gasm-sdk/debug/watchpoint_linux_loong64.go

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

//go:build linux && loong64

package debug

import (
	"fmt"
	"syscall"
	"unsafe"
)

// Hardware watchpoint support via the NT_LOONGARCH_HW_WATCH regset.
//
// The kernel's PTRACE_POKEUSER on loong64 accepts only the user_pt_regs
// indices 0-34 (GPRs, orig_a0, era, badv, per
// arch/loongarch/include/uapi/asm/ptrace.h), so there is no debug-register
// window to poke.  The real interface is PTRACE_GETREGSET/SETREGSET on
// NT_LOONGARCH_HW_WATCH (0xa06, include/uapi/linux/elf.h) with struct
// user_watch_state_v2 (arch/loongarch/include/uapi/asm/ptrace.h): a dbg_info
// word followed by 14 slots of {addr u64, mask u64, ctrl u32, pad u32}.
// hw_break_get puts the slot count in the low byte of dbg_info
// (arch/loongarch/kernel/ptrace.c, ptrace_hbp_get_resource_info) and
// hw_break_set ignores dbg_info, reading addr, mask and ctrl per slot.

const ntLoongHWWatch = 0xa06

// loongWatchState mirrors the kernel's struct user_watch_state_v2.
type loongWatchState struct {
	DbgInfo uint64
	DbgRegs [14]loongWatchReg
}

type loongWatchReg struct {
	Addr uint64
	Mask uint64
	Ctrl uint32
	Pad  uint32
}

// Control word bit layout (arch/loongarch/include/asm/hw_breakpoint.h):
// bits 1-4 privilege enables (CTRL_PLV3_ENABLE, 0x10, covers user mode),
// bits 8-9 access type (LOAD 1<<0, STORE 1<<1), bits 10-11 length
// (0=8 bytes, 1=4, 2=2, 3=1, inverted like the hardware FWP cfg).
const (
	loongCtrlPLV3Enable = 0x10
	loongTypeLoad       = 1 << 8
	loongTypeStore      = 2 << 8
	loongLenShift       = 10
)

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

const (
	WatchWrite WatchpointType = 1
	WatchRead  WatchpointType = 3
)

// maxWatchpoints reports the slot capacity of the regset struct; the number
// the hardware actually provides is read from dbg_info at arm time.
func maxWatchpoints() int { return len(loongWatchState{}.DbgRegs) }

// archWatchpointAddr resolves the address of the watchpoint that fired:
// the loongarch kernel already reports the accessed address as si_addr.
func archWatchpointAddr(s *Session, siAddr uint64) uint64 { return siAddr }

func (s *Session) FindFreeWatchpointSlot() int {
	for i := range maxWatchpoints() {
		if !s.wpSlots[i] {
			return i
		}
	}
	return -1
}

func (s *Session) IsWatchpointSlotUsed(slot int) bool {
	if slot < 0 || slot >= maxWatchpoints() {
		return false
	}
	return s.wpSlots[slot]
}

// SetWatchpoint installs a hardware watchpoint on the given address.
func (s *Session) SetWatchpoint(slot int, addr uint64, typ WatchpointType, size int) error {
	if slot < 0 || slot >= maxWatchpoints() {
		return fmt.Errorf("debug: watchpoint slot must be 0-%d", maxWatchpoints()-1)
	}
	if s.wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d already in use", slot)
	}

	var ctrlType uint32
	switch typ {
	case WatchWrite:
		ctrlType = loongTypeStore
	case WatchRead:
		ctrlType = loongTypeLoad | loongTypeStore
	}
	var lenBits uint32
	switch size {
	case 1:
		lenBits = 3
	case 2:
		lenBits = 2
	case 4:
		lenBits = 1
	case 8:
		lenBits = 0
	default:
		return fmt.Errorf("debug: watchpoint size must be 1, 2, 4, or 8")
	}

	state, err := s.getLoongWatchState()
	if err != nil {
		return fmt.Errorf("debug: read watchpoint state: %w", err)
	}
	if uint64(slot) >= state.DbgInfo&0xff {
		return fmt.Errorf("debug: slot %d exceeds available watchpoints (%d)", slot, state.DbgInfo&0xff)
	}

	state.DbgRegs[slot].Addr = addr
	state.DbgRegs[slot].Mask = 0
	state.DbgRegs[slot].Ctrl = loongCtrlPLV3Enable | ctrlType | lenBits<<loongLenShift

	if err := s.setLoongWatchState(state); err != nil {
		return fmt.Errorf("debug: set watchpoint: %w", err)
	}

	s.wpSlots[slot] = true
	return nil
}

func (s *Session) ClearWatchpoint(slot int) error {
	if slot < 0 || slot >= maxWatchpoints() {
		return fmt.Errorf("debug: watchpoint slot must be 0-%d", maxWatchpoints()-1)
	}
	if !s.wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d is not in use", slot)
	}

	state, err := s.getLoongWatchState()
	if err != nil {
		return err
	}
	state.DbgRegs[slot].Addr = 0
	state.DbgRegs[slot].Mask = 0
	state.DbgRegs[slot].Ctrl = 0
	if err := s.setLoongWatchState(state); err != nil {
		return err
	}
	s.wpSlots[slot] = false
	return nil
}

func (s *Session) ClearAllWatchpoints() error {
	for slot := range maxWatchpoints() {
		if s.wpSlots[slot] {
			if err := s.ClearWatchpoint(slot); err != nil {
				return err
			}
		}
	}
	return nil
}

func (s *Session) getLoongWatchState() (*loongWatchState, error) {
	var state loongWatchState
	iovec := syscall.Iovec{
		Base: (*byte)(unsafe.Pointer(&state)),
		Len:  uint64(unsafe.Sizeof(state)),
	}
	_, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(syscall.PTRACE_GETREGSET),
		uintptr(s.pid),
		uintptr(ntLoongHWWatch),
		uintptr(unsafe.Pointer(&iovec)),
		0, 0,
	)
	if errno != 0 {
		return nil, errno
	}
	return &state, nil
}

func (s *Session) setLoongWatchState(state *loongWatchState) error {
	iovec := syscall.Iovec{
		Base: (*byte)(unsafe.Pointer(state)),
		Len:  uint64(unsafe.Sizeof(*state)),
	}
	_, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(syscall.PTRACE_SETREGSET),
		uintptr(s.pid),
		uintptr(ntLoongHWWatch),
		uintptr(unsafe.Pointer(&iovec)),
		0, 0,
	)
	if errno != 0 {
		return errno
	}
	return nil
}