table.go

package frame

import (
	"bytes"
	"encoding/binary"
	"fmt"

	"github.com/derekparker/delve/pkg/dwarf/util"
)

type DWRule struct {
	Rule       Rule
	Offset     int64
	Reg        uint64
	Expression []byte
}

type FrameContext struct {
	loc           uint64
	order         binary.ByteOrder
	address       uint64
	CFA           DWRule
	Regs          map[uint64]DWRule
	initialRegs   map[uint64]DWRule
	prevRegs      map[uint64]DWRule
	buf           *bytes.Buffer
	cie           *CommonInformationEntry
	RetAddrReg    uint64
	codeAlignment uint64
	dataAlignment int64
}

// Instructions used to recreate the table from the .debug_frame data.
const (
	DW_CFA_nop                = 0x0        // No ops
	DW_CFA_set_loc            = 0x01       // op1: address
	DW_CFA_advance_loc1       = iota       // op1: 1-bytes delta
	DW_CFA_advance_loc2                    // op1: 2-byte delta
	DW_CFA_advance_loc4                    // op1: 4-byte delta
	DW_CFA_offset_extended                 // op1: ULEB128 register, op2: ULEB128 offset
	DW_CFA_restore_extended                // op1: ULEB128 register
	DW_CFA_undefined                       // op1: ULEB128 register
	DW_CFA_same_value                      // op1: ULEB128 register
	DW_CFA_register                        // op1: ULEB128 register, op2: ULEB128 register
	DW_CFA_remember_state                  // No ops
	DW_CFA_restore_state                   // No ops
	DW_CFA_def_cfa                         // op1: ULEB128 register, op2: ULEB128 offset
	DW_CFA_def_cfa_register                // op1: ULEB128 register
	DW_CFA_def_cfa_offset                  // op1: ULEB128 offset
	DW_CFA_def_cfa_expression              // op1: BLOCK
	DW_CFA_expression                      // op1: ULEB128 register, op2: BLOCK
	DW_CFA_offset_extended_sf              // op1: ULEB128 register, op2: SLEB128 BLOCK
	DW_CFA_def_cfa_sf                      // op1: ULEB128 register, op2: SLEB128 offset
	DW_CFA_def_cfa_offset_sf               // op1: SLEB128 offset
	DW_CFA_val_offset                      // op1: ULEB128, op2: ULEB128
	DW_CFA_val_offset_sf                   // op1: ULEB128, op2: SLEB128
	DW_CFA_val_expression                  // op1: ULEB128, op2: BLOCK
	DW_CFA_lo_user            = 0x1c       // op1: BLOCK
	DW_CFA_hi_user            = 0x3f       // op1: ULEB128 register, op2: BLOCK
	DW_CFA_advance_loc        = (0x1 << 6) // High 2 bits: 0x1, low 6: delta
	DW_CFA_offset             = (0x2 << 6) // High 2 bits: 0x2, low 6: register
	DW_CFA_restore            = (0x3 << 6) // High 2 bits: 0x3, low 6: register
)

// Rules defined for register values.
type Rule byte

const (
	RuleUndefined Rule = iota
	RuleSameVal
	RuleOffset
	RuleValOffset
	RuleRegister
	RuleExpression
	RuleValExpression
	RuleArchitectural
	RuleCFA          // Value is rule.Reg + rule.Offset
	RuleFramePointer // Value is stored at address rule.Reg + rule.Offset, but only if it's less than the current CFA, otherwise same value
)

const low_6_offset = 0x3f

type instruction func(frame *FrameContext)

// // Mapping from DWARF opcode to function.
var fnlookup = map[byte]instruction{
	DW_CFA_advance_loc:        advanceloc,
	DW_CFA_offset:             offset,
	DW_CFA_restore:            restore,
	DW_CFA_set_loc:            setloc,
	DW_CFA_advance_loc1:       advanceloc1,
	DW_CFA_advance_loc2:       advanceloc2,
	DW_CFA_advance_loc4:       advanceloc4,
	DW_CFA_offset_extended:    offsetextended,
	DW_CFA_restore_extended:   restoreextended,
	DW_CFA_undefined:          undefined,
	DW_CFA_same_value:         samevalue,
	DW_CFA_register:           register,
	DW_CFA_remember_state:     rememberstate,
	DW_CFA_restore_state:      restorestate,
	DW_CFA_def_cfa:            defcfa,
	DW_CFA_def_cfa_register:   defcfaregister,
	DW_CFA_def_cfa_offset:     defcfaoffset,
	DW_CFA_def_cfa_expression: defcfaexpression,
	DW_CFA_expression:         expression,
	DW_CFA_offset_extended_sf: offsetextendedsf,
	DW_CFA_def_cfa_sf:         defcfasf,
	DW_CFA_def_cfa_offset_sf:  defcfaoffsetsf,
	DW_CFA_val_offset:         valoffset,
	DW_CFA_val_offset_sf:      valoffsetsf,
	DW_CFA_val_expression:     valexpression,
	DW_CFA_lo_user:            louser,
	DW_CFA_hi_user:            hiuser,
}

func executeCIEInstructions(cie *CommonInformationEntry) *FrameContext {
	initialInstructions := make([]byte, len(cie.InitialInstructions))
	copy(initialInstructions, cie.InitialInstructions)
	frame := &FrameContext{
		cie:           cie,
		Regs:          make(map[uint64]DWRule),
		RetAddrReg:    cie.ReturnAddressRegister,
		initialRegs:   make(map[uint64]DWRule),
		prevRegs:      make(map[uint64]DWRule),
		codeAlignment: cie.CodeAlignmentFactor,
		dataAlignment: cie.DataAlignmentFactor,
		buf:           bytes.NewBuffer(initialInstructions),
	}

	frame.ExecuteDwarfProgram()
	return frame
}

// Unwind the stack to find the return address register.
func executeDwarfProgramUntilPC(fde *FrameDescriptionEntry, pc uint64) *FrameContext {
	frame := executeCIEInstructions(fde.CIE)
	frame.order = fde.order
	frame.loc = fde.Begin()
	frame.address = pc
	frame.ExecuteUntilPC(fde.Instructions)

	return frame
}

func (frame *FrameContext) ExecuteDwarfProgram() {
	for frame.buf.Len() > 0 {
		executeDwarfInstruction(frame)
	}
}

// Execute dwarf instructions.
func (frame *FrameContext) ExecuteUntilPC(instructions []byte) {
	frame.buf.Truncate(0)
	frame.buf.Write(instructions)

	// We only need to execute the instructions until
	// ctx.loc > ctx.address (which is the address we
	// are currently at in the traced process).
	for frame.address >= frame.loc && frame.buf.Len() > 0 {
		executeDwarfInstruction(frame)
	}
}

func executeDwarfInstruction(frame *FrameContext) {
	instruction, err := frame.buf.ReadByte()
	if err != nil {
		panic("Could not read from instruction buffer")
	}

	if instruction == DW_CFA_nop {
		return
	}

	fn := lookupFunc(instruction, frame.buf)

	fn(frame)
}

func lookupFunc(instruction byte, buf *bytes.Buffer) instruction {
	const high_2_bits = 0xc0
	var restore bool

	// Special case the 3 opcodes that have their argument encoded in the opcode itself.
	switch instruction & high_2_bits {
	case DW_CFA_advance_loc:
		instruction = DW_CFA_advance_loc
		restore = true

	case DW_CFA_offset:
		instruction = DW_CFA_offset
		restore = true

	case DW_CFA_restore:
		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")
		}
	}

	fn, ok := fnlookup[instruction]
	if !ok {
		panic(fmt.Sprintf("Encountered an unexpected DWARF CFA opcode: %#v", instruction))
	}

	return fn
}

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)

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

func advanceloc4(frame *FrameContext) {
	var delta uint32
	binary.Read(frame.buf, frame.order, &delta)

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

func offset(frame *FrameContext) {
	b, err := frame.buf.ReadByte()
	if err != nil {
		panic(err)
	}

	var (
		reg       = b & low_6_offset
		offset, _ = util.DecodeULEB128(frame.buf)
	)

	frame.Regs[uint64(reg)] = DWRule{Offset: int64(offset) * frame.dataAlignment, Rule: RuleOffset}
}

func restore(frame *FrameContext) {
	b, err := frame.buf.ReadByte()
	if err != nil {
		panic(err)
	}

	reg := uint64(b & low_6_offset)
	oldrule, ok := frame.initialRegs[reg]
	if ok {
		frame.Regs[reg] = DWRule{Offset: oldrule.Offset, Rule: RuleOffset}
	} else {
		frame.Regs[reg] = DWRule{Rule: RuleUndefined}
	}
}

func setloc(frame *FrameContext) {
	var loc uint64
	binary.Read(frame.buf, frame.order, &loc)

	frame.loc = loc
}

func offsetextended(frame *FrameContext) {
	var (
		reg, _    = util.DecodeULEB128(frame.buf)
		offset, _ = util.DecodeULEB128(frame.buf)
	)

	frame.Regs[reg] = DWRule{Offset: int64(offset) * frame.dataAlignment, Rule: RuleOffset}
}

func undefined(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)
	frame.Regs[reg] = DWRule{Rule: RuleUndefined}
}

func samevalue(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)
	frame.Regs[reg] = DWRule{Rule: RuleSameVal}
}

func register(frame *FrameContext) {
	reg1, _ := util.DecodeULEB128(frame.buf)
	reg2, _ := util.DecodeULEB128(frame.buf)
	frame.Regs[reg1] = DWRule{Reg: reg2, Rule: RuleRegister}
}

func rememberstate(frame *FrameContext) {
	frame.prevRegs = frame.Regs
}

func restorestate(frame *FrameContext) {
	frame.Regs = frame.prevRegs
}

func restoreextended(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)

	oldrule, ok := frame.initialRegs[reg]
	if ok {
		frame.Regs[reg] = DWRule{Offset: oldrule.Offset, Rule: RuleOffset}
	} else {
		frame.Regs[reg] = DWRule{Rule: RuleUndefined}
	}
}

func defcfa(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)
	offset, _ := util.DecodeULEB128(frame.buf)

	frame.CFA.Rule = RuleCFA
	frame.CFA.Reg = reg
	frame.CFA.Offset = int64(offset)
}

func defcfaregister(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)
	frame.CFA.Reg = reg
}

func defcfaoffset(frame *FrameContext) {
	offset, _ := util.DecodeULEB128(frame.buf)
	frame.CFA.Offset = int64(offset)
}

func defcfasf(frame *FrameContext) {
	reg, _ := util.DecodeULEB128(frame.buf)
	offset, _ := util.DecodeSLEB128(frame.buf)

	frame.CFA.Rule = RuleCFA
	frame.CFA.Reg = reg
	frame.CFA.Offset = offset * frame.dataAlignment
}

func defcfaoffsetsf(frame *FrameContext) {
	offset, _ := util.DecodeSLEB128(frame.buf)
	offset *= frame.dataAlignment
	frame.CFA.Offset = offset
}

func defcfaexpression(frame *FrameContext) {
	var (
		l, _ = util.DecodeULEB128(frame.buf)
		expr = frame.buf.Next(int(l))
	)

	frame.CFA.Expression = expr
	frame.CFA.Rule = RuleExpression
}

func expression(frame *FrameContext) {
	var (
		reg, _ = util.DecodeULEB128(frame.buf)
		l, _   = util.DecodeULEB128(frame.buf)
		expr   = frame.buf.Next(int(l))
	)

	frame.Regs[reg] = DWRule{Rule: RuleExpression, Expression: expr}
}

func offsetextendedsf(frame *FrameContext) {
	var (
		reg, _    = util.DecodeULEB128(frame.buf)
		offset, _ = util.DecodeSLEB128(frame.buf)
	)

	frame.Regs[reg] = DWRule{Offset: offset * frame.dataAlignment, Rule: RuleOffset}
}

func valoffset(frame *FrameContext) {
	var (
		reg, _    = util.DecodeULEB128(frame.buf)
		offset, _ = util.DecodeULEB128(frame.buf)
	)

	frame.Regs[reg] = DWRule{Offset: int64(offset), Rule: RuleValOffset}
}

func valoffsetsf(frame *FrameContext) {
	var (
		reg, _    = util.DecodeULEB128(frame.buf)
		offset, _ = util.DecodeSLEB128(frame.buf)
	)

	frame.Regs[reg] = DWRule{Offset: offset * frame.dataAlignment, Rule: RuleValOffset}
}

func valexpression(frame *FrameContext) {
	var (
		reg, _ = util.DecodeULEB128(frame.buf)
		l, _   = util.DecodeULEB128(frame.buf)
		expr   = frame.buf.Next(int(l))
	)

	frame.Regs[reg] = DWRule{Rule: RuleValExpression, Expression: expr}
}

func louser(frame *FrameContext) {
	frame.buf.Next(1)
}

func hiuser(frame *FrameContext) {
	frame.buf.Next(1)
}