserror.EOVERFLOW
+		}
+		if f.offset+offset < 0 {
+			return f.offset, syserror.EINVAL
+		}
+		f.offset += offset
+
+	case linux.SEEK_END:
+		var s syscall.Stat_t
+		if err := syscall.Fstat(f.inode.hostFD, &s); err != nil {
+			return f.offset, err
+		}
+		size := s.Size
+
+		// Check for overflow. Note that underflow cannot occur, since size >= 0.
+		if offset > math.MaxInt64-size {
+			return f.offset, syserror.EOVERFLOW
+		}
+		if size+offset < 0 {
+			return f.offset, syserror.EINVAL
+		}
+		f.offset = size + offset
+
+	case linux.SEEK_DATA, linux.SEEK_HOLE:
+		// Modifying the offset in the host file table should not matter, since
+		// this is the only place where we use it.
+		//
+		// For reading and writing, we always rely on our internal offset.
+		n, err := unix.Seek(f.inode.hostFD, offset, int(whence))
+		if err != nil {
+			return f.offset, err
+		}
+		f.offset = n
+
+	default:
+		// Invalid whence.
+		return f.offset, syserror.EINVAL
+	}
+
+	return f.offset, nil
+}
+
+// Sync implements FileDescriptionImpl.
+func (f *defaultFileFD) Sync(context.Context) error {
+	// TODO(gvisor.dev/issue/1672): Currently we do not support the SyncData optimization, so we always sync everything.
+	return unix.Fsync(f.inode.hostFD)
+}
+
+// ConfigureMMap implements FileDescriptionImpl.
+func (f *defaultFileFD) ConfigureMMap(_ context.Context, opts *memmap.MMapOpts) error {
+	if !f.canMap {
+		return syserror.ENODEV
+	}
+	// TODO(gvisor.dev/issue/1672): Implement ConfigureMMap and Mappable interface.
+	return syserror.ENODEV
+}
diff --git a/pkg/sentry/fsimpl/host/host.go b/pkg/sentry/fsimpl/host/host.go
new file mode 100644
index 000000000..c205e6a0b
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/host.go
@@ -0,0 +1,286 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package host provides a filesystem implementation for host files imported as
+// file descriptors.
+package host
+
+import (
+	"errors"
+	"fmt"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/refs"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	kernfs.Filesystem
+}
+
+// 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)
+	if !ok {
+		return nil, fmt.Errorf("can't import host FDs into filesystems of type %T", mnt.Filesystem().Impl())
+	}
+
+	// Retrieve metadata.
+	var s syscall.Stat_t
+	if err := syscall.Fstat(hostFD, &s); err != nil {
+		return nil, err
+	}
+
+	fileMode := linux.FileMode(s.Mode)
+	fileType := fileMode.FileType()
+	// Pipes, character devices, and sockets can return EWOULDBLOCK for
+	// operations that would block.
+	isStream := fileType == syscall.S_IFIFO || fileType == syscall.S_IFCHR || fileType == syscall.S_IFSOCK
+
+	i := &inode{
+		hostFD:   hostFD,
+		isStream: isStream,
+		isTTY:    isTTY,
+		ino:      fs.NextIno(),
+		mode:     fileMode,
+		uid:      ownerUID,
+		gid:      ownerGID,
+	}
+
+	d := &kernfs.Dentry{}
+	d.Init(i)
+	// i.open will take a reference on d.
+	defer d.DecRef()
+
+	return i.open(d.VFSDentry(), mnt)
+}
+
+// inode implements kernfs.Inode.
+type inode struct {
+	kernfs.InodeNotDirectory
+	kernfs.InodeNotSymlink
+
+	// When the reference count reaches zero, the host fd is closed.
+	refs.AtomicRefCount
+
+	// hostFD contains the host fd that this file was originally created from,
+	// which must be available at time of restore.
+	//
+	// This field is initialized at creation time and is immutable.
+	hostFD int
+
+	// isStream is true if the host fd points to a file representing a stream,
+	// e.g. a socket or a pipe. Such files are not seekable and can return
+	// EWOULDBLOCK for I/O operations.
+	//
+	// This field is initialized at creation time and is immutable.
+	isStream bool
+
+	// isTTY is true if this file represents a TTY.
+	//
+	// This field is initialized at creation time and is immutable.
+	isTTY bool
+
+	// ino is an inode number unique within this filesystem.
+	ino uint64
+
+	// mu protects the inode metadata below.
+	mu sync.Mutex
+
+	// mode is the file mode of this inode. Note that this value may become out
+	// of date if the mode is changed on the host, e.g. with chmod.
+	mode linux.FileMode
+
+	// uid and gid of the file owner. Note that these refer to the owner of the
+	// file created on import, not the fd on the host.
+	uid auth.KUID
+	gid auth.KGID
+}
+
+// Note that these flags may become out of date, since they can be modified
+// on the host, e.g. with fcntl.
+func fileFlagsFromHostFD(fd int) (int, error) {
+	flags, err := unix.FcntlInt(uintptr(fd), syscall.F_GETFL, 0)
+	if err != nil {
+		log.Warningf("Failed to get file flags for donated FD %d: %v", fd, err)
+		return 0, err
+	}
+	// TODO(gvisor.dev/issue/1672): implement behavior corresponding to these allowed flags.
+	flags &= syscall.O_ACCMODE | syscall.O_DIRECT | syscall.O_NONBLOCK | syscall.O_DSYNC | syscall.O_SYNC | syscall.O_APPEND
+	return flags, nil
+}
+
+// CheckPermissions implements kernfs.Inode.
+func (i *inode) CheckPermissions(ctx context.Context, creds *auth.Credentials, atx vfs.AccessTypes) error {
+	return vfs.GenericCheckPermissions(creds, atx, false /* isDir */, uint16(i.mode), i.uid, i.gid)
+}
+
+// Mode implements kernfs.Inode.
+func (i *inode) Mode() linux.FileMode {
+	return i.mode
+}
+
+// Stat implements kernfs.Inode.
+func (i *inode) Stat(_ *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error) {
+	var s unix.Statx_t
+	if err := unix.Statx(i.hostFD, "", int(unix.AT_EMPTY_PATH|opts.Sync), int(opts.Mask), &s); err != nil {
+		return linux.Statx{}, err
+	}
+	ls := unixToLinuxStatx(s)
+
+	// Use our own internal inode number and file owner.
+	//
+	// TODO(gvisor.dev/issue/1672): Use a kernfs-specific device number as well.
+	// 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.Ino = i.ino
+	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
+}
+
+// SetStat implements kernfs.Inode.
+func (i *inode) SetStat(_ *vfs.Filesystem, opts vfs.SetStatOptions) error {
+	s := opts.Stat
+
+	m := s.Mask
+	if m == 0 {
+		return nil
+	}
+	if m&(linux.STATX_UID|linux.STATX_GID) != 0 {
+		return syserror.EPERM
+	}
+	if m&linux.STATX_MODE != 0 {
+		if err := syscall.Fchmod(i.hostFD, uint32(s.Mode)); err != nil {
+			return err
+		}
+		i.mode = linux.FileMode(s.Mode)
+	}
+	if m&linux.STATX_SIZE != 0 {
+		if err := syscall.Ftruncate(i.hostFD, int64(s.Size)); err != nil {
+			return err
+		}
+	}
+	if m&(linux.STATX_ATIME|linux.STATX_MTIME) != 0 {
+		timestamps := []unix.Timespec{
+			toTimespec(s.Atime, m&linux.STATX_ATIME == 0),
+			toTimespec(s.Mtime, m&linux.STATX_MTIME == 0),
+		}
+		if err := unix.UtimesNanoAt(i.hostFD, "", timestamps, unix.AT_EMPTY_PATH); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// DecRef implements kernfs.Inode.
+func (i *inode) DecRef() {
+	i.AtomicRefCount.DecRefWithDestructor(i.Destroy)
+}
+
+// Destroy implements kernfs.Inode.
+func (i *inode) Destroy() {
+	if err := unix.Close(i.hostFD); err != nil {
+		log.Warningf("failed to close host fd %d: %v", i.hostFD, err)
+	}
+}
+
+// Open implements kernfs.Inode.
+func (i *inode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	return i.open(vfsd, rp.Mount())
+}
+
+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 {
+			return nil, errors.New("cannot use host socket as TTY")
+		}
+		// TODO(gvisor.dev/issue/1672): support importing sockets.
+		return nil, errors.New("importing host sockets not supported")
+	}
+
+	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")
+	}
+
+	// For simplicity, set offset to 0. Technically, we should
+	// only set to 0 on files that are not seekable (sockets, pipes, etc.),
+	// and use the offset from the host fd otherwise.
+	fd := &defaultFileFD{
+		fileDescription: fileDescription{
+			inode: i,
+		},
+		canMap: canMap(uint32(fileType)),
+		mu:     sync.Mutex{},
+		offset: 0,
+	}
+
+	vfsfd := &fd.vfsfd
+	flags, err := fileFlagsFromHostFD(i.hostFD)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := vfsfd.Init(fd, uint32(flags), mnt, d, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
+	return vfsfd, nil
+}
+
+// fileDescription is embedded by host fd implementations of FileDescriptionImpl.
+type fileDescription struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+
+	// inode is vfsfd.Dentry().Impl().(*kernfs.Dentry).Inode().(*inode), but
+	// cached to reduce indirections and casting. fileDescription does not hold
+	// a reference on the inode through the inode field (since one is already
+	// held via the Dentry).
+	//
+	// inode is immutable after fileDescription creation.
+	inode *inode
+}
+
+// SetStat implements vfs.FileDescriptionImpl.
+func (f *fileDescription) SetStat(_ context.Context, opts vfs.SetStatOptions) error {
+	return f.inode.SetStat(nil, opts)
+}
+
+// Stat implements vfs.FileDescriptionImpl.
+func (f *fileDescription) Stat(_ context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	return f.inode.Stat(nil, opts)
+}
+
+// Release implements vfs.FileDescriptionImpl.
+func (f *fileDescription) Release() {
+	// noop
+}
diff --git a/pkg/sentry/fsimpl/host/util.go b/pkg/sentry/fsimpl/host/util.go
new file mode 100644
index 000000000..e1ccacb4d
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/util.go
@@ -0,0 +1,86 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package host
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+func toTimespec(ts linux.StatxTimestamp, omit bool) unix.Timespec {
+	if omit {
+		return unix.Timespec{
+			Sec:  0,
+			Nsec: unix.UTIME_OMIT,
+		}
+	}
+	return unix.Timespec{
+		Sec:  int64(ts.Sec),
+		Nsec: int64(ts.Nsec),
+	}
+}
+
+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}
+}
+
+// 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 {
+	return fileType == syscall.S_IFIFO || fileType == syscall.S_IFCHR || fileType == syscall.S_IFSOCK
+}
+
+// canMap returns true if a file with fileType is allowed to be memory mapped.
+// This is ported over from VFS1, but it's probably not the best way for us
+// to check if a file can be memory mapped.
+func canMap(fileType uint32) bool {
+	// TODO(gvisor.dev/issue/1672): Also allow "special files" to be mapped (see fs/host:canMap()).
+	//
+	// TODO(b/38213152): Some obscure character devices can be mapped.
+	return fileType == syscall.S_IFREG
+}
+
+// isBlockError checks if an error is EAGAIN or EWOULDBLOCK.
+// If so, they can be transformed into syserror.ErrWouldBlock.
+func isBlockError(err error) bool {
+	return err == syserror.EAGAIN || err == syserror.EWOULDBLOCK
+}