Implement FUSE_READ

Fixes #3206

13 files changed, 862 insertions, 36 deletions

diff --git a/pkg/abi/linux/fuse.go b/pkg/abi/linux/fuse.go
index 4ef0ab9a7..0ece7b756 100644
--- a/pkg/abi/linux/fuse.go
+++ b/pkg/abi/linux/fuse.go
@@ -184,6 +184,13 @@ const (
 	FUSE_KERNEL_MINOR_VERSION = 31
 )
 
+// Constants relevant to FUSE operations.
+const (
+	FUSE_NAME_MAX     = 1024
+	FUSE_PAGE_SIZE    = 4096
+	FUSE_DIRENT_ALIGN = 8
+)
+
 // FUSEInitIn is the request sent by the kernel to the daemon,
 // to negotiate the version and flags.
 //
@@ -392,6 +399,36 @@ type FUSEOpenOut struct {
 
 	// OpenFlag for the opened file.
 	OpenFlag uint32
+}
+
+// FUSE_READ flags, consistent with the ones in include/uapi/linux/fuse.h.
+const (
+	FUSE_READ_LOCKOWNER = 1 << 1
+)
+
+// FUSEReadIn is the request sent by the kernel to the daemon
+// for FUSE_READ.
+//
+// +marshal
+type FUSEReadIn struct {
+	// Fh is the file handle in userspace.
+	Fh uint64
+
+	// Offset is the read offset.
+	Offset uint64
+
+	// Size is the number of bytes to read.
+	Size uint32
+
+	// ReadFlags for this FUSE_READ request.
+	// Currently only contains FUSE_READ_LOCKOWNER.
+	ReadFlags uint32
+
+	// LockOwner is the id of the lock owner if there is one.
+	LockOwner uint64
+
+	// Flags for the underlying file.
+	Flags uint32
 
 	_ uint32
 }
diff --git a/pkg/sentry/fsimpl/fuse/BUILD b/pkg/sentry/fsimpl/fuse/BUILD
index b9dcee96a..241d50c3d 100644
--- a/pkg/sentry/fsimpl/fuse/BUILD
+++ b/pkg/sentry/fsimpl/fuse/BUILD
@@ -36,7 +36,9 @@ go_library(
         "fusefs.go",
         "init.go",
         "inode_refs.go",
+        "read_write.go",
         "register.go",
+        "regular_file.go",
         "request_list.go",
     ],
     visibility = ["//pkg/sentry:internal"],
@@ -46,6 +48,7 @@ go_library(
         "//pkg/log",
         "//pkg/marshal",
         "//pkg/refs",
+        "//pkg/safemem",
         "//pkg/sentry/fsimpl/devtmpfs",
         "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/kernel",
diff --git a/pkg/sentry/fsimpl/fuse/connection.go b/pkg/sentry/fsimpl/fuse/connection.go
index 9cfd67158..236165652 100644
--- a/pkg/sentry/fsimpl/fuse/connection.go
+++ b/pkg/sentry/fsimpl/fuse/connection.go
@@ -161,6 +161,7 @@ type connection struct {
 	bgLock sync.Mutex
 
 	// maxRead is the maximum size of a read buffer in in bytes.
+	// Initialized from a fuse fs parameter.
 	maxRead uint32
 
 	// maxWrite is the maximum size of a write buffer in bytes.
@@ -206,7 +207,7 @@ type connection struct {
 }
 
 // newFUSEConnection creates a FUSE connection to fd.
-func newFUSEConnection(_ context.Context, fd *vfs.FileDescription, maxInFlightRequests uint64) (*connection, error) {
+func newFUSEConnection(_ context.Context, fd *vfs.FileDescription, opts *filesystemOptions) (*connection, error) {
 	// Mark the device as ready so it can be used. /dev/fuse can only be used if the FD was used to
 	// mount a FUSE filesystem.
 	fuseFD := fd.Impl().(*DeviceFD)
@@ -216,13 +217,14 @@ func newFUSEConnection(_ context.Context, fd *vfs.FileDescription, maxInFlightRe
 	hdrLen := uint32((*linux.FUSEHeaderOut)(nil).SizeBytes())
 	fuseFD.writeBuf = make([]byte, hdrLen)
 	fuseFD.completions = make(map[linux.FUSEOpID]*futureResponse)
-	fuseFD.fullQueueCh = make(chan struct{}, maxInFlightRequests)
+	fuseFD.fullQueueCh = make(chan struct{}, opts.maxActiveRequests)
 	fuseFD.writeCursor = 0
 
 	return &connection{
 		fd:                  fuseFD,
 		maxBackground:       fuseDefaultMaxBackground,
 		congestionThreshold: fuseDefaultCongestionThreshold,
+		maxRead:             opts.maxRead,
 		maxPages:            fuseDefaultMaxPagesPerReq,
 		initializedChan:     make(chan struct{}),
 		connected:           true,
diff --git a/pkg/sentry/fsimpl/fuse/dev.go b/pkg/sentry/fsimpl/fuse/dev.go
index e2de8e097..fd3592e32 100644
--- a/pkg/sentry/fsimpl/fuse/dev.go
+++ b/pkg/sentry/fsimpl/fuse/dev.go
@@ -401,10 +401,12 @@ func (fd *DeviceFD) sendError(ctx context.Context, errno int32, req *Request) er
 // receiver is going to be waiting on the future channel. This is to be used by:
 // FUSE_INIT.
 func (fd *DeviceFD) noReceiverAction(ctx context.Context, r *Response) error {
-	if r.opcode == linux.FUSE_INIT {
+	switch r.opcode {
+	case linux.FUSE_INIT:
 		creds := auth.CredentialsFromContext(ctx)
 		rootUserNs := kernel.KernelFromContext(ctx).RootUserNamespace()
 		return fd.fs.conn.InitRecv(r, creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, rootUserNs))
+		// TODO(gvisor.dev/issue/3247): support async read: correctly process the response using information from r.options.
 	}
 
 	return nil
diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index 4dc8ef993..65e22ba4d 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -16,6 +16,7 @@
 package fuse
 
 import (
+	"math"
 	"strconv"
 	"sync/atomic"
 
@@ -58,6 +59,11 @@ type filesystemOptions struct {
 	// exist at any time. Any further requests will block when trying to
 	// Call the server.
 	maxActiveRequests uint64
+
+	// maxRead is the max number of bytes to read,
+	// specified as "max_read" in fs parameters.
+	// If not specified by user, use math.MaxUint32 as default value.
+	maxRead uint32
 }
 
 // filesystem implements vfs.FilesystemImpl.
@@ -144,6 +150,21 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	// Set the maxInFlightRequests option.
 	fsopts.maxActiveRequests = maxActiveRequestsDefault
 
+	if maxReadStr, ok := mopts["max_read"]; ok {
+		delete(mopts, "max_read")
+		maxRead, err := strconv.ParseUint(maxReadStr, 10, 32)
+		if err != nil {
+			log.Warningf("%s.GetFilesystem: invalid max_read: max_read=%s", fsType.Name(), maxReadStr)
+			return nil, nil, syserror.EINVAL
+		}
+		if maxRead < fuseMinMaxRead {
+			maxRead = fuseMinMaxRead
+		}
+		fsopts.maxRead = uint32(maxRead)
+	} else {
+		fsopts.maxRead = math.MaxUint32
+	}
+
 	// Check for unparsed options.
 	if len(mopts) != 0 {
 		log.Warningf("%s.GetFilesystem: unknown options: %v", fsType.Name(), mopts)
@@ -179,7 +200,7 @@ func NewFUSEFilesystem(ctx context.Context, devMinor uint32, opts *filesystemOpt
 		opts:     opts,
 	}
 
-	conn, err := newFUSEConnection(ctx, device, opts.maxActiveRequests)
+	conn, err := newFUSEConnection(ctx, device, opts)
 	if err != nil {
 		log.Warningf("fuse.NewFUSEFilesystem: NewFUSEConnection failed with error: %v", err)
 		return nil, syserror.EINVAL
@@ -244,6 +265,7 @@ func (fs *filesystem) newInode(nodeID uint64, attr linux.FUSEAttr) *kernfs.Dentr
 	i := &inode{fs: fs, NodeID: nodeID}
 	creds := auth.Credentials{EffectiveKGID: auth.KGID(attr.UID), EffectiveKUID: auth.KUID(attr.UID)}
 	i.InodeAttrs.Init(&creds, linux.UNNAMED_MAJOR, fs.devMinor, fs.NextIno(), linux.FileMode(attr.Mode))
+	atomic.StoreUint64(&i.size, attr.Size)
 	i.OrderedChildren.Init(kernfs.OrderedChildrenOptions{})
 	i.EnableLeakCheck()
 	i.dentry.Init(i)
@@ -269,10 +291,13 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 		fd = &(directoryFD.fileDescription)
 		fdImpl = directoryFD
 	} else {
-		// FOPEN_KEEP_CACHE is the defualt flag for noOpen.
-		fd = &fileDescription{OpenFlag: linux.FOPEN_KEEP_CACHE}
-		fdImpl = fd
+		regularFD := &regularFileFD{}
+		fd = &(regularFD.fileDescription)
+		fdImpl = regularFD
 	}
+	// FOPEN_KEEP_CACHE is the defualt flag for noOpen.
+	fd.OpenFlag = linux.FOPEN_KEEP_CACHE
+
 	// Only send open request when FUSE server support open or is opening a directory.
 	if !i.fs.conn.noOpen || isDir {
 		kernelTask := kernel.TaskFromContext(ctx)
@@ -281,21 +306,25 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 			return nil, syserror.EINVAL
 		}
 
+		// Build the request.
 		var opcode linux.FUSEOpcode
 		if isDir {
 			opcode = linux.FUSE_OPENDIR
 		} else {
 			opcode = linux.FUSE_OPEN
 		}
+
 		in := linux.FUSEOpenIn{Flags: opts.Flags & ^uint32(linux.O_CREAT|linux.O_EXCL|linux.O_NOCTTY)}
 		if !i.fs.conn.atomicOTrunc {
 			in.Flags &= ^uint32(linux.O_TRUNC)
 		}
+
 		req, err := i.fs.conn.NewRequest(auth.CredentialsFromContext(ctx), uint32(kernelTask.ThreadID()), i.NodeID, opcode, &in)
 		if err != nil {
 			return nil, err
 		}
 
+		// Send the request and receive the reply.
 		res, err := i.fs.conn.Call(kernelTask, req)
 		if err != nil {
 			return nil, err
@@ -309,15 +338,17 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentr
 			if err := res.UnmarshalPayload(&out); err != nil {
 				return nil, err
 			}
+
+			// Process the reply.
 			fd.OpenFlag = out.OpenFlag
+			if isDir {
+				fd.OpenFlag &= ^uint32(linux.FOPEN_DIRECT_IO)
+			}
+
 			fd.Fh = out.Fh
 		}
 	}
 
-	if isDir {
-		fd.OpenFlag &= ^uint32(linux.FOPEN_DIRECT_IO)
-	}
-
 	// TODO(gvisor.dev/issue/3234): invalidate mmap after implemented it for FUSE Inode
 	fd.DirectIO = fd.OpenFlag&linux.FOPEN_DIRECT_IO != 0
 	fdOptions := &vfs.FileDescriptionOptions{}
@@ -457,6 +488,16 @@ func (i *inode) Readlink(ctx context.Context, mnt *vfs.Mount) (string, error) {
 	return i.link, nil
 }
 
+// getFUSEAttr returns a linux.FUSEAttr of this inode stored in local cache.
+// TODO(gvisor.dev/issue/3679): Add support for other fields.
+func (i *inode) getFUSEAttr() linux.FUSEAttr {
+	return linux.FUSEAttr{
+		Ino:  i.Ino(),
+		Size: atomic.LoadUint64(&i.size),
+		Mode: uint32(i.Mode()),
+	}
+}
+
 // statFromFUSEAttr makes attributes from linux.FUSEAttr to linux.Statx. The
 // opts.Sync attribute is ignored since the synchronization is handled by the
 // FUSE server.
@@ -510,47 +551,90 @@ func statFromFUSEAttr(attr linux.FUSEAttr, mask, devMinor uint32) linux.Statx {
 	return stat
 }
 
-// Stat implements kernfs.Inode.Stat.
-func (i *inode) Stat(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error) {
-	fusefs := fs.Impl().(*filesystem)
-	conn := fusefs.conn
-	task, creds := kernel.TaskFromContext(ctx), auth.CredentialsFromContext(ctx)
+// getAttr gets the attribute of this inode by issuing a FUSE_GETATTR request
+// or read from local cache.
+// It updates the corresponding attributes if necessary.
+func (i *inode) getAttr(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions) (linux.FUSEAttr, error) {
+	attributeVersion := atomic.LoadUint64(&i.fs.conn.attributeVersion)
+
+	// TODO(gvisor.dev/issue/3679): send the request only if
+	// - invalid local cache for fields specified in the opts.Mask
+	// - forced update
+	// - i.attributeTime expired
+	// If local cache is still valid, return local cache.
+	// Currently we always send a request,
+	// and we always set the metadata with the new result,
+	// unless attributeVersion has changed.
+
+	task := kernel.TaskFromContext(ctx)
 	if task == nil {
 		log.Warningf("couldn't get kernel task from context")
-		return linux.Statx{}, syserror.EINVAL
+		return linux.FUSEAttr{}, syserror.EINVAL
 	}
 
+	creds := auth.CredentialsFromContext(ctx)
+
 	var in linux.FUSEGetAttrIn
 	// We don't set any attribute in the request, because in VFS2 fstat(2) will
 	// finally be translated into vfs.FilesystemImpl.StatAt() (see
 	// pkg/sentry/syscalls/linux/vfs2/stat.go), resulting in the same flow
 	// as stat(2). Thus GetAttrFlags and Fh variable will never be used in VFS2.
-	req, err := conn.NewRequest(creds, uint32(task.ThreadID()), i.NodeID, linux.FUSE_GETATTR, &in)
+	req, err := i.fs.conn.NewRequest(creds, uint32(task.ThreadID()), i.NodeID, linux.FUSE_GETATTR, &in)
 	if err != nil {
-		return linux.Statx{}, err
+		return linux.FUSEAttr{}, err
 	}
 
-	res, err := conn.Call(task, req)
+	res, err := i.fs.conn.Call(task, req)
 	if err != nil {
-		return linux.Statx{}, err
+		return linux.FUSEAttr{}, err
 	}
 	if err := res.Error(); err != nil {
-		return linux.Statx{}, err
+		return linux.FUSEAttr{}, err
 	}
 
 	var out linux.FUSEGetAttrOut
 	if err := res.UnmarshalPayload(&out); err != nil {
-		return linux.Statx{}, err
+		return linux.FUSEAttr{}, err
 	}
 
-	// Set all metadata into kernfs.InodeAttrs.
+	// Local version is newer, return the local one.
+	// Skip the update.
+	if attributeVersion != 0 && atomic.LoadUint64(&i.attributeVersion) > attributeVersion {
+		return i.getFUSEAttr(), nil
+	}
+
+	// Set the metadata of kernfs.InodeAttrs.
 	if err := i.SetStat(ctx, fs, creds, vfs.SetStatOptions{
-		Stat: statFromFUSEAttr(out.Attr, linux.STATX_ALL, fusefs.devMinor),
+		Stat: statFromFUSEAttr(out.Attr, linux.STATX_ALL, i.fs.devMinor),
 	}); err != nil {
+		return linux.FUSEAttr{}, err
+	}
+
+	// Set the size if no error (after SetStat() check).
+	atomic.StoreUint64(&i.size, out.Attr.Size)
+
+	return out.Attr, nil
+}
+
+// reviseAttr attempts to update the attributes for internal purposes
+// by calling getAttr with a pre-specified mask.
+// Used by read, write, lseek.
+func (i *inode) reviseAttr(ctx context.Context) error {
+	// Never need atime for internal purposes.
+	_, err := i.getAttr(ctx, i.fs.VFSFilesystem(), vfs.StatOptions{
+		Mask: linux.STATX_BASIC_STATS &^ linux.STATX_ATIME,
+	})
+	return err
+}
+
+// Stat implements kernfs.Inode.Stat.
+func (i *inode) Stat(ctx context.Context, fs *vfs.Filesystem, opts vfs.StatOptions) (linux.Statx, error) {
+	attr, err := i.getAttr(ctx, fs, opts)
+	if err != nil {
 		return linux.Statx{}, err
 	}
 
-	return statFromFUSEAttr(out.Attr, opts.Mask, fusefs.devMinor), nil
+	return statFromFUSEAttr(attr, opts.Mask, i.fs.devMinor), nil
 }
 
 // DecRef implements kernfs.Inode.
diff --git a/pkg/sentry/fsimpl/fuse/init.go b/pkg/sentry/fsimpl/fuse/init.go
index 779c2bd3f..2ff2542b6 100644
--- a/pkg/sentry/fsimpl/fuse/init.go
+++ b/pkg/sentry/fsimpl/fuse/init.go
@@ -29,9 +29,10 @@ const (
 	// Follow the same behavior as unix fuse implementation.
 	fuseMaxTimeGranNs = 1000000000
 
-	// Minimum value for MaxWrite.
+	// Minimum value for MaxWrite and MaxRead.
 	// Follow the same behavior as unix fuse implementation.
 	fuseMinMaxWrite = 4096
+	fuseMinMaxRead  = 4096
 
 	// Temporary default value for max readahead, 128kb.
 	fuseDefaultMaxReadahead = 131072
diff --git a/pkg/sentry/fsimpl/fuse/read_write.go b/pkg/sentry/fsimpl/fuse/read_write.go
new file mode 100644
index 000000000..4ef8531dc
--- /dev/null
+++ b/pkg/sentry/fsimpl/fuse/read_write.go
@@ -0,0 +1,152 @@
+// 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 fuse
+
+import (
+	"io"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// ReadInPages sends FUSE_READ requests for the size after round it up to
+// a multiple of page size, blocks on it for reply, processes the reply
+// and returns the payload (or joined payloads) as a byte slice.
+// This is used for the general purpose reading.
+// We do not support direct IO (which read the exact number of bytes)
+// at this moment.
+func (fs *filesystem) ReadInPages(ctx context.Context, fd *regularFileFD, off uint64, size uint32) ([][]byte, uint32, error) {
+	attributeVersion := atomic.LoadUint64(&fs.conn.attributeVersion)
+
+	t := kernel.TaskFromContext(ctx)
+	if t == nil {
+		log.Warningf("fusefs.Read: couldn't get kernel task from context")
+		return nil, 0, syserror.EINVAL
+	}
+
+	// Round up to a multiple of page size.
+	readSize, _ := usermem.PageRoundUp(uint64(size))
+
+	// One request cannnot exceed either maxRead or maxPages.
+	maxPages := fs.conn.maxRead >> usermem.PageShift
+	if maxPages > uint32(fs.conn.maxPages) {
+		maxPages = uint32(fs.conn.maxPages)
+	}
+
+	var outs [][]byte
+	var sizeRead uint32
+
+	// readSize is a multiple of usermem.PageSize.
+	// Always request bytes as a multiple of pages.
+	pagesRead, pagesToRead := uint32(0), uint32(readSize>>usermem.PageShift)
+
+	// Reuse the same struct for unmarshalling to avoid unnecessary memory allocation.
+	in := linux.FUSEReadIn{
+		Fh:        fd.Fh,
+		LockOwner: 0, // TODO(gvisor.dev/issue/3245): file lock
+		ReadFlags: 0, // TODO(gvisor.dev/issue/3245): |= linux.FUSE_READ_LOCKOWNER
+		Flags:     fd.statusFlags(),
+	}
+
+	// This loop is intended for fragmented read where the bytes to read is
+	// larger than either the maxPages or maxRead.
+	// For the majority of reads with normal size, this loop should only
+	// execute once.
+	for pagesRead < pagesToRead {
+		pagesCanRead := pagesToRead - pagesRead
+		if pagesCanRead > maxPages {
+			pagesCanRead = maxPages
+		}
+
+		in.Offset = off + (uint64(pagesRead) << usermem.PageShift)
+		in.Size = pagesCanRead << usermem.PageShift
+
+		req, err := fs.conn.NewRequest(auth.CredentialsFromContext(ctx), uint32(t.ThreadID()), fd.inode().NodeID, linux.FUSE_READ, &in)
+		if err != nil {
+			return nil, 0, err
+		}
+
+		// TODO(gvisor.dev/issue/3247): support async read.
+
+		res, err := fs.conn.Call(t, req)
+		if err != nil {
+			return nil, 0, err
+		}
+		if err := res.Error(); err != nil {
+			return nil, 0, err
+		}
+
+		// Not enough bytes in response,
+		// either we reached EOF,
+		// or the FUSE server sends back a response
+		// that cannot even fit the hdr.
+		if len(res.data) <= res.hdr.SizeBytes() {
+			// We treat both case as EOF here for now
+			// since there is no reliable way to detect
+			// the over-short hdr case.
+			break
+		}
+
+		// Directly using the slice to avoid extra copy.
+		out := res.data[res.hdr.SizeBytes():]
+
+		outs = append(outs, out)
+		sizeRead += uint32(len(out))
+
+		pagesRead += pagesCanRead
+	}
+
+	defer fs.ReadCallback(ctx, fd, off, size, sizeRead, attributeVersion)
+
+	// No bytes returned: offset >= EOF.
+	if len(outs) == 0 {
+		return nil, 0, io.EOF
+	}
+
+	return outs, sizeRead, nil
+}
+
+// ReadCallback updates several information after receiving a read response.
+// Due to readahead, sizeRead can be larger than size.
+func (fs *filesystem) ReadCallback(ctx context.Context, fd *regularFileFD, off uint64, size uint32, sizeRead uint32, attributeVersion uint64) {
+	// TODO(gvisor.dev/issue/3247): support async read.
+	// If this is called by an async read, correctly process it.
+	// May need to update the signature.
+
+	i := fd.inode()
+	// TODO(gvisor.dev/issue/1193): Invalidate or update atime.
+
+	// Reached EOF.
+	if sizeRead < size {
+		// TODO(gvisor.dev/issue/3630): If we have writeback cache, then we need to fill this hole.
+		// Might need to update the buf to be returned from the Read().
+
+		// Update existing size.
+		newSize := off + uint64(sizeRead)
+		fs.conn.mu.Lock()
+		if attributeVersion == i.attributeVersion && newSize < atomic.LoadUint64(&i.size) {
+			fs.conn.attributeVersion++
+			i.attributeVersion = i.fs.conn.attributeVersion
+			atomic.StoreUint64(&i.size, newSize)
+		}
+		fs.conn.mu.Unlock()
+	}
+}
diff --git a/pkg/sentry/fsimpl/fuse/regular_file.go b/pkg/sentry/fsimpl/fuse/regular_file.go
new file mode 100644
index 000000000..37ce4e268
--- /dev/null
+++ b/pkg/sentry/fsimpl/fuse/regular_file.go
@@ -0,0 +1,125 @@
+// 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 fuse
+
+import (
+	"io"
+	"math"
+	"sync"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+type regularFileFD struct {
+	fileDescription
+
+	// off is the file offset.
+	off int64
+	// offMu protects off.
+	offMu sync.Mutex
+}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *regularFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+
+	// Check that flags are supported.
+	//
+	// TODO(gvisor.dev/issue/2601): Support select preadv2 flags.
+	if opts.Flags&^linux.RWF_HIPRI != 0 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	size := dst.NumBytes()
+	if size == 0 {
+		// Early return if count is 0.
+		return 0, nil
+	} else if size > math.MaxUint32 {
+		// FUSE only supports uint32 for size.
+		// Overflow.
+		return 0, syserror.EINVAL
+	}
+
+	// TODO(gvisor.dev/issue/3678): Add direct IO support.
+
+	inode := fd.inode()
+
+	// Reading beyond EOF, update file size if outdated.
+	if uint64(offset+size) > atomic.LoadUint64(&inode.size) {
+		if err := inode.reviseAttr(ctx); err != nil {
+			return 0, err
+		}
+		// If the offset after update is still too large, return error.
+		if uint64(offset) >= atomic.LoadUint64(&inode.size) {
+			return 0, io.EOF
+		}
+	}
+
+	// Truncate the read with updated file size.
+	fileSize := atomic.LoadUint64(&inode.size)
+	if uint64(offset+size) > fileSize {
+		size = int64(fileSize) - offset
+	}
+
+	buffers, n, err := inode.fs.ReadInPages(ctx, fd, uint64(offset), uint32(size))
+	if err != nil {
+		return 0, err
+	}
+
+	// TODO(gvisor.dev/issue/3237): support indirect IO (e.g. caching),
+	// store the bytes that were read ahead.
+
+	// Update the number of bytes to copy for short read.
+	if n < uint32(size) {
+		size = int64(n)
+	}
+
+	// Copy the bytes read to the dst.
+	// This loop is intended for fragmented reads.
+	// For the majority of reads, this loop only execute once.
+	var copied int64
+	for _, buffer := range buffers {
+		toCopy := int64(len(buffer))
+		if copied+toCopy > size {
+			toCopy = size - copied
+		}
+		cp, err := dst.DropFirst64(copied).CopyOut(ctx, buffer[:toCopy])
+		if err != nil {
+			return 0, err
+		}
+		if int64(cp) != toCopy {
+			return 0, syserror.EIO
+		}
+		copied += toCopy
+	}
+
+	return copied, nil
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *regularFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	fd.offMu.Lock()
+	n, err := fd.PRead(ctx, dst, fd.off, opts)
+	fd.off += n
+	fd.offMu.Unlock()
+	return n, err
+}
diff --git a/test/fuse/BUILD b/test/fuse/BUILD
index 8bde81e3c..cae51ce49 100644
--- a/test/fuse/BUILD
+++ b/test/fuse/BUILD
@@ -36,3 +36,8 @@ syscall_test(
     fuse = "True",
     test = "//test/fuse/linux:mkdir_test",
 )
+
+syscall_test(
+    fuse = "True",
+    test = "//test/fuse/linux:read_test",
+)
diff --git a/test/fuse/linux/BUILD b/test/fuse/linux/BUILD
index 298ea11f8..8afd28f16 100644
--- a/test/fuse/linux/BUILD
+++ b/test/fuse/linux/BUILD
@@ -112,3 +112,16 @@ cc_library(
         "@com_google_absl//absl/strings:str_format",
     ],
 )
+
+cc_binary(
+    name = "read_test",
+    testonly = 1,
+    srcs = ["read_test.cc"],
+    deps = [
+        gtest,
+        ":fuse_base",
+        "//test/util:fuse_util",
+        "//test/util:test_main",
+        "//test/util:test_util",
+    ],
+)
\ No newline at end of file
diff --git a/test/fuse/linux/fuse_base.cc b/test/fuse/linux/fuse_base.cc
index 98b4e1466..e3c6b585c 100644
--- a/test/fuse/linux/fuse_base.cc
+++ b/test/fuse/linux/fuse_base.cc
@@ -129,7 +129,8 @@ void FuseTest::SkipServerActualRequest() {
 
 // Sends the `kSetInodeLookup` command, expected mode, and the path of the
 // inode to create under the mount point.
-void FuseTest::SetServerInodeLookup(const std::string& path, mode_t mode) {
+void FuseTest::SetServerInodeLookup(const std::string& path, mode_t mode,
+                                    uint64_t size) {
   uint32_t cmd = static_cast<uint32_t>(FuseTestCmd::kSetInodeLookup);
   EXPECT_THAT(RetryEINTR(write)(sock_[0], &cmd, sizeof(cmd)),
               SyscallSucceedsWithValue(sizeof(cmd)));
@@ -137,6 +138,9 @@ void FuseTest::SetServerInodeLookup(const std::string& path, mode_t mode) {
   EXPECT_THAT(RetryEINTR(write)(sock_[0], &mode, sizeof(mode)),
               SyscallSucceedsWithValue(sizeof(mode)));
 
+  EXPECT_THAT(RetryEINTR(write)(sock_[0], &size, sizeof(size)),
+              SyscallSucceedsWithValue(sizeof(size)));
+
   // Pad 1 byte for null-terminate c-string.
   EXPECT_THAT(RetryEINTR(write)(sock_[0], path.c_str(), path.size() + 1),
               SyscallSucceedsWithValue(path.size() + 1));
@@ -144,10 +148,10 @@ void FuseTest::SetServerInodeLookup(const std::string& path, mode_t mode) {
   WaitServerComplete();
 }
 
-void FuseTest::MountFuse() {
+void FuseTest::MountFuse(const char* mountOpts) {
   EXPECT_THAT(dev_fd_ = open("/dev/fuse", O_RDWR), SyscallSucceeds());
 
-  std::string mount_opts = absl::StrFormat("fd=%d,%s", dev_fd_, kMountOpts);
+  std::string mount_opts = absl::StrFormat("fd=%d,%s", dev_fd_, mountOpts);
   mount_point_ = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
   EXPECT_THAT(mount("fuse", mount_point_.path().c_str(), "fuse",
                     MS_NODEV | MS_NOSUID, mount_opts.c_str()),
@@ -311,11 +315,15 @@ void FuseTest::ServerHandleCommand() {
 // request with this specific path comes in.
 void FuseTest::ServerReceiveInodeLookup() {
   mode_t mode;
+  uint64_t size;
   std::vector<char> buf(FUSE_MIN_READ_BUFFER);
 
   EXPECT_THAT(RetryEINTR(read)(sock_[1], &mode, sizeof(mode)),
               SyscallSucceedsWithValue(sizeof(mode)));
 
+  EXPECT_THAT(RetryEINTR(read)(sock_[1], &size, sizeof(size)),
+              SyscallSucceedsWithValue(sizeof(size)));
+
   EXPECT_THAT(RetryEINTR(read)(sock_[1], buf.data(), buf.size()),
               SyscallSucceeds());
 
@@ -332,6 +340,9 @@ void FuseTest::ServerReceiveInodeLookup() {
   // comply with the unqiueness of different path.
   ++nodeid_;
 
+  // Set the size.
+  out_payload.attr.size = size;
+
   memcpy(buf.data(), &out_header, sizeof(out_header));
   memcpy(buf.data() + sizeof(out_header), &out_payload, sizeof(out_payload));
   lookups_.AddMemBlock(FUSE_LOOKUP, buf.data(), out_len);
diff --git a/test/fuse/linux/fuse_base.h b/test/fuse/linux/fuse_base.h
index ff4c4499d..452748d6d 100644
--- a/test/fuse/linux/fuse_base.h
+++ b/test/fuse/linux/fuse_base.h
@@ -137,7 +137,8 @@ class FuseTest : public ::testing::Test {
   // path, pretending there is an inode and avoid ENOENT when testing. If mode
   // is not given, it creates a regular file with mode 0600.
   void SetServerInodeLookup(const std::string& path,
-                            mode_t mode = S_IFREG | S_IRUSR | S_IWUSR);
+                            mode_t mode = S_IFREG | S_IRUSR | S_IWUSR,
+                            uint64_t size = 512);
 
   // Called by the testing thread to ask the FUSE server for its next received
   // FUSE request. Be sure to use the corresponding struct of iovec to receive
@@ -166,16 +167,16 @@ class FuseTest : public ::testing::Test {
  protected:
   TempPath mount_point_;
 
-  // Unmounts the mountpoint of the FUSE server.
-  void UnmountFuse();
-
- private:
   // Opens /dev/fuse and inherit the file descriptor for the FUSE server.
-  void MountFuse();
+  void MountFuse(const char* mountOpts = kMountOpts);
 
   // Creates a socketpair for communication and forks FUSE server.
   void SetUpFuseServer();
 
+  // Unmounts the mountpoint of the FUSE server.
+  void UnmountFuse();
+
+ private:
   // Sends a FuseTestCmd and gets a uint32_t data from the FUSE server.
   inline uint32_t GetServerData(uint32_t cmd);
 
diff --git a/test/fuse/linux/read_test.cc b/test/fuse/linux/read_test.cc
new file mode 100644
index 000000000..c702651bd
--- /dev/null
+++ b/test/fuse/linux/read_test.cc
@@ -0,0 +1,390 @@
+// 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.
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/fuse.h>
+#include <sys/stat.h>
+#include <sys/statfs.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "test/fuse/linux/fuse_base.h"
+#include "test/util/fuse_util.h"
+#include "test/util/test_util.h"
+
+namespace gvisor {
+namespace testing {
+
+namespace {
+
+class ReadTest : public FuseTest {
+  void SetUp() override {
+    FuseTest::SetUp();
+    test_file_path_ = JoinPath(mount_point_.path().c_str(), test_file_);
+  }
+
+  // TearDown overrides the parent's function
+  // to skip checking the unconsumed release request at the end.
+  void TearDown() override { UnmountFuse(); }
+
+ protected:
+  const std::string test_file_ = "test_file";
+  const mode_t test_file_mode_ = S_IFREG | S_IRWXU | S_IRWXG | S_IRWXO;
+  const uint64_t test_fh_ = 1;
+  const uint32_t open_flag_ = O_RDWR;
+
+  std::string test_file_path_;
+
+  PosixErrorOr<FileDescriptor> OpenTestFile(const std::string &path,
+                                            uint64_t size = 512) {
+    SetServerInodeLookup(test_file_, test_file_mode_, size);
+
+    struct fuse_out_header out_header_open = {
+        .len = sizeof(struct fuse_out_header) + sizeof(struct fuse_open_out),
+    };
+    struct fuse_open_out out_payload_open = {
+        .fh = test_fh_,
+        .open_flags = open_flag_,
+    };
+    auto iov_out_open = FuseGenerateIovecs(out_header_open, out_payload_open);
+    SetServerResponse(FUSE_OPEN, iov_out_open);
+
+    auto res = Open(path.c_str(), open_flag_);
+    if (res.ok()) {
+      SkipServerActualRequest();
+    }
+    return res;
+  }
+};
+
+class ReadTestSmallMaxRead : public ReadTest {
+  void SetUp() override {
+    MountFuse(mountOpts);
+    SetUpFuseServer();
+    test_file_path_ = JoinPath(mount_point_.path().c_str(), test_file_);
+  }
+
+ protected:
+  constexpr static char mountOpts[] =
+      "rootmode=755,user_id=0,group_id=0,max_read=4096";
+  // 4096 is hard-coded as the max_read in mount options.
+  const int size_fragment = 4096;
+};
+
+TEST_F(ReadTest, ReadWhole) {
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_));
+
+  // Prepare for the read.
+  const int n_read = 5;
+  std::vector<char> data(n_read);
+  RandomizeBuffer(data.data(), data.size());
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+  SetServerResponse(FUSE_READ, iov_out_read);
+
+  // Read the whole "file".
+  std::vector<char> buf(n_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), n_read),
+              SyscallSucceedsWithValue(n_read));
+
+  // Check the read request.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+  GetServerActualRequest(iov_in);
+
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_header_read.len,
+            sizeof(in_header_read) + sizeof(in_payload_read));
+  EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+  EXPECT_EQ(in_payload_read.offset, 0);
+  EXPECT_EQ(buf, data);
+}
+
+TEST_F(ReadTest, ReadPartial) {
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_));
+
+  // Prepare for the read.
+  const int n_data = 10;
+  std::vector<char> data(n_data);
+  RandomizeBuffer(data.data(), data.size());
+  // Note: due to read ahead, current read implementation will treat any
+  // response that is longer than requested as correct (i.e. not reach the EOF).
+  // Therefore, the test below should make sure the size to read does not exceed
+  // n_data.
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+
+  std::vector<char> buf(n_data);
+
+  // Read 1 bytes.
+  SetServerResponse(FUSE_READ, iov_out_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), 1), SyscallSucceedsWithValue(1));
+
+  // Check the 1-byte read request.
+  GetServerActualRequest(iov_in);
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_header_read.len,
+            sizeof(in_header_read) + sizeof(in_payload_read));
+  EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+  EXPECT_EQ(in_payload_read.offset, 0);
+
+  // Read 3 bytes.
+  SetServerResponse(FUSE_READ, iov_out_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), 3), SyscallSucceedsWithValue(3));
+
+  // Check the 3-byte read request.
+  GetServerActualRequest(iov_in);
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_payload_read.offset, 1);
+
+  // Read 5 bytes.
+  SetServerResponse(FUSE_READ, iov_out_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), 5), SyscallSucceedsWithValue(5));
+
+  // Check the 5-byte read request.
+  GetServerActualRequest(iov_in);
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_payload_read.offset, 4);
+}
+
+TEST_F(ReadTest, PRead) {
+  const int file_size = 512;
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_, file_size));
+
+  // Prepare for the read.
+  const int n_read = 5;
+  std::vector<char> data(n_read);
+  RandomizeBuffer(data.data(), data.size());
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+  SetServerResponse(FUSE_READ, iov_out_read);
+
+  // Read some bytes.
+  std::vector<char> buf(n_read);
+  const int offset_read = file_size >> 1;
+  EXPECT_THAT(pread(fd.get(), buf.data(), n_read, offset_read),
+              SyscallSucceedsWithValue(n_read));
+
+  // Check the read request.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+  GetServerActualRequest(iov_in);
+
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_header_read.len,
+            sizeof(in_header_read) + sizeof(in_payload_read));
+  EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+  EXPECT_EQ(in_payload_read.offset, offset_read);
+  EXPECT_EQ(buf, data);
+}
+
+TEST_F(ReadTest, ReadZero) {
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_));
+
+  // Issue the read.
+  std::vector<char> buf;
+  EXPECT_THAT(read(fd.get(), buf.data(), 0), SyscallSucceedsWithValue(0));
+}
+
+TEST_F(ReadTest, ReadShort) {
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_));
+
+  // Prepare for the short read.
+  const int n_read = 5;
+  std::vector<char> data(n_read >> 1);
+  RandomizeBuffer(data.data(), data.size());
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+  SetServerResponse(FUSE_READ, iov_out_read);
+
+  // Read the whole "file".
+  std::vector<char> buf(n_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), n_read),
+              SyscallSucceedsWithValue(data.size()));
+
+  // Check the read request.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+  GetServerActualRequest(iov_in);
+
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_header_read.len,
+            sizeof(in_header_read) + sizeof(in_payload_read));
+  EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+  EXPECT_EQ(in_payload_read.offset, 0);
+  std::vector<char> short_buf(buf.begin(), buf.begin() + data.size());
+  EXPECT_EQ(short_buf, data);
+}
+
+TEST_F(ReadTest, ReadShortEOF) {
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_));
+
+  // Prepare for the short read.
+  struct fuse_out_header out_header_read = {
+      .len = static_cast<uint32_t>(sizeof(struct fuse_out_header)),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read);
+  SetServerResponse(FUSE_READ, iov_out_read);
+
+  // Read the whole "file".
+  const int n_read = 10;
+  std::vector<char> buf(n_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), n_read), SyscallSucceedsWithValue(0));
+
+  // Check the read request.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+  GetServerActualRequest(iov_in);
+
+  EXPECT_EQ(in_payload_read.fh, test_fh_);
+  EXPECT_EQ(in_header_read.len,
+            sizeof(in_header_read) + sizeof(in_payload_read));
+  EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+  EXPECT_EQ(in_payload_read.offset, 0);
+}
+
+TEST_F(ReadTestSmallMaxRead, ReadSmallMaxRead) {
+  const int n_fragment = 10;
+  const int n_read = size_fragment * n_fragment;
+
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_, n_read));
+
+  // Prepare for the read.
+  std::vector<char> data(size_fragment);
+  RandomizeBuffer(data.data(), data.size());
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+
+  for (int i = 0; i < n_fragment; ++i) {
+    SetServerResponse(FUSE_READ, iov_out_read);
+  }
+
+  // Read the whole "file".
+  std::vector<char> buf(n_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), n_read),
+              SyscallSucceedsWithValue(n_read));
+
+  ASSERT_EQ(GetServerNumUnsentResponses(), 0);
+  ASSERT_EQ(GetServerNumUnconsumedRequests(), n_fragment);
+
+  // Check each read segment.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+
+  for (int i = 0; i < n_fragment; ++i) {
+    GetServerActualRequest(iov_in);
+    EXPECT_EQ(in_payload_read.fh, test_fh_);
+    EXPECT_EQ(in_header_read.len,
+              sizeof(in_header_read) + sizeof(in_payload_read));
+    EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+    EXPECT_EQ(in_payload_read.offset, i * size_fragment);
+    EXPECT_EQ(in_payload_read.size, size_fragment);
+
+    auto it = buf.begin() + i * size_fragment;
+    EXPECT_EQ(std::vector<char>(it, it + size_fragment), data);
+  }
+}
+
+TEST_F(ReadTestSmallMaxRead, ReadSmallMaxReadShort) {
+  const int n_fragment = 10;
+  const int n_read = size_fragment * n_fragment;
+
+  auto fd = ASSERT_NO_ERRNO_AND_VALUE(OpenTestFile(test_file_path_, n_read));
+
+  // Prepare for the read.
+  std::vector<char> data(size_fragment);
+  RandomizeBuffer(data.data(), data.size());
+  struct fuse_out_header out_header_read = {
+      .len =
+          static_cast<uint32_t>(sizeof(struct fuse_out_header) + data.size()),
+  };
+  auto iov_out_read = FuseGenerateIovecs(out_header_read, data);
+
+  for (int i = 0; i < n_fragment - 1; ++i) {
+    SetServerResponse(FUSE_READ, iov_out_read);
+  }
+
+  // The last fragment is a short read.
+  std::vector<char> half_data(data.begin(), data.begin() + (data.size() >> 1));
+  struct fuse_out_header out_header_read_short = {
+      .len = static_cast<uint32_t>(sizeof(struct fuse_out_header) +
+                                   half_data.size()),
+  };
+  auto iov_out_read_short =
+      FuseGenerateIovecs(out_header_read_short, half_data);
+  SetServerResponse(FUSE_READ, iov_out_read_short);
+
+  // Read the whole "file".
+  std::vector<char> buf(n_read);
+  EXPECT_THAT(read(fd.get(), buf.data(), n_read),
+              SyscallSucceedsWithValue(n_read - (data.size() >> 1)));
+
+  ASSERT_EQ(GetServerNumUnsentResponses(), 0);
+  ASSERT_EQ(GetServerNumUnconsumedRequests(), n_fragment);
+
+  // Check each read segment.
+  struct fuse_in_header in_header_read;
+  struct fuse_read_in in_payload_read;
+  auto iov_in = FuseGenerateIovecs(in_header_read, in_payload_read);
+
+  for (int i = 0; i < n_fragment; ++i) {
+    GetServerActualRequest(iov_in);
+    EXPECT_EQ(in_payload_read.fh, test_fh_);
+    EXPECT_EQ(in_header_read.len,
+              sizeof(in_header_read) + sizeof(in_payload_read));
+    EXPECT_EQ(in_header_read.opcode, FUSE_READ);
+    EXPECT_EQ(in_payload_read.offset, i * size_fragment);
+    EXPECT_EQ(in_payload_read.size, size_fragment);
+
+    auto it = buf.begin() + i * size_fragment;
+    if (i != n_fragment - 1) {
+      EXPECT_EQ(std::vector<char>(it, it + data.size()), data);
+    } else {
+      EXPECT_EQ(std::vector<char>(it, it + half_data.size()), half_data);
+    }
+  }
+}
+
+}  // namespace
+
+}  // namespace testing
+}  // namespace gvisor
\ No newline at end of file