diff --git a/unix/mkpost.go b/unix/mkpost.go
index 76075c34..783f11d9 100644
--- a/unix/mkpost.go
+++ b/unix/mkpost.go
@@ -89,10 +89,12 @@ func main() {
 			return out
 		})
 
-		// Inside PtraceIoDesc replace all *byte with uintptr
+		// Inside PtraceIoDesc replace the Offs field, which refers to an address
+		// in the child process (not the Go parent), with a uintptr.
 		ptraceIoDescStruct := regexp.MustCompile(`(?s:type PtraceIoDesc struct \{.*?\})`)
+		addrField := regexp.MustCompile(`(\bOffs\s+)\*byte`)
 		b = ptraceIoDescStruct.ReplaceAllFunc(b, func(in []byte) []byte {
-			return bytes.ReplaceAll(in, []byte("*byte"), []byte("uintptr"))
+			return addrField.ReplaceAll(in, []byte(`${1}uintptr`))
 		})
 	}
 
diff --git a/unix/syscall_freebsd.go b/unix/syscall_freebsd.go
index d50b9dc2..19e71c22 100644
--- a/unix/syscall_freebsd.go
+++ b/unix/syscall_freebsd.go
@@ -274,6 +274,25 @@ func PtraceGetRegs(pid int, regsout *Reg) (err error) {
 	return ptrace(PT_GETREGS, pid, uintptr(unsafe.Pointer(regsout)), 0)
 }
 
+func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
+	ioDesc := PtraceIoDesc{
+		Op:   int32(req),
+		Offs: offs,
+	}
+	if countin > 0 {
+		_ = out[:countin] // check bounds
+		ioDesc.Addr = &out[0]
+	} else if out != nil {
+		ioDesc.Addr = (*byte)(unsafe.Pointer(&_zero))
+	}
+	ioDesc.SetLen(countin)
+
+	// TODO(#58387): It's not actually safe to pass &ioDesc as a uintptr here.
+	// Pass it as an unsafe.Pointer instead.
+	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
+	return int(ioDesc.Len), err
+}
+
 func PtraceLwpEvents(pid int, enable int) (err error) {
 	return ptrace(PT_LWP_EVENTS, pid, 0, enable)
 }
diff --git a/unix/syscall_freebsd_386.go b/unix/syscall_freebsd_386.go
index 6a91d471..2f798dbe 100644
--- a/unix/syscall_freebsd_386.go
+++ b/unix/syscall_freebsd_386.go
@@ -42,6 +42,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (d *PtraceIoDesc) SetLen(length int) {
+	d.Len = uint32(length)
+}
+
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
 	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr((*offset)>>32), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0)
@@ -59,14 +63,3 @@ func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr,
 func PtraceGetFsBase(pid int, fsbase *int64) (err error) {
 	return ptrace(PT_GETFSBASE, pid, uintptr(unsafe.Pointer(fsbase)), 0)
 }
-
-func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
-	ioDesc := PtraceIoDesc{
-		Op:   int32(req),
-		Offs: offs,
-		Addr: uintptr(unsafe.Pointer(&out[0])), // TODO(#58351): this is not safe.
-		Len:  uint32(countin),
-	}
-	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
-	return int(ioDesc.Len), err
-}
diff --git a/unix/syscall_freebsd_amd64.go b/unix/syscall_freebsd_amd64.go
index 48110a0a..1ed93b69 100644
--- a/unix/syscall_freebsd_amd64.go
+++ b/unix/syscall_freebsd_amd64.go
@@ -42,6 +42,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (d *PtraceIoDesc) SetLen(length int) {
+	d.Len = uint64(length)
+}
+
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
 	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0, 0)
@@ -59,14 +63,3 @@ func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr,
 func PtraceGetFsBase(pid int, fsbase *int64) (err error) {
 	return ptrace(PT_GETFSBASE, pid, uintptr(unsafe.Pointer(fsbase)), 0)
 }
-
-func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
-	ioDesc := PtraceIoDesc{
-		Op:   int32(req),
-		Offs: offs,
-		Addr: uintptr(unsafe.Pointer(&out[0])), // TODO(#58351): this is not safe.
-		Len:  uint64(countin),
-	}
-	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
-	return int(ioDesc.Len), err
-}
diff --git a/unix/syscall_freebsd_arm.go b/unix/syscall_freebsd_arm.go
index 52f1d4b7..08932093 100644
--- a/unix/syscall_freebsd_arm.go
+++ b/unix/syscall_freebsd_arm.go
@@ -42,6 +42,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (d *PtraceIoDesc) SetLen(length int) {
+	d.Len = uint32(length)
+}
+
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
 	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr((*offset)>>32), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0)
@@ -55,14 +59,3 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
-
-func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
-	ioDesc := PtraceIoDesc{
-		Op:   int32(req),
-		Offs: offs,
-		Addr: uintptr(unsafe.Pointer(&out[0])), // TODO(#58351): this is not safe.
-		Len:  uint32(countin),
-	}
-	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
-	return int(ioDesc.Len), err
-}
diff --git a/unix/syscall_freebsd_arm64.go b/unix/syscall_freebsd_arm64.go
index 5537ee4f..d151a0d0 100644
--- a/unix/syscall_freebsd_arm64.go
+++ b/unix/syscall_freebsd_arm64.go
@@ -42,6 +42,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (d *PtraceIoDesc) SetLen(length int) {
+	d.Len = uint64(length)
+}
+
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
 	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0, 0)
@@ -55,14 +59,3 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
-
-func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
-	ioDesc := PtraceIoDesc{
-		Op:   int32(req),
-		Offs: offs,
-		Addr: uintptr(unsafe.Pointer(&out[0])), // TODO(#58351): this is not safe.
-		Len:  uint64(countin),
-	}
-	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
-	return int(ioDesc.Len), err
-}
diff --git a/unix/syscall_freebsd_riscv64.go b/unix/syscall_freebsd_riscv64.go
index 164abd5d..d5cd64b3 100644
--- a/unix/syscall_freebsd_riscv64.go
+++ b/unix/syscall_freebsd_riscv64.go
@@ -42,6 +42,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (d *PtraceIoDesc) SetLen(length int) {
+	d.Len = uint64(length)
+}
+
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
 	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0, 0)
@@ -55,14 +59,3 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 func Syscall9(num, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
-
-func PtraceIO(req int, pid int, offs uintptr, out []byte, countin int) (count int, err error) {
-	ioDesc := PtraceIoDesc{
-		Op:   int32(req),
-		Offs: offs,
-		Addr: uintptr(unsafe.Pointer(&out[0])), // TODO(#58351): this is not safe.
-		Len:  uint64(countin),
-	}
-	err = ptrace(PT_IO, pid, uintptr(unsafe.Pointer(&ioDesc)), 0)
-	return int(ioDesc.Len), err
-}
diff --git a/unix/ztypes_freebsd_386.go b/unix/ztypes_freebsd_386.go
index d9c78cdc..29dc4833 100644
--- a/unix/ztypes_freebsd_386.go
+++ b/unix/ztypes_freebsd_386.go
@@ -362,7 +362,7 @@ type FpExtendedPrecision struct{}
 type PtraceIoDesc struct {
 	Op   int32
 	Offs uintptr
-	Addr uintptr
+	Addr *byte
 	Len  uint32
 }
 
diff --git a/unix/ztypes_freebsd_amd64.go b/unix/ztypes_freebsd_amd64.go
index 26991b16..0a89b289 100644
--- a/unix/ztypes_freebsd_amd64.go
+++ b/unix/ztypes_freebsd_amd64.go
@@ -367,7 +367,7 @@ type FpExtendedPrecision struct{}
 type PtraceIoDesc struct {
 	Op   int32
 	Offs uintptr
-	Addr uintptr
+	Addr *byte
 	Len  uint64
 }
 
diff --git a/unix/ztypes_freebsd_arm.go b/unix/ztypes_freebsd_arm.go
index f8324e7e..c8666bb1 100644
--- a/unix/ztypes_freebsd_arm.go
+++ b/unix/ztypes_freebsd_arm.go
@@ -350,7 +350,7 @@ type FpExtendedPrecision struct {
 type PtraceIoDesc struct {
 	Op   int32
 	Offs uintptr
-	Addr uintptr
+	Addr *byte
 	Len  uint32
 }
 
diff --git a/unix/ztypes_freebsd_arm64.go b/unix/ztypes_freebsd_arm64.go
index 4220411f..88fb48a8 100644
--- a/unix/ztypes_freebsd_arm64.go
+++ b/unix/ztypes_freebsd_arm64.go
@@ -347,7 +347,7 @@ type FpExtendedPrecision struct{}
 type PtraceIoDesc struct {
 	Op   int32
 	Offs uintptr
-	Addr uintptr
+	Addr *byte
 	Len  uint64
 }
 
diff --git a/unix/ztypes_freebsd_riscv64.go b/unix/ztypes_freebsd_riscv64.go
index 0660fd45..698dc975 100644
--- a/unix/ztypes_freebsd_riscv64.go
+++ b/unix/ztypes_freebsd_riscv64.go
@@ -348,7 +348,7 @@ type FpExtendedPrecision struct{}
 type PtraceIoDesc struct {
 	Op   int32
 	Offs uintptr
-	Addr uintptr
+	Addr *byte
 	Len  uint64
 }