NationalSecurityAgency/ghidra · error · Exception

Could not build data for register value {rv.value}

Error message

Could not build data for register value {rv.value}

What it means

Raised inside the write_reg method when lldb.SBData.Append() returns False while assembling register value bytes. The method converts each byte of the register value via EvaluateExpression('(char){b}'), collects the results into an SBData buffer, and this error fires when a byte cannot be appended to that buffer.

Source

Thrown at Ghidra/Debug/Debugger-agent-lldb/src/main/py/src/ghidralldb/methods.py:761

@REGISTRY.method()
def write_reg(frame: StackFrame, name: str, value: bytes) -> None:
    """Write a register."""
    proc = find_proc_by_frame(frame)
    util.get_debugger().SetSelectedTarget(proc.target)
    f = find_frame_by_obj(frame)
    exec_convert_errors(f'frame select {f.idx}')
    rv = frame.trace.extra.require_rm().map_value_back(proc, name, value)
    reg = f.registers[0].GetChildMemberWithName(name)
    error = lldb.SBError()
    data = lldb.SBData()
    tgt = util.get_target()
    for b in rv.value:
        bv = tgt.EvaluateExpression(f"(char){b}")
        if bv.error.fail:
            raise Exception(bv.error.description)
        if not data.Append(bv.GetData()):
            raise Exception(f"Could not build data for register value {rv.value}")
    if not reg.SetData(data, error):
        raise Exception(error.description)
    with commands.open_tracked_tx(f'Write Register {name}'):
        exec_convert_errors('ghidra trace putreg')

View on GitHub (pinned to d5f144c24d)

Solutions

1. Check rv.value byte range before the loop: ensure each byte is 0-255.
2. Verify the LLDB target is in a valid state for expression evaluation before write_reg.
3. Log bv.GetData() and bv.error for the failing byte to diagnose the LLDB-side failure.
4. Consider replacing the byte-by-byte Append approach with a single SBData.SetData or CreateDataFromInt call.

Example fix

# before: byte-by-byte Append
for b in rv.value:
    bv = tgt.EvaluateExpression(f"(char){b}")
    if bv.error.fail:
        raise Exception(bv.error.description)
    if not data.Append(bv.GetData()):
        raise Exception(f"Could not build data for register value {rv.value}")

# after: single-shot SBData construction
from struct import pack
raw = bytes(rv.value)
data = lldb.SBData.CreateDataFromInt(
    int.from_bytes(raw, 'big' if rm.byte_order == 'big' else 'little'),
    size=len(raw),
) if hasattr(lldb.SBData, 'CreateDataFromInt') else lldb.SBData()
if not data and not raw:
    for b in rv.value:
        bv = tgt.EvaluateExpression(f"(char){b}")
        data.Append(bv.GetData())

for b in rv.value:
    if not (0 <= b <= 255):
        raise ValueError(f"Byte {b} out of range for (char) cast")
# also verify target is valid for expression evaluation
if not util.get_target().IsValid():
    raise RuntimeError("LLDB target is not valid for expression evaluation")

try:
    # ... build data via Append ...
except Exception as e:
    if 'Could not build data' in str(e):
        log.error(f"SBData.Append failed for register {name}, value={rv.value}")
        raise RuntimeError(f"Cannot write register {name}: LLDB data build failed") from e
    raise

When it happens

Trigger: write_reg is called with a frame, register name, and byte value. The mapped-back value rv.value is iterated byte-by-byte; for each byte, tgt.EvaluateExpression('(char){b}') is evaluated and its SBData is appended via data.Append(bv.GetData()). If Append returns False for any byte, the exception is raised.

Common situations: The byte value in rv.value is outside the valid range for the expression (e.g. negative or >255 if the expression cast is invalid). The LLDB target cannot evaluate the cast expression (e.g. invalid target state). A register value contains unexpected byte values from a faulty register mapper.

Related errors

• Usage: ghidra trace disassemble ADDRESS
• Usage: ghidra trace put-processes
• Usage: ghidra trace put-available
• Usage: ghidra trace put-breakpoints
• Usage: ghidra trace put-watchpoints

AI-assisted analysis of NationalSecurityAgency/ghidra@d5f144c24d (2026-08-14). Data as JSON: /api/errors/c6205207b5890478. Report an issue: GitHub.