gasm-sdk/debug/watchpoint_linux_riscv64.go

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

//go:build linux && riscv64

package debug

import (
	"fmt"
	"syscall"
)

// Hardware watchpoint support for RISC-V via Sdtrig trigger registers.
// Uses PTRACE_POKEUSER/PEEKUSER to access debug registers.

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

const (
	WatchWrite WatchpointType = 1
	WatchRead  WatchpointType = 3
)

// wpSlots tracks watchpoint slot occupancy.
var wpSlots [4]bool

const maxWatchpoints = 4

func (s *Session) FindFreeWatchpointSlot() int {
	for i := 0; i < maxWatchpoints; i++ {
		if !wpSlots[i] {
			return i
		}
	}
	return -1
}

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

// SetWatchpoint installs a hardware watchpoint.
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 wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d already in use", slot)
	}
	if size != 1 && size != 2 && size != 4 && size != 8 {
		return fmt.Errorf("debug: watchpoint size must be 1, 2, 4, or 8")
	}

	// RISC-V trigger registers: tdata1 encodes type/control, tdata2 holds address.
	// The exact encoding depends on the trigger implementation (Sdtrig).
	// Use PTRACE_POKEUSER to write to the trigger CSRs via the kernel's
	// debug register interface.
	if err := ptracePokeUser(s.pid, uintptr(0x1000+slot*8), addr); err != nil {
		return fmt.Errorf("debug: set watchpoint address: %w", err)
	}

	// tdata1: set match control.  Mode=2 (data match), select=0, action=1 (debug exception).
	var tdata1 uint64 = 2 << 60 // type = match (2)
	tdata1 |= 1 << 0            // action = enter debug mode
	tdata1 |= 1 << 7            // store (write) trigger
	if typ == WatchRead {
		tdata1 |= 1 << 6 // load trigger
	}
	// Size encoding: 0=1byte, 1=2byte, 2=4byte, 3=8byte.
	var sizeBits uint64
	switch size {
	case 1:
		sizeBits = 0
	case 2:
		sizeBits = 1
	case 4:
		sizeBits = 2
	case 8:
		sizeBits = 3
	}
	tdata1 |= sizeBits << 16 // size field

	if err := ptracePokeUser(s.pid, uintptr(0x1001+slot*8), tdata1); err != nil {
		return fmt.Errorf("debug: set watchpoint control: %w", err)
	}

	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 !wpSlots[slot] {
		return fmt.Errorf("debug: watchpoint slot %d is not in use", slot)
	}

	// Disable by clearing tdata1.
	if err := ptracePokeUser(s.pid, uintptr(0x1001+slot*8), 0); err != nil {
		return err
	}
	wpSlots[slot] = false
	return nil
}

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

func ptracePokeUser(pid int, offset uintptr, val uint64) error {
	const ptracePokeuser = 6
	_, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(ptracePokeuser),
		uintptr(pid),
		offset,
		uintptr(val),
		0, 0,
	)
	if errno != 0 {
		return errno
	}
	return nil
}

func ptracePeekUser(pid int, offset uintptr) (uint64, error) {
	const ptracePeekuser = 3
	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
}