rust: qom: change instance_init to take a ParentInit<>

This removes undefined behavior associated to writing to uninitialized
fields, and makes it possible to remove "unsafe" from the instance_init
implementation.

However, the init function itself is still unsafe, because it must promise
(as a sort as MaybeUninit::assume_init) that all fields have been
initialized.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 21 insertions, 29 deletions

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index be8387f..2d416cd 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -2,7 +2,7 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use std::{ffi::CStr, mem::size_of, ptr::addr_of_mut};
+use std::{ffi::CStr, mem::size_of};
 
 use qemu_api::{
     chardev::{CharBackend, Chardev, Event},
@@ -11,9 +11,10 @@ use qemu_api::{
     memory::{hwaddr, MemoryRegion, MemoryRegionOps, MemoryRegionOpsBuilder},
     prelude::*,
     qdev::{Clock, ClockEvent, DeviceImpl, DeviceState, Property, ResetType, ResettablePhasesImpl},
-    qom::{ObjectImpl, Owned, ParentField},
+    qom::{ObjectImpl, Owned, ParentField, ParentInit},
     static_assert,
     sysbus::{SysBusDevice, SysBusDeviceImpl},
+    uninit_field_mut,
     vmstate::VMStateDescription,
 };
 
@@ -163,7 +164,7 @@ impl PL011Impl for PL011State {
 impl ObjectImpl for PL011State {
     type ParentType = SysBusDevice;
 
-    const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
+    const INSTANCE_INIT: Option<unsafe fn(ParentInit<Self>)> = Some(Self::init);
     const INSTANCE_POST_INIT: Option<fn(&Self)> = Some(Self::post_init);
     const CLASS_INIT: fn(&mut Self::Class) = Self::Class::class_init::<Self>;
 }
@@ -488,7 +489,7 @@ impl PL011State {
     /// `PL011State` type. It must not be called more than once on the same
     /// location/instance. All its fields are expected to hold uninitialized
     /// values with the sole exception of `parent_obj`.
-    unsafe fn init(&mut self) {
+    unsafe fn init(mut this: ParentInit<Self>) {
         static PL011_OPS: MemoryRegionOps<PL011State> = MemoryRegionOpsBuilder::<PL011State>::new()
             .read(&PL011State::read)
             .write(&PL011State::write)
@@ -496,28 +497,23 @@ impl PL011State {
             .impl_sizes(4, 4)
             .build();
 
-        // SAFETY:
-        //
-        // self and self.iomem are guaranteed to be valid at this point since callers
-        // must make sure the `self` reference is valid.
+        // SAFETY: this and this.iomem are guaranteed to be valid at this point
         MemoryRegion::init_io(
-            unsafe { &mut *addr_of_mut!(self.iomem) },
-            addr_of_mut!(*self),
+            &mut uninit_field_mut!(*this, iomem),
             &PL011_OPS,
             "pl011",
             0x1000,
         );
 
-        self.regs = Default::default();
+        uninit_field_mut!(*this, regs).write(Default::default());
 
-        // SAFETY:
-        //
-        // self.clock is not initialized at this point; but since `Owned<_>` is
-        // not Drop, we can overwrite the undefined value without side effects;
-        // it's not sound but, because for all PL011State instances are created
-        // by QOM code which calls this function to initialize the fields, at
-        // leastno code is able to access an invalid self.clock value.
-        self.clock = self.init_clock_in("clk", &Self::clock_update, ClockEvent::ClockUpdate);
+        let clock = DeviceState::init_clock_in(
+            &mut this,
+            "clk",
+            &Self::clock_update,
+            ClockEvent::ClockUpdate,
+        );
+        uninit_field_mut!(*this, clock).write(clock);
     }
 
     const fn clock_update(&self, _event: ClockEvent) {
diff --git a/rust/hw/timer/hpet/src/device.rs b/rust/hw/timer/hpet/src/device.rs
index 340ca1d..a281927 100644
--- a/rust/hw/timer/hpet/src/device.rs
+++ b/rust/hw/timer/hpet/src/device.rs
@@ -21,8 +21,8 @@ use qemu_api::{
         hwaddr, MemoryRegion, MemoryRegionOps, MemoryRegionOpsBuilder, MEMTXATTRS_UNSPECIFIED,
     },
     prelude::*,
-    qdev::{DeviceImpl, DeviceMethods, DeviceState, Property, ResetType, ResettablePhasesImpl},
-    qom::{ObjectImpl, ObjectType, ParentField},
+    qdev::{DeviceImpl, DeviceState, Property, ResetType, ResettablePhasesImpl},
+    qom::{ObjectImpl, ObjectType, ParentField, ParentInit},
     qom_isa,
     sysbus::{SysBusDevice, SysBusDeviceImpl},
     timer::{Timer, CLOCK_VIRTUAL, NANOSECONDS_PER_SECOND},
@@ -697,7 +697,7 @@ impl HPETState {
             .set(self.counter.get().deposit(shift, len, val));
     }
 
-    unsafe fn init(&mut self) {
+    unsafe fn init(mut this: ParentInit<Self>) {
         static HPET_RAM_OPS: MemoryRegionOps<HPETState> =
             MemoryRegionOpsBuilder::<HPETState>::new()
                 .read(&HPETState::read)
@@ -707,18 +707,14 @@ impl HPETState {
                 .impl_sizes(4, 8)
                 .build();
 
-        // SAFETY:
-        // self and self.iomem are guaranteed to be valid at this point since callers
-        // must make sure the `self` reference is valid.
         MemoryRegion::init_io(
-            unsafe { &mut *addr_of_mut!(self.iomem) },
-            addr_of_mut!(*self),
+            &mut uninit_field_mut!(*this, iomem),
             &HPET_RAM_OPS,
             "hpet",
             HPET_REG_SPACE_LEN,
         );
 
-        Self::init_timers(unsafe { &mut *((self as *mut Self).cast::<MaybeUninit<Self>>()) });
+        Self::init_timers(&mut this);
     }
 
     fn post_init(&self) {
@@ -900,7 +896,7 @@ unsafe impl ObjectType for HPETState {
 impl ObjectImpl for HPETState {
     type ParentType = SysBusDevice;
 
-    const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
+    const INSTANCE_INIT: Option<unsafe fn(ParentInit<Self>)> = Some(Self::init);
     const INSTANCE_POST_INIT: Option<fn(&Self)> = Some(Self::post_init);
     const CLASS_INIT: fn(&mut Self::Class) = Self::Class::class_init::<Self>;
 }