Update lisafs to handle size=0 for FGetXattr RPC.

As per getxattr(2), if size=0 the syscall should return the size of the
attribute value. In the case of lisafs (and even 9P), the GetXattr RPC returns
a string, not an integer. The semantics here should be that if size=0, then the
entire attribute value is returned. 9P already implements this. Update lisafs
to do so.

PiperOrigin-RevId: 429717050

pkg/lisafs/client.go

@@ -101,7 +101,7 @@ func NewClient(sock *unet.Socket) (*Client, Inode, error) {
 		mountReq  MountReq
 		mountResp MountResp
 	)
-	if err := c.SndRcvMessage(Mount, uint32(mountReq.SizeBytes()), mountReq.MarshalBytes, mountResp.CheckedUnmarshal, nil, mountResp.String, mountResp.String); err != nil {
+	if err := c.SndRcvMessage(Mount, uint32(mountReq.SizeBytes()), mountReq.MarshalBytes, mountResp.CheckedUnmarshal, nil, mountReq.String, mountResp.String); err != nil {
 		return nil, Inode{}, err
 	}
 

pkg/lisafs/fd.go

@@ -458,13 +458,18 @@ type ControlFDImpl interface {
 	Renamed()
 
 	// GetXattr returns extended attributes of this file. It returns the number
-	// of bytes written into dataBuf.
+	// of bytes written into the buffer returned by getValueBuf which can be used
+	// to request buffer for some size.
 	//
-	// If the value is larger than len(dataBuf), implementations may return
-	// ERANGE to indicate that the buffer is too small.
+	// If the value is larger than size, implementations may return ERANGE to
+	// indicate that the buffer is too small.
+	//
+	// N.B. size may be 0, in which can the implementation must first find out
+	// the attribute value size using getxattr(2) by passing size=0. Then request
+	// a buffer large enough using getValueBuf and write the value there.
 	//
 	// On the server, GetXattr has a read concurrency guarantee.
-	GetXattr(name string, dataBuf []byte) (uint16, error)
+	GetXattr(name string, size uint32, getValueBuf func(uint32) []byte) (uint16, error)
 
 	// SetXattr sets extended attributes on this file.
 	//

pkg/lisafs/handlers.go

@@ -1295,17 +1295,19 @@ func FGetXattrHandler(c *Connection, comm Communicator, payloadLen uint32) (uint
 	// FGetXattrResp simply is a wrapper around SizedString.
 	var valueLen primitive.Uint16
 	respMetaSize := uint32(valueLen.SizeBytes())
-	payloadBuf := comm.PayloadBuf(respMetaSize + uint32(req.BufSize))
 	var n uint16
 	if err := fd.safelyRead(func() error {
 		if fd.node.isDeleted() {
 			return unix.EINVAL
 		}
-		n, err = fd.impl.GetXattr(string(req.Name), payloadBuf[respMetaSize:])
+		n, err = fd.impl.GetXattr(string(req.Name), uint32(req.BufSize), func(dataLen uint32) []byte {
+			return comm.PayloadBuf(dataLen + respMetaSize)[respMetaSize:]
+		})
 		return err
 	}); err != nil {
 		return 0, err
 	}
+	payloadBuf := comm.PayloadBuf(respMetaSize)
 	valueLen = primitive.Uint16(n)
 	valueLen.MarshalBytes(payloadBuf)
 	return respMetaSize + uint32(n), nil

runsc/fsgofer/lisafs.go

@@ -668,14 +668,21 @@ func (fd *controlFDLisa) Renamed() {
 }
 
 // GetXattr implements lisafs.ControlFDImpl.GetXattr.
-func (fd *controlFDLisa) GetXattr(name string, dataBuf []byte) (uint16, error) {
+func (fd *controlFDLisa) GetXattr(name string, size uint32, getValueBuf func(uint32) []byte) (uint16, error) {
 	if !fd.Conn().ServerImpl().(*LisafsServer).config.EnableVerityXattr {
 		return 0, unix.EOPNOTSUPP
 	}
 	if _, ok := verityXattrs[name]; !ok {
 		return 0, unix.EOPNOTSUPP
 	}
-	n, err := unix.Fgetxattr(fd.hostFD, name, dataBuf)
+	if size == 0 {
+		n, err := unix.Fgetxattr(fd.hostFD, name, nil)
+		if err != nil {
+			return 0, err
+		}
+		size = uint32(n)
+	}
+	n, err := unix.Fgetxattr(fd.hostFD, name, getValueBuf(size))
 	return uint16(n), err
 }