Port aio to VFS2.

In order to make sure all aio goroutines have stopped during S/R, a new
WaitGroup was added to TaskSet, analagous to runningGoroutines. This WaitGroup
is incremented with each aio goroutine, and waited on during kernel.Pause.

The old VFS1 aio code was changed to use this new WaitGroup, rather than
fs.Async. The only uses of fs.Async are now inode and mount Release operations,
which do not call fs.Async recursively. This fixes a lock-ordering violation
that can cause deadlocks.

Updates #1035.

PiperOrigin-RevId: 316689380

9 files changed, 373 insertions, 173 deletions

diff --git a/pkg/sentry/kernel/BUILD b/pkg/sentry/kernel/BUILD
index a28eab8b8..1510a7c26 100644
--- a/pkg/sentry/kernel/BUILD
+++ b/pkg/sentry/kernel/BUILD
@@ -85,6 +85,7 @@ go_library(
     name = "kernel",
     srcs = [
         "abstract_socket_namespace.go",
+        "aio.go",
         "context.go",
         "fd_table.go",
         "fd_table_unsafe.go",
diff --git a/pkg/sentry/kernel/aio.go b/pkg/sentry/kernel/aio.go
new file mode 100644
index 000000000..0ac78c0b8
--- /dev/null
+++ b/pkg/sentry/kernel/aio.go
@@ -0,0 +1,81 @@
+// 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 kernel
+
+import (
+	"time"
+
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+// AIOCallback is an function that does asynchronous I/O on behalf of a task.
+type AIOCallback func(context.Context)
+
+// QueueAIO queues an AIOCallback which will be run asynchronously.
+func (t *Task) QueueAIO(cb AIOCallback) {
+	ctx := taskAsyncContext{t: t}
+	wg := &t.TaskSet().aioGoroutines
+	wg.Add(1)
+	go func() {
+		cb(ctx)
+		wg.Done()
+	}()
+}
+
+type taskAsyncContext struct {
+	context.NoopSleeper
+	t *Task
+}
+
+// Debugf implements log.Logger.Debugf.
+func (ctx taskAsyncContext) Debugf(format string, v ...interface{}) {
+	ctx.t.Debugf(format, v...)
+}
+
+// Infof implements log.Logger.Infof.
+func (ctx taskAsyncContext) Infof(format string, v ...interface{}) {
+	ctx.t.Infof(format, v...)
+}
+
+// Warningf implements log.Logger.Warningf.
+func (ctx taskAsyncContext) Warningf(format string, v ...interface{}) {
+	ctx.t.Warningf(format, v...)
+}
+
+// IsLogging implements log.Logger.IsLogging.
+func (ctx taskAsyncContext) IsLogging(level log.Level) bool {
+	return ctx.t.IsLogging(level)
+}
+
+// Deadline implements context.Context.Deadline.
+func (ctx taskAsyncContext) Deadline() (time.Time, bool) {
+	return ctx.t.Deadline()
+}
+
+// Done implements context.Context.Done.
+func (ctx taskAsyncContext) Done() <-chan struct{} {
+	return ctx.t.Done()
+}
+
+// Err implements context.Context.Err.
+func (ctx taskAsyncContext) Err() error {
+	return ctx.t.Err()
+}
+
+// Value implements context.Context.Value.
+func (ctx taskAsyncContext) Value(key interface{}) interface{} {
+	return ctx.t.Value(key)
+}
diff --git a/pkg/sentry/kernel/context.go b/pkg/sentry/kernel/context.go
index 0c40bf315..dd5f0f5fa 100644
--- a/pkg/sentry/kernel/context.go
+++ b/pkg/sentry/kernel/context.go
@@ -18,7 +18,6 @@ import (
 	"time"
 
 	"gvisor.dev/gvisor/pkg/context"
-	"gvisor.dev/gvisor/pkg/log"
 )
 
 // contextID is the kernel package's type for context.Context.Value keys.
@@ -113,55 +112,3 @@ func (*Task) Done() <-chan struct{} {
 func (*Task) Err() error {
 	return nil
 }
-
-// AsyncContext returns a context.Context that may be used by goroutines that
-// do work on behalf of t and therefore share its contextual values, but are
-// not t's task goroutine (e.g. asynchronous I/O).
-func (t *Task) AsyncContext() context.Context {
-	return taskAsyncContext{t: t}
-}
-
-type taskAsyncContext struct {
-	context.NoopSleeper
-	t *Task
-}
-
-// Debugf implements log.Logger.Debugf.
-func (ctx taskAsyncContext) Debugf(format string, v ...interface{}) {
-	ctx.t.Debugf(format, v...)
-}
-
-// Infof implements log.Logger.Infof.
-func (ctx taskAsyncContext) Infof(format string, v ...interface{}) {
-	ctx.t.Infof(format, v...)
-}
-
-// Warningf implements log.Logger.Warningf.
-func (ctx taskAsyncContext) Warningf(format string, v ...interface{}) {
-	ctx.t.Warningf(format, v...)
-}
-
-// IsLogging implements log.Logger.IsLogging.
-func (ctx taskAsyncContext) IsLogging(level log.Level) bool {
-	return ctx.t.IsLogging(level)
-}
-
-// Deadline implements context.Context.Deadline.
-func (ctx taskAsyncContext) Deadline() (time.Time, bool) {
-	return ctx.t.Deadline()
-}
-
-// Done implements context.Context.Done.
-func (ctx taskAsyncContext) Done() <-chan struct{} {
-	return ctx.t.Done()
-}
-
-// Err implements context.Context.Err.
-func (ctx taskAsyncContext) Err() error {
-	return ctx.t.Err()
-}
-
-// Value implements context.Context.Value.
-func (ctx taskAsyncContext) Value(key interface{}) interface{} {
-	return ctx.t.Value(key)
-}
diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 52491da7a..554a42e05 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -452,9 +452,7 @@ func (k *Kernel) SaveTo(w io.Writer) error {
 		return err
 	}
 
-	// Ensure that all pending asynchronous work is complete:
-	//   - inode and mount release
-	//   - asynchronuous IO
+	// Ensure that all inode and mount release operations have completed.
 	fs.AsyncBarrier()
 
 	// Once all fs work has completed (flushed references have all been released),
@@ -1249,13 +1247,15 @@ func (k *Kernel) Kill(es ExitStatus) {
 }
 
 // Pause requests that all tasks in k temporarily stop executing, and blocks
-// until all tasks in k have stopped. Multiple calls to Pause nest and require
-// an equal number of calls to Unpause to resume execution.
+// until all tasks and asynchronous I/O operations in k have stopped. Multiple
+// calls to Pause nest and require an equal number of calls to Unpause to
+// resume execution.
 func (k *Kernel) Pause() {
 	k.extMu.Lock()
 	k.tasks.BeginExternalStop()
 	k.extMu.Unlock()
 	k.tasks.runningGoroutines.Wait()
+	k.tasks.aioGoroutines.Wait()
 }
 
 // Unpause ends the effect of a previous call to Pause. If Unpause is called
diff --git a/pkg/sentry/kernel/threads.go b/pkg/sentry/kernel/threads.go
index bf2dabb6e..872e1a82d 100644
--- a/pkg/sentry/kernel/threads.go
+++ b/pkg/sentry/kernel/threads.go
@@ -87,6 +87,13 @@ type TaskSet struct {
 	// at time of save (but note that this is not necessarily the same thing as
 	// sync.WaitGroup's zero value).
 	runningGoroutines sync.WaitGroup `state:"nosave"`
+
+	// aioGoroutines is the number of goroutines running async I/O
+	// callbacks.
+	//
+	// aioGoroutines is not saved but is required to be zero at the time of
+	// save.
+	aioGoroutines sync.WaitGroup `state:"nosave"`
 }
 
 // newTaskSet returns a new, empty TaskSet.
diff --git a/pkg/sentry/syscalls/linux/sys_aio.go b/pkg/sentry/syscalls/linux/sys_aio.go
index d781d6a04..ba2557c52 100644
--- a/pkg/sentry/syscalls/linux/sys_aio.go
+++ b/pkg/sentry/syscalls/linux/sys_aio.go
@@ -15,8 +15,8 @@
 package linux
 
 import (
-	"encoding/binary"
-
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
@@ -27,59 +27,6 @@ import (
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
-// I/O commands.
-const (
-	_IOCB_CMD_PREAD   = 0
-	_IOCB_CMD_PWRITE  = 1
-	_IOCB_CMD_FSYNC   = 2
-	_IOCB_CMD_FDSYNC  = 3
-	_IOCB_CMD_NOOP    = 6
-	_IOCB_CMD_PREADV  = 7
-	_IOCB_CMD_PWRITEV = 8
-)
-
-// I/O flags.
-const (
-	_IOCB_FLAG_RESFD = 1
-)
-
-// ioCallback describes an I/O request.
-//
-// The priority field is currently ignored in the implementation below. Also
-// note that the IOCB_FLAG_RESFD feature is not supported.
-type ioCallback struct {
-	Data      uint64
-	Key       uint32
-	Reserved1 uint32
-
-	OpCode  uint16
-	ReqPrio int16
-	FD      int32
-
-	Buf    uint64
-	Bytes  uint64
-	Offset int64
-
-	Reserved2 uint64
-	Flags     uint32
-
-	// eventfd to signal if IOCB_FLAG_RESFD is set in flags.
-	ResFD int32
-}
-
-// ioEvent describes an I/O result.
-//
-// +stateify savable
-type ioEvent struct {
-	Data    uint64
-	Obj     uint64
-	Result  int64
-	Result2 int64
-}
-
-// ioEventSize is the size of an ioEvent encoded.
-var ioEventSize = binary.Size(ioEvent{})
-
 // IoSetup implements linux syscall io_setup(2).
 func IoSetup(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
 	nrEvents := args[0].Int()
@@ -192,7 +139,7 @@ func IoGetevents(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.S
 			}
 		}
 
-		ev := v.(*ioEvent)
+		ev := v.(*linux.IOEvent)
 
 		// Copy out the result.
 		if _, err := t.CopyOut(eventsAddr, ev); err != nil {
@@ -204,7 +151,7 @@ func IoGetevents(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.S
 		}
 
 		// Keep rolling.
-		eventsAddr += usermem.Addr(ioEventSize)
+		eventsAddr += usermem.Addr(linux.IOEventSize)
 	}
 
 	// Everything finished.
@@ -231,7 +178,7 @@ func waitForRequest(ctx *mm.AIOContext, t *kernel.Task, haveDeadline bool, deadl
 }
 
 // memoryFor returns appropriate memory for the given callback.
-func memoryFor(t *kernel.Task, cb *ioCallback) (usermem.IOSequence, error) {
+func memoryFor(t *kernel.Task, cb *linux.IOCallback) (usermem.IOSequence, error) {
 	bytes := int(cb.Bytes)
 	if bytes < 0 {
 		// Linux also requires that this field fit in ssize_t.
@@ -242,17 +189,17 @@ func memoryFor(t *kernel.Task, cb *ioCallback) (usermem.IOSequence, error) {
 	// we have no guarantee that t's AddressSpace will be active during the
 	// I/O.
 	switch cb.OpCode {
-	case _IOCB_CMD_PREAD, _IOCB_CMD_PWRITE:
+	case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PWRITE:
 		return t.SingleIOSequence(usermem.Addr(cb.Buf), bytes, usermem.IOOpts{
 			AddressSpaceActive: false,
 		})
 
-	case _IOCB_CMD_PREADV, _IOCB_CMD_PWRITEV:
+	case linux.IOCB_CMD_PREADV, linux.IOCB_CMD_PWRITEV:
 		return t.IovecsIOSequence(usermem.Addr(cb.Buf), bytes, usermem.IOOpts{
 			AddressSpaceActive: false,
 		})
 
-	case _IOCB_CMD_FSYNC, _IOCB_CMD_FDSYNC, _IOCB_CMD_NOOP:
+	case linux.IOCB_CMD_FSYNC, linux.IOCB_CMD_FDSYNC, linux.IOCB_CMD_NOOP:
 		return usermem.IOSequence{}, nil
 
 	default:
@@ -261,54 +208,62 @@ func memoryFor(t *kernel.Task, cb *ioCallback) (usermem.IOSequence, error) {
 	}
 }
 
-func performCallback(t *kernel.Task, file *fs.File, cbAddr usermem.Addr, cb *ioCallback, ioseq usermem.IOSequence, ctx *mm.AIOContext, eventFile *fs.File) {
-	if ctx.Dead() {
-		ctx.CancelPendingRequest()
-		return
-	}
-	ev := &ioEvent{
-		Data: cb.Data,
-		Obj:  uint64(cbAddr),
-	}
+// IoCancel implements linux syscall io_cancel(2).
+//
+// It is not presently supported (ENOSYS indicates no support on this
+// architecture).
+func IoCancel(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	return 0, nil, syserror.ENOSYS
+}
 
-	// Construct a context.Context that will not be interrupted if t is
-	// interrupted.
-	c := t.AsyncContext()
+// LINT.IfChange
 
-	var err error
-	switch cb.OpCode {
-	case _IOCB_CMD_PREAD, _IOCB_CMD_PREADV:
-		ev.Result, err = file.Preadv(c, ioseq, cb.Offset)
-	case _IOCB_CMD_PWRITE, _IOCB_CMD_PWRITEV:
-		ev.Result, err = file.Pwritev(c, ioseq, cb.Offset)
-	case _IOCB_CMD_FSYNC:
-		err = file.Fsync(c, 0, fs.FileMaxOffset, fs.SyncAll)
-	case _IOCB_CMD_FDSYNC:
-		err = file.Fsync(c, 0, fs.FileMaxOffset, fs.SyncData)
-	}
+func getAIOCallback(t *kernel.Task, file *fs.File, cbAddr usermem.Addr, cb *linux.IOCallback, ioseq usermem.IOSequence, actx *mm.AIOContext, eventFile *fs.File) kernel.AIOCallback {
+	return func(ctx context.Context) {
+		if actx.Dead() {
+			actx.CancelPendingRequest()
+			return
+		}
+		ev := &linux.IOEvent{
+			Data: cb.Data,
+			Obj:  uint64(cbAddr),
+		}
 
-	// Update the result.
-	if err != nil {
-		err = handleIOError(t, ev.Result != 0 /* partial */, err, nil /* never interrupted */, "aio", file)
-		ev.Result = -int64(kernel.ExtractErrno(err, 0))
-	}
+		var err error
+		switch cb.OpCode {
+		case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PREADV:
+			ev.Result, err = file.Preadv(ctx, ioseq, cb.Offset)
+		case linux.IOCB_CMD_PWRITE, linux.IOCB_CMD_PWRITEV:
+			ev.Result, err = file.Pwritev(ctx, ioseq, cb.Offset)
+		case linux.IOCB_CMD_FSYNC:
+			err = file.Fsync(ctx, 0, fs.FileMaxOffset, fs.SyncAll)
+		case linux.IOCB_CMD_FDSYNC:
+			err = file.Fsync(ctx, 0, fs.FileMaxOffset, fs.SyncData)
+		}
+
+		// Update the result.
+		if err != nil {
+			err = handleIOError(t, ev.Result != 0 /* partial */, err, nil /* never interrupted */, "aio", file)
+			ev.Result = -int64(kernel.ExtractErrno(err, 0))
+		}
 
-	file.DecRef()
+		file.DecRef()
 
-	// Queue the result for delivery.
-	ctx.FinishRequest(ev)
+		// Queue the result for delivery.
+		actx.FinishRequest(ev)
 
-	// Notify the event file if one was specified. This needs to happen
-	// *after* queueing the result to avoid racing with the thread we may
-	// wake up.
-	if eventFile != nil {
-		eventFile.FileOperations.(*eventfd.EventOperations).Signal(1)
-		eventFile.DecRef()
+		// Notify the event file if one was specified. This needs to happen
+		// *after* queueing the result to avoid racing with the thread we may
+		// wake up.
+		if eventFile != nil {
+			eventFile.FileOperations.(*eventfd.EventOperations).Signal(1)
+			eventFile.DecRef()
+		}
 	}
 }
 
 // submitCallback processes a single callback.
-func submitCallback(t *kernel.Task, id uint64, cb *ioCallback, cbAddr usermem.Addr) error {
+func submitCallback(t *kernel.Task, id uint64, cb *linux.IOCallback, cbAddr usermem.Addr) error {
 	file := t.GetFile(cb.FD)
 	if file == nil {
 		// File not found.
@@ -318,7 +273,7 @@ func submitCallback(t *kernel.Task, id uint64, cb *ioCallback, cbAddr usermem.Ad
 
 	// Was there an eventFD? Extract it.
 	var eventFile *fs.File
-	if cb.Flags&_IOCB_FLAG_RESFD != 0 {
+	if cb.Flags&linux.IOCB_FLAG_RESFD != 0 {
 		eventFile = t.GetFile(cb.ResFD)
 		if eventFile == nil {
 			// Bad FD.
@@ -340,7 +295,7 @@ func submitCallback(t *kernel.Task, id uint64, cb *ioCallback, cbAddr usermem.Ad
 
 	// Check offset for reads/writes.
 	switch cb.OpCode {
-	case _IOCB_CMD_PREAD, _IOCB_CMD_PREADV, _IOCB_CMD_PWRITE, _IOCB_CMD_PWRITEV:
+	case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PREADV, linux.IOCB_CMD_PWRITE, linux.IOCB_CMD_PWRITEV:
 		if cb.Offset < 0 {
 			return syserror.EINVAL
 		}
@@ -366,7 +321,7 @@ func submitCallback(t *kernel.Task, id uint64, cb *ioCallback, cbAddr usermem.Ad
 
 	// Perform the request asynchronously.
 	file.IncRef()
-	fs.Async(func() { performCallback(t, file, cbAddr, cb, ioseq, ctx, eventFile) })
+	t.QueueAIO(getAIOCallback(t, file, cbAddr, cb, ioseq, ctx, eventFile))
 
 	// All set.
 	return nil
@@ -395,7 +350,7 @@ func IoSubmit(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 		}
 
 		// Copy in this callback.
-		var cb ioCallback
+		var cb linux.IOCallback
 		cbAddr := usermem.Addr(t.Arch().Value(cbAddrNative))
 		if _, err := t.CopyIn(cbAddr, &cb); err != nil {
 
@@ -424,10 +379,4 @@ func IoSubmit(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 	return uintptr(nrEvents), nil, nil
 }
 
-// IoCancel implements linux syscall io_cancel(2).
-//
-// It is not presently supported (ENOSYS indicates no support on this
-// architecture).
-func IoCancel(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
-	return 0, nil, syserror.ENOSYS
-}
+// LINT.ThenChange(vfs2/aio.go)
diff --git a/pkg/sentry/syscalls/linux/vfs2/BUILD b/pkg/sentry/syscalls/linux/vfs2/BUILD
index 9f93f4354..c301a0991 100644
--- a/pkg/sentry/syscalls/linux/vfs2/BUILD
+++ b/pkg/sentry/syscalls/linux/vfs2/BUILD
@@ -5,6 +5,7 @@ package(licenses = ["notice"])
 go_library(
     name = "vfs2",
     srcs = [
+        "aio.go",
         "epoll.go",
         "eventfd.go",
         "execve.go",
@@ -40,6 +41,7 @@ go_library(
         "//pkg/abi/linux",
         "//pkg/binary",
         "//pkg/bits",
+        "//pkg/context",
         "//pkg/fspath",
         "//pkg/gohacks",
         "//pkg/sentry/arch",
@@ -57,6 +59,7 @@ go_library(
         "//pkg/sentry/limits",
         "//pkg/sentry/loader",
         "//pkg/sentry/memmap",
+        "//pkg/sentry/mm",
         "//pkg/sentry/socket",
         "//pkg/sentry/socket/control",
         "//pkg/sentry/socket/unix/transport",
diff --git a/pkg/sentry/syscalls/linux/vfs2/aio.go b/pkg/sentry/syscalls/linux/vfs2/aio.go
new file mode 100644
index 000000000..e5cdefc50
--- /dev/null
+++ b/pkg/sentry/syscalls/linux/vfs2/aio.go
@@ -0,0 +1,216 @@
+// Copyright 2018 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 vfs2
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/eventfd"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/mm"
+	slinux "gvisor.dev/gvisor/pkg/sentry/syscalls/linux"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// IoSubmit implements linux syscall io_submit(2).
+func IoSubmit(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	id := args[0].Uint64()
+	nrEvents := args[1].Int()
+	addr := args[2].Pointer()
+
+	if nrEvents < 0 {
+		return 0, nil, syserror.EINVAL
+	}
+
+	for i := int32(0); i < nrEvents; i++ {
+		// Copy in the address.
+		cbAddrNative := t.Arch().Native(0)
+		if _, err := t.CopyIn(addr, cbAddrNative); err != nil {
+			if i > 0 {
+				// Some successful.
+				return uintptr(i), nil, nil
+			}
+			// Nothing done.
+			return 0, nil, err
+		}
+
+		// Copy in this callback.
+		var cb linux.IOCallback
+		cbAddr := usermem.Addr(t.Arch().Value(cbAddrNative))
+		if _, err := t.CopyIn(cbAddr, &cb); err != nil {
+			if i > 0 {
+				// Some have been successful.
+				return uintptr(i), nil, nil
+			}
+			// Nothing done.
+			return 0, nil, err
+		}
+
+		// Process this callback.
+		if err := submitCallback(t, id, &cb, cbAddr); err != nil {
+			if i > 0 {
+				// Partial success.
+				return uintptr(i), nil, nil
+			}
+			// Nothing done.
+			return 0, nil, err
+		}
+
+		// Advance to the next one.
+		addr += usermem.Addr(t.Arch().Width())
+	}
+
+	return uintptr(nrEvents), nil, nil
+}
+
+// submitCallback processes a single callback.
+func submitCallback(t *kernel.Task, id uint64, cb *linux.IOCallback, cbAddr usermem.Addr) error {
+	if cb.Reserved2 != 0 {
+		return syserror.EINVAL
+	}
+
+	fd := t.GetFileVFS2(cb.FD)
+	if fd == nil {
+		return syserror.EBADF
+	}
+	defer fd.DecRef()
+
+	// Was there an eventFD? Extract it.
+	var eventFD *vfs.FileDescription
+	if cb.Flags&linux.IOCB_FLAG_RESFD != 0 {
+		eventFD = t.GetFileVFS2(cb.ResFD)
+		if eventFD == nil {
+			return syserror.EBADF
+		}
+		defer eventFD.DecRef()
+
+		// Check that it is an eventfd.
+		if _, ok := eventFD.Impl().(*eventfd.EventFileDescription); !ok {
+			return syserror.EINVAL
+		}
+	}
+
+	ioseq, err := memoryFor(t, cb)
+	if err != nil {
+		return err
+	}
+
+	// Check offset for reads/writes.
+	switch cb.OpCode {
+	case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PREADV, linux.IOCB_CMD_PWRITE, linux.IOCB_CMD_PWRITEV:
+		if cb.Offset < 0 {
+			return syserror.EINVAL
+		}
+	}
+
+	// Prepare the request.
+	aioCtx, ok := t.MemoryManager().LookupAIOContext(t, id)
+	if !ok {
+		return syserror.EINVAL
+	}
+	if ready := aioCtx.Prepare(); !ready {
+		// Context is busy.
+		return syserror.EAGAIN
+	}
+
+	if eventFD != nil {
+		// The request is set. Make sure there's a ref on the file.
+		//
+		// This is necessary when the callback executes on completion,
+		// which is also what will release this reference.
+		eventFD.IncRef()
+	}
+
+	// Perform the request asynchronously.
+	fd.IncRef()
+	t.QueueAIO(getAIOCallback(t, fd, eventFD, cbAddr, cb, ioseq, aioCtx))
+	return nil
+}
+
+func getAIOCallback(t *kernel.Task, fd, eventFD *vfs.FileDescription, cbAddr usermem.Addr, cb *linux.IOCallback, ioseq usermem.IOSequence, aioCtx *mm.AIOContext) kernel.AIOCallback {
+	return func(ctx context.Context) {
+		if aioCtx.Dead() {
+			aioCtx.CancelPendingRequest()
+			return
+		}
+		ev := &linux.IOEvent{
+			Data: cb.Data,
+			Obj:  uint64(cbAddr),
+		}
+
+		var err error
+		switch cb.OpCode {
+		case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PREADV:
+			ev.Result, err = fd.PRead(ctx, ioseq, cb.Offset, vfs.ReadOptions{})
+		case linux.IOCB_CMD_PWRITE, linux.IOCB_CMD_PWRITEV:
+			ev.Result, err = fd.PWrite(ctx, ioseq, cb.Offset, vfs.WriteOptions{})
+		case linux.IOCB_CMD_FSYNC, linux.IOCB_CMD_FDSYNC:
+			err = fd.Sync(ctx)
+		}
+
+		// Update the result.
+		if err != nil {
+			err = slinux.HandleIOErrorVFS2(t, ev.Result != 0 /* partial */, err, nil /* never interrupted */, "aio", fd)
+			ev.Result = -int64(kernel.ExtractErrno(err, 0))
+		}
+
+		fd.DecRef()
+
+		// Queue the result for delivery.
+		aioCtx.FinishRequest(ev)
+
+		// Notify the event file if one was specified. This needs to happen
+		// *after* queueing the result to avoid racing with the thread we may
+		// wake up.
+		if eventFD != nil {
+			eventFD.Impl().(*eventfd.EventFileDescription).Signal(1)
+			eventFD.DecRef()
+		}
+	}
+}
+
+// memoryFor returns appropriate memory for the given callback.
+func memoryFor(t *kernel.Task, cb *linux.IOCallback) (usermem.IOSequence, error) {
+	bytes := int(cb.Bytes)
+	if bytes < 0 {
+		// Linux also requires that this field fit in ssize_t.
+		return usermem.IOSequence{}, syserror.EINVAL
+	}
+
+	// Since this I/O will be asynchronous with respect to t's task goroutine,
+	// we have no guarantee that t's AddressSpace will be active during the
+	// I/O.
+	switch cb.OpCode {
+	case linux.IOCB_CMD_PREAD, linux.IOCB_CMD_PWRITE:
+		return t.SingleIOSequence(usermem.Addr(cb.Buf), bytes, usermem.IOOpts{
+			AddressSpaceActive: false,
+		})
+
+	case linux.IOCB_CMD_PREADV, linux.IOCB_CMD_PWRITEV:
+		return t.IovecsIOSequence(usermem.Addr(cb.Buf), bytes, usermem.IOOpts{
+			AddressSpaceActive: false,
+		})
+
+	case linux.IOCB_CMD_FSYNC, linux.IOCB_CMD_FDSYNC, linux.IOCB_CMD_NOOP:
+		return usermem.IOSequence{}, nil
+
+	default:
+		// Not a supported command.
+		return usermem.IOSequence{}, syserror.EINVAL
+	}
+}
diff --git a/pkg/sentry/syscalls/linux/vfs2/vfs2.go b/pkg/sentry/syscalls/linux/vfs2/vfs2.go
index 954c82f97..caa6a98ff 100644
--- a/pkg/sentry/syscalls/linux/vfs2/vfs2.go
+++ b/pkg/sentry/syscalls/linux/vfs2/vfs2.go
@@ -105,11 +105,7 @@ func Override() {
 	s.Table[197] = syscalls.Supported("removexattr", Removexattr)
 	s.Table[198] = syscalls.Supported("lremovexattr", Lremovexattr)
 	s.Table[199] = syscalls.Supported("fremovexattr", Fremovexattr)
-	delete(s.Table, 206) // io_setup
-	delete(s.Table, 207) // io_destroy
-	delete(s.Table, 208) // io_getevents
-	delete(s.Table, 209) // io_submit
-	delete(s.Table, 210) // io_cancel
+	s.Table[209] = syscalls.PartiallySupported("io_submit", IoSubmit, "Generally supported with exceptions. User ring optimizations are not implemented.", []string{"gvisor.dev/issue/204"})
 	s.Table[213] = syscalls.Supported("epoll_create", EpollCreate)
 	s.Table[217] = syscalls.Supported("getdents64", Getdents64)
 	delete(s.Table, 221) // fdavise64