go-delve/delve · critical

Could not read byte

Error message

Could not read byte

What it means

advanceloc panics when it cannot read the delta byte for a DW_CFA_advance_loc1-style one-byte location advance. The frame instruction stream ended exactly where an operand byte was expected, meaning the CFI program is truncated relative to its declared length.

Source

Thrown at pkg/dwarf/frame/table.go:255

		instruction = DW_CFA_restore
		restore = true
	}

	if restore {
		// Restore the last byte as it actually contains the argument for the opcode.
		err := buf.UnreadByte()
		if err != nil {
			panic("Could not unread byte")
		}
	}

	return fnlookup[instruction]
}

func advanceloc(frame *FrameContext) {
	b, err := frame.buf.ReadByte()
	if err != nil {
		panic("Could not read byte")
	}

	delta := b & low_6_offset
	frame.loc += uint64(delta) * frame.codeAlignment
}

func advanceloc1(frame *FrameContext) {
	delta, err := frame.buf.ReadByte()
	if err != nil {
		panic("Could not read byte")
	}

	frame.loc += uint64(delta) * frame.codeAlignment
}

func advanceloc2(frame *FrameContext) {
	var delta uint16
	binary.Read(frame.buf, frame.order, &delta)

View on GitHub (pinned to a23773e6c3)

Solutions

1. Verify the binary/core integrity and re-acquire it from a trusted source
2. Check that the binary and core dump match (same build ID)
3. Report to delve maintainers — this should be an error return, not a panic
4. Recover from the panic at the call site and skip frame unwinding for that frame

Example fix

// before
frames, err := t.ExecuteUntilPC(pc) // panic on malformed stream
// after
func safeFrames(t *frame.Table, pc uint64) (frames []frame.Frame, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("invalid CFI stream: %v", r)
        }
    }()
    fde, e := t.ExecuteUntilPC(pc)
    if e != nil {
        return nil, e
    }
    frames, err = fde.EstablishFrame(t.CIE) // illustrative decode
    return
}

func safeAdvance(t *frame.FrameDescriptionEntry) (err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("truncated CFI stream: %v", r)
        }
    }()
    // trigger the decode path
    return nil
}

When it happens

Trigger: Parsing a frame description entry whose declared length extends past the end of the section data; corrupted .debug_frame/.eh_frame sections; decode bugs where buf was not rewound between CFI programs.

Common situations: Analyzing a truncated or partially downloaded binary; core dump files whose mapped frame sections are incomplete; third-party binaries with hand-written assembler frames.

Related errors

• Could not read from instruction buffer
• Could not unread byte
• panic(err)
• Could not parse ULEB128 value
• Could not parse SLEB128 value

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/486ded9fbcafeef8. Report an issue: GitHub.