gvisor/fs: don't update file.offset for sockets, pipes, etc

sockets, pipes and other non-seekable file descriptors don't
use file.offset, so we don't need to update it.

With this change, we will be able to call file operations
without locking the file.mu mutex. This is already used for
pipes in the splice system call.

PiperOrigin-RevId: 253746644

pkg/sentry/fs/dev/null.go

@@ -97,6 +97,7 @@ func newZeroDevice(ctx context.Context, owner fs.FileOwner, mode linux.FileMode)
 func (zd *zeroDevice) GetFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
 	flags.Pread = true
 	flags.Pwrite = true
+	flags.NonSeekable = true
 
 	return fs.NewFile(ctx, dirent, flags, &zeroFileOperations{}), nil
 }

pkg/sentry/fs/file.go

@@ -267,7 +267,7 @@ func (f *File) Readv(ctx context.Context, dst usermem.IOSequence) (int64, error)
 
 	reads.Increment()
 	n, err := f.FileOperations.Read(ctx, f, dst, f.offset)
-	if n > 0 {
+	if n > 0 && !f.flags.NonSeekable {
 		atomic.AddInt64(&f.offset, n)
 	}
 	f.mu.Unlock()
@@ -330,7 +330,7 @@ func (f *File) Writev(ctx context.Context, src usermem.IOSequence) (int64, error
 
 	// We must hold the lock during the write.
 	n, err := f.FileOperations.Write(ctx, f, src, f.offset)
-	if n >= 0 {
+	if n >= 0 && !f.flags.NonSeekable {
 		atomic.StoreInt64(&f.offset, f.offset+n)
 	}
 	f.mu.Unlock()

pkg/sentry/fs/flags.go

@@ -57,6 +57,9 @@ type FileFlags struct {
 	// Linux sets this flag for all files. Since gVisor is only compatible
 	// with 64-bit Linux, it also sets this flag for all files.
 	LargeFile bool
+
+	// NonSeekable indicates that file.offset isn't used.
+	NonSeekable bool
 }
 
 // SettableFileFlags is a subset of FileFlags above that can be changed

pkg/sentry/kernel/pipe/pipe.go

@@ -162,6 +162,7 @@ func NewConnectedPipe(ctx context.Context, sizeBytes, atomicIOBytes int64) (*fs.
 //
 // Precondition: at least one of flags.Read or flags.Write must be set.
 func (p *Pipe) Open(ctx context.Context, d *fs.Dirent, flags fs.FileFlags) *fs.File {
+	flags.NonSeekable = true
 	switch {
 	case flags.Read && flags.Write:
 		p.rOpen()

pkg/sentry/socket/epsocket/epsocket.go

@@ -262,7 +262,7 @@ func New(t *kernel.Task, family int, skType linux.SockType, protocol int, queue
 
 	dirent := socket.NewDirent(t, epsocketDevice)
 	defer dirent.DecRef()
-	return fs.NewFile(t, dirent, fs.FileFlags{Read: true, Write: true}, &SocketOperations{
+	return fs.NewFile(t, dirent, fs.FileFlags{Read: true, Write: true, NonSeekable: true}, &SocketOperations{
 		Queue:    queue,
 		family:   family,
 		Endpoint: endpoint,

pkg/sentry/socket/hostinet/socket.go

@@ -77,7 +77,7 @@ func newSocketFile(ctx context.Context, family int, stype linux.SockType, protoc
 	}
 	dirent := socket.NewDirent(ctx, socketDevice)
 	defer dirent.DecRef()
-	return fs.NewFile(ctx, dirent, fs.FileFlags{NonBlocking: nonblock, Read: true, Write: true}, s), nil
+	return fs.NewFile(ctx, dirent, fs.FileFlags{NonBlocking: nonblock, Read: true, Write: true, NonSeekable: true}, s), nil
 }
 
 // Release implements fs.FileOperations.Release.

pkg/sentry/socket/netlink/provider.go

@@ -89,7 +89,7 @@ func (*socketProvider) Socket(t *kernel.Task, stype linux.SockType, protocol int
 
 	d := socket.NewDirent(t, netlinkSocketDevice)
 	defer d.DecRef()
-	return fs.NewFile(t, d, fs.FileFlags{Read: true, Write: true}, s), nil
+	return fs.NewFile(t, d, fs.FileFlags{Read: true, Write: true, NonSeekable: true}, s), nil
 }
 
 // Pair implements socket.Provider.Pair by returning an error.

pkg/sentry/socket/rpcinet/socket.go

@@ -322,7 +322,13 @@ func (s *socketOperations) Accept(t *kernel.Task, peerRequested bool, flags int,
 
 	dirent := socket.NewDirent(t, socketDevice)
 	defer dirent.DecRef()
-	file := fs.NewFile(t, dirent, fs.FileFlags{Read: true, Write: true, NonBlocking: flags&linux.SOCK_NONBLOCK != 0}, &socketOperations{
+	fileFlags := fs.FileFlags{
+		Read:        true,
+		Write:       true,
+		NonSeekable: true,
+		NonBlocking: flags&linux.SOCK_NONBLOCK != 0,
+	}
+	file := fs.NewFile(t, dirent, fileFlags, &socketOperations{
 		wq:       &wq,
 		fd:       payload.Fd,
 		rpcConn:  s.rpcConn,

pkg/sentry/socket/unix/unix.go

@@ -64,7 +64,7 @@ type SocketOperations struct {
 func New(ctx context.Context, endpoint transport.Endpoint, stype linux.SockType) *fs.File {
 	dirent := socket.NewDirent(ctx, unixSocketDevice)
 	defer dirent.DecRef()
-	return NewWithDirent(ctx, dirent, endpoint, stype, fs.FileFlags{Read: true, Write: true})
+	return NewWithDirent(ctx, dirent, endpoint, stype, fs.FileFlags{Read: true, Write: true, NonSeekable: true})
 }
 
 // NewWithDirent creates a new unix socket using an existing dirent.