fix(debug): hardware watchpoints, signal stops and breakpoint restore

Assisted-by: GLM 5.3

debug/watchpoint_linux_amd64.go

@@ -8,10 +8,48 @@ package debug
 import (
 	"fmt"
 	"syscall"
+	"unsafe"
 )
 
 // Hardware watchpoint support via x86-64 debug registers (DR0-DR3, DR7).
 
+// The kernel translates PTRACE_POKEUSER/PEEKUSER offsets inside
+// [offsetof(struct user, u_debugreg[0]), u_debugreg[7]] to DR0-DR7
+// (arch/x86/kernel/ptrace.c, arch_ptrace).  sys/user.h places u_debugreg at
+// 0x350: DR0-DR3 are 0x350/0x358/0x360/0x368, DR6 (status) is 0x380 and
+// DR7 (control) is 0x388.  Offsets below 0x350 write user_regs_struct
+// fields (r15 at 0x0, r10 at 0x38), not debug registers.
+const (
+	drOffset = 0x350 // offsetof(struct user, u_debugreg[0]), DR0
+	dr6Off   = 0x380 // offsetof(struct user, u_debugreg[6]), DR6
+	dr7Off   = 0x388 // offsetof(struct user, u_debugreg[7]), DR7
+)
+
+// archWatchpointAddr resolves the address of the watchpoint that fired.
+// x86 delivers si_addr = the instruction pointer of the trapping access
+// (arch/x86/kernel/ptrace.c send_sigtrap passes regs->ip), so the watched
+// data address is recovered from DR6's slot bits (B0-B3, positive polarity
+// through PEEKUSER) and the matching DR0-DR3.
+func archWatchpointAddr(s *Session, siAddr uint64) uint64 {
+	dr6, err := ptracePeekUser(s.pid, dr6Off)
+	if err != nil {
+		return siAddr
+	}
+	for slot := range 4 {
+		if dr6&(1<<slot) != 0 {
+			addr, err := ptracePeekUser(s.pid, drOffset+uintptr(slot*8))
+			if err == nil && addr != 0 {
+				return addr
+			}
+		}
+	}
+	return siAddr
+}
+
+// maxWatchpoints reports the number of hardware watchpoint slots the
+// architecture provides: four address registers, DR0-DR3.
+func maxWatchpoints() int { return 4 }
+
 // WatchpointType selects what triggers the watchpoint.
 type WatchpointType int
 
@@ -62,23 +100,11 @@ func (s *Session) SetWatchpoint(slot int, addr uint64, typ WatchpointType, size
 		return fmt.Errorf("debug: watchpoint size must be 1, 2, 4, or 8")
 	}
 
-	var drAddr uintptr
-	switch slot {
-	case 0:
-		drAddr = 0x0
-	case 1:
-		drAddr = 0x8
-	case 2:
-		drAddr = 0x10
-	case 3:
-		drAddr = 0x18
-	}
-
-	if err := ptracePokeUser(s.pid, drAddr, addr); err != nil {
+	if err := ptracePokeUser(s.pid, drOffset+uintptr(slot*8), addr); err != nil {
 		return fmt.Errorf("debug: set DR%d: %w", slot, err)
 	}
 
-	dr7, err := ptracePeekUser(s.pid, 0x38)
+	dr7, err := ptracePeekUser(s.pid, dr7Off)
 	if err != nil {
 		return fmt.Errorf("debug: read DR7: %w", err)
 	}
@@ -90,7 +116,7 @@ func (s *Session) SetWatchpoint(slot int, addr uint64, typ WatchpointType, size
 	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 {
+	if err := ptracePokeUser(s.pid, dr7Off, dr7); err != nil {
 		return fmt.Errorf("debug: set DR7: %w", err)
 	}
 	s.wpSlots[slot] = true
@@ -105,12 +131,12 @@ func (s *Session) ClearWatchpoint(slot int) error {
 	if !s.wpSlots[slot] {
 		return fmt.Errorf("debug: watchpoint slot %d is not in use", slot)
 	}
-	dr7, err := ptracePeekUser(s.pid, 0x38)
+	dr7, err := ptracePeekUser(s.pid, dr7Off)
 	if err != nil {
 		return err
 	}
 	dr7 &^= uint64(1) << (2 * slot)
-	if err := ptracePokeUser(s.pid, 0x38, dr7); err != nil {
+	if err := ptracePokeUser(s.pid, dr7Off, dr7); err != nil {
 		return err
 	}
 	s.wpSlots[slot] = false
@@ -119,7 +145,7 @@ func (s *Session) ClearWatchpoint(slot int) error {
 
 // ClearAllWatchpoints removes all hardware watchpoints.
 func (s *Session) ClearAllWatchpoints() error {
-	for slot := range 4 {
+	for slot := range maxWatchpoints() {
 		if s.wpSlots[slot] {
 			if err := s.ClearWatchpoint(slot); err != nil {
 				return err
@@ -146,16 +172,21 @@ func ptracePokeUser(pid int, offset uintptr, val uint64) error {
 }
 
 func ptracePeekUser(pid int, offset uintptr) (uint64, error) {
+	// x86 PEEKUSR writes the word to the user-space pointer in data
+	// (arch/x86/kernel/ptrace.c uses put_user); passing 0 there fails with
+	// EFAULT, so the word is read through a real address.
 	const ptracePeekuser = 3
-	val, _, errno := syscall.Syscall6(
+	var word uint64
+	_, _, errno := syscall.Syscall6(
 		syscall.SYS_PTRACE,
 		uintptr(ptracePeekuser),
 		uintptr(pid),
 		offset,
-		0, 0, 0,
+		uintptr(unsafe.Pointer(&word)),
+		0, 0,
 	)
 	if errno != 0 {
 		return 0, errno
 	}
-	return uint64(val), nil
+	return word, nil
 }