gasm-sdk/debug/watchpoint_linux_amd64.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

//go:build linux && amd64

package debug

import (
	"fmt"
	"syscall"
)

// Hardware watchpoint support via x86-64 debug registers (DR0-DR3, DR7).
//
// DR0-DR3 hold the watched addresses.  DR7 is the control register:
//   bits 0,2,4,6: local enable for DR0-DR3
//   bits 16-17,20-21,24-25,28-29: R/W type (00=exec, 01=write, 11=read/write)
//   bits 18-19,22-23,26-27,30-31: length (00=1, 01=2, 10=8, 11=4)

// WatchpointType selects what triggers the watchpoint.
type WatchpointType int

const (
	WatchWrite WatchpointType = 1 // trigger on write
	WatchRead  WatchpointType = 3 // trigger on read or write
)

// FindFreeWatchpointSlot returns the index of the first free watchpoint slot
// (0-3), or -1 if all four hardware watchpoints are in use.
func (s *Session) FindFreeWatchpointSlot() int {
	for i := 0; i < 4; i++ {
		if !s.wpSlots[i] {
			return i
		}
	}
	return -1
}

// IsWatchpointSlotUsed reports whether slot (0-3) currently holds a watchpoint.
func (s *Session) IsWatchpointSlotUsed(slot int) bool {
	if slot < 0 || slot > 3 {
		return false
	}
	return s.wpSlots[slot]
}

// SetWatchpoint installs a hardware watchpoint on the given address.
// slot is 0-3 (four hardware watchpoints available); the slot must be free.
func (s *Session) SetWatchpoint(slot int, addr uint64, typ WatchpointType, size int) error {
	if slot < 0 || slot > 3 {
		return fmt.Errorf("debug: watchpoint slot must be 0-3")
	}
	if s.wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d already in use", slot)
	}

	// Determine the length encoding.
	var lenBits uint64
	switch size {
	case 1:
		lenBits = 0
	case 2:
		lenBits = 1
	case 4:
		lenBits = 3
	case 8:
		lenBits = 2
	default:
		return fmt.Errorf("debug: watchpoint size must be 1, 2, 4, or 8")
	}

	// Write the watched address to DR0-DR3.
	var drAddr uintptr
	switch slot {
	case 0:
		drAddr = 0x0 // DR0 offset in user_regs_struct
	case 1:
		drAddr = 0x8 // DR1
	case 2:
		drAddr = 0x10 // DR2
	case 3:
		drAddr = 0x18 // DR3
	}

	// PTRACE_POKEUSER writes to the debuggee's user area (includes debug regs).
	if err := ptracePokeUser(s.pid, drAddr, addr); err != nil {
		return fmt.Errorf("debug: set DR%d: %w", slot, err)
	}

	// Read the current DR7, set the enable and type bits, write it back.
	dr7, err := ptracePeekUser(s.pid, 0x38) // DR7 offset
	if err != nil {
		return fmt.Errorf("debug: read DR7: %w", err)
	}

	enableBit := uint64(1) << (2 * slot)   // local enable
	rwBits := uint64(typ) << (16 + 4*slot) // R/W type
	lenField := lenBits << (18 + 4*slot)   // length

	// Clear the existing bits for this slot, then set the new ones.
	mask := ^((uint64(1) << (2 * slot)) | (uint64(3) << (16 + 4*slot)) | (uint64(3) << (18 + 4*slot)))
	dr7 = (dr7 & mask) | enableBit | rwBits | lenField

	if err := ptracePokeUser(s.pid, 0x38, dr7); err != nil {
		return fmt.Errorf("debug: set DR7: %w", err)
	}
	s.wpSlots[slot] = true
	return nil
}

// ClearWatchpoint removes a hardware watchpoint.
func (s *Session) ClearWatchpoint(slot int) error {
	if slot < 0 || slot > 3 {
		return fmt.Errorf("debug: watchpoint slot must be 0-3")
	}
	if !s.wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d is not in use", slot)
	}
	// Read DR7, clear the enable bit for this slot.
	dr7, err := ptracePeekUser(s.pid, 0x38)
	if err != nil {
		return err
	}
	dr7 &^= uint64(1) << (2 * slot) // disable
	if err := ptracePokeUser(s.pid, 0x38, dr7); err != nil {
		return err
	}
	s.wpSlots[slot] = false
	return nil
}

// ClearAllWatchpoints removes all hardware watchpoints.
func (s *Session) ClearAllWatchpoints() error {
	for slot := 0; slot < 4; slot++ {
		if s.wpSlots[slot] {
			if err := s.ClearWatchpoint(slot); err != nil {
				return err
			}
		}
	}
	return nil
}

// ptracePokeUser writes a value to the debuggee's user area at the given offset.
func ptracePokeUser(pid int, offset uintptr, val uint64) error {
	const ptracePokeuser = 6 // PTRACE_POKEUSER
	_, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(ptracePokeuser),
		uintptr(pid),
		offset,
		uintptr(val),
		0, 0,
	)
	if errno != 0 {
		return errno
	}
	return nil
}

// ptracePeekUser reads a value from the debuggee's user area at the given offset.
func ptracePeekUser(pid int, offset uintptr) (uint64, error) {
	const ptracePeekuser = 3 // PTRACE_PEEKUSER
	val, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(ptracePeekuser),
		uintptr(pid),
		offset,
		0, 0, 0,
	)
	if errno != 0 {
		return 0, errno
	}
	return uint64(val), nil
}