So far Devres uses an inner memory allocation and reference count, i.e.
an inner Arc, in order to ensure that the devres callback can't run into
a use-after-free in case where the Devres object is dropped while the
devres callback runs concurrently.
Instead, use a completion in order to avoid a potential UAF: In
Devres::drop(), if we detect that we can't remove the devres action
anymore, we wait for the completion that is completed from the devres
callback. If, in turn, we were able to successfully remove the devres
action, we can just go ahead.
This, again, allows us to get rid of the internal Arc, and instead let
Devres consume an `impl PinInit<T, E>` in order to return an
`impl PinInit<Devres<T>, E>`, which enables us to get away with less
memory allocations.
Additionally, having the resulting explicit synchronization in
Devres::drop() prevents potential subtle undesired side effects of the
devres callback dropping the final Arc reference asynchronously within
the devres callback.
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
drivers/gpu/nova-core/driver.rs | 7 +-
drivers/gpu/nova-core/gpu.rs | 6 +-
rust/kernel/devres.rs | 208 +++++++++++++++++++-------------
rust/kernel/pci.rs | 20 +--
samples/rust/rust_driver_pci.rs | 19 +--
5 files changed, 149 insertions(+), 111 deletions(-)
diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 8c86101c26cb..110f2b355db4 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -1,6 +1,6 @@
// SPDX-License-Identifier: GPL-2.0
-use kernel::{auxiliary, bindings, c_str, device::Core, pci, prelude::*};
+use kernel::{auxiliary, bindings, c_str, device::Core, pci, prelude::*, sync::Arc};
use crate::gpu::Gpu;
@@ -34,7 +34,10 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
pdev.enable_device_mem()?;
pdev.set_master();
- let bar = pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0"))?;
+ let bar = Arc::pin_init(
+ pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0")),
+ GFP_KERNEL,
+ )?;
let this = KBox::pin_init(
try_pin_init!(Self {
diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 60b86f370284..47653c14838b 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -1,6 +1,6 @@
// SPDX-License-Identifier: GPL-2.0
-use kernel::{device, devres::Devres, error::code::*, pci, prelude::*};
+use kernel::{device, devres::Devres, error::code::*, pci, prelude::*, sync::Arc};
use crate::driver::Bar0;
use crate::firmware::{Firmware, FIRMWARE_VERSION};
@@ -161,14 +161,14 @@ fn new(bar: &Bar0) -> Result<Spec> {
pub(crate) struct Gpu {
spec: Spec,
/// MMIO mapping of PCI BAR 0
- bar: Devres<Bar0>,
+ bar: Arc<Devres<Bar0>>,
fw: Firmware,
}
impl Gpu {
pub(crate) fn new(
pdev: &pci::Device<device::Bound>,
- devres_bar: Devres<Bar0>,
+ devres_bar: Arc<Devres<Bar0>>,
) -> Result<impl PinInit<Self>> {
let bar = devres_bar.access(pdev.as_ref())?;
let spec = Spec::new(bar)?;
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 250073749279..15a0a94e992b 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -13,20 +13,31 @@
ffi::c_void,
prelude::*,
revocable::{Revocable, RevocableGuard},
- sync::{rcu, Arc, Completion},
- types::{ARef, ForeignOwnable},
+ sync::{rcu, Completion},
+ types::{ARef, ForeignOwnable, Opaque},
};
+use pin_init::Wrapper;
+
+/// [`Devres`] inner data accessed from [`Devres::callback`].
#[pin_data]
-struct DevresInner<T> {
- dev: ARef<Device>,
- callback: unsafe extern "C" fn(*mut c_void),
+struct Inner<T> {
#[pin]
data: Revocable<T>,
+ /// Tracks whether [`Devres::callback`] has been completed.
+ #[pin]
+ devm: Completion,
+ /// Tracks whether revoking [`Self::data`] has been completed.
#[pin]
revoke: Completion,
}
+impl<T> Inner<T> {
+ fn as_ptr(&self) -> *const Self {
+ self
+ }
+}
+
/// This abstraction is meant to be used by subsystems to containerize [`Device`] bound resources to
/// manage their lifetime.
///
@@ -88,100 +99,106 @@ struct DevresInner<T> {
/// # fn no_run(dev: &Device<Bound>) -> Result<(), Error> {
/// // SAFETY: Invalid usage for example purposes.
/// let iomem = unsafe { IoMem::<{ core::mem::size_of::<u32>() }>::new(0xBAAAAAAD)? };
-/// let devres = Devres::new(dev, iomem, GFP_KERNEL)?;
+/// let devres = KBox::pin_init(Devres::new(dev, iomem), GFP_KERNEL)?;
///
/// let res = devres.try_access().ok_or(ENXIO)?;
/// res.write8(0x42, 0x0);
/// # Ok(())
/// # }
/// ```
-pub struct Devres<T>(Arc<DevresInner<T>>);
+#[pin_data(PinnedDrop)]
+pub struct Devres<T> {
+ dev: ARef<Device>,
+ /// Pointer to [`Self::devres_callback`].
+ ///
+ /// Has to be stored, since Rust does not guarantee to always return the same address for a
+ /// function. However, the C API uses the address as a key.
+ callback: unsafe extern "C" fn(*mut c_void),
+ /// Contains all the fields shared with [`Self::callback`].
+ // TODO: Replace with `UnsafePinned`, once available.
+ #[pin]
+ inner: Opaque<Inner<T>>,
+}
-impl<T> DevresInner<T> {
- fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
- let inner = Arc::pin_init(
- try_pin_init!( DevresInner {
- dev: dev.into(),
- callback: Self::devres_callback,
+impl<T> Devres<T> {
+ /// Creates a new [`Devres`] instance of the given `data`.
+ ///
+ /// The `data` encapsulated within the returned `Devres` instance' `data` will be
+ /// (revoked)[`Revocable`] once the device is detached.
+ pub fn new<'a, E>(
+ dev: &'a Device<Bound>,
+ data: impl PinInit<T, E> + 'a,
+ ) -> impl PinInit<Self, Error> + 'a
+ where
+ T: 'a,
+ Error: From<E>,
+ {
+ let callback = Self::devres_callback;
+
+ try_pin_init!(&this in Self {
+ inner <- Opaque::pin_init(try_pin_init!(Inner {
data <- Revocable::new(data),
+ devm <- Completion::new(),
revoke <- Completion::new(),
- }),
- flags,
- )?;
-
- // Convert `Arc<DevresInner>` into a raw pointer and make devres own this reference until
- // `Self::devres_callback` is called.
- let data = inner.clone().into_raw();
+ })),
+ callback,
+ dev: {
+ // SAFETY: It is valid to dereference `this` to find the address of `inner`.
+ let inner = unsafe { core::ptr::addr_of_mut!((*this.as_ptr()).inner) };
- // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
- // detached.
- let ret =
- unsafe { bindings::devm_add_action(dev.as_raw(), Some(inner.callback), data as _) };
-
- if ret != 0 {
- // SAFETY: We just created another reference to `inner` in order to pass it to
- // `bindings::devm_add_action`. If `bindings::devm_add_action` fails, we have to drop
- // this reference accordingly.
- let _ = unsafe { Arc::from_raw(data) };
- return Err(Error::from_errno(ret));
- }
+ // SAFETY:
+ // - `dev.as_raw()` is a pointer to a valid bound device.
+ // - `inner` is guaranteed to be a valid for the duration of the lifetime of `Self`.
+ // - `devm_add_action()` is guaranteed not to call `callback` until `this` has been
+ // properly initialized, because we require `dev` (i.e. the *bound* device) to
+ // live at least as long as the returned `impl PinInit<Self, Error>`.
+ to_result(unsafe {
+ bindings::devm_add_action(dev.as_raw(), Some(callback), inner.cast())
+ })?;
- Ok(inner)
+ dev.into()
+ },
+ })
}
- fn as_ptr(&self) -> *const Self {
- self as _
+ fn inner(&self) -> &Inner<T> {
+ // SAFETY: `inner` is valid and properly initialized.
+ unsafe { &*self.inner.get() }
}
- fn remove_action(this: &Arc<Self>) -> bool {
- // SAFETY:
- // - `self.inner.dev` is a valid `Device`,
- // - the `action` and `data` pointers are the exact same ones as given to devm_add_action()
- // previously,
- // - `self` is always valid, even if the action has been released already.
- let success = unsafe {
- bindings::devm_remove_action_nowarn(
- this.dev.as_raw(),
- Some(this.callback),
- this.as_ptr() as _,
- )
- } == 0;
-
- if success {
- // SAFETY: We leaked an `Arc` reference to devm_add_action() in `DevresInner::new`; if
- // devm_remove_action_nowarn() was successful we can (and have to) claim back ownership
- // of this reference.
- let _ = unsafe { Arc::from_raw(this.as_ptr()) };
- }
-
- success
+ fn data(&self) -> &Revocable<T> {
+ &self.inner().data
}
#[allow(clippy::missing_safety_doc)]
unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
- let ptr = ptr as *mut DevresInner<T>;
- // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
- // reference.
- // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
- // `DevresInner::new`.
- let inner = unsafe { Arc::from_raw(ptr) };
+ // SAFETY: In `Self::new` we've passed a valid pointer to `Inner` to `devm_add_action()`,
+ // hence `ptr` must be a valid pointer to `Inner`.
+ let inner = unsafe { &*ptr.cast::<Inner<T>>() };
if !inner.data.revoke() {
// If `revoke()` returns false, it means that `Devres::drop` already started revoking
- // `inner.data` for us. Hence we have to wait until `Devres::drop()` signals that it
- // completed revoking `inner.data`.
+ // `data` for us. Hence we have to wait until `Devres::drop` signals that it
+ // completed revoking `data`.
inner.revoke.wait_for_completion();
}
- }
-}
-impl<T> Devres<T> {
- /// Creates a new [`Devres`] instance of the given `data`. The `data` encapsulated within the
- /// returned `Devres` instance' `data` will be revoked once the device is detached.
- pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
- let inner = DevresInner::new(dev, data, flags)?;
+ // Signal that we're done using `inner`.
+ inner.devm.complete_all();
+ }
- Ok(Devres(inner))
+ fn remove_action(&self) -> bool {
+ // SAFETY:
+ // - `self.dev` is a valid `Device`,
+ // - the `action` and `data` pointers are the exact same ones as given to
+ // `devm_add_action()` previously,
+ (unsafe {
+ bindings::devm_remove_action_nowarn(
+ self.dev.as_raw(),
+ Some(self.callback),
+ self.inner().as_ptr().cast_mut().cast(),
+ )
+ } == 0)
}
/// Obtain `&'a T`, bypassing the [`Revocable`].
@@ -213,44 +230,61 @@ pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
/// }
/// ```
pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a T> {
- if self.0.dev.as_raw() != dev.as_raw() {
+ if self.dev.as_raw() != dev.as_raw() {
return Err(EINVAL);
}
// SAFETY: `dev` being the same device as the device this `Devres` has been created for
- // proves that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as
- // long as `dev` lives; `dev` lives at least as long as `self`.
- Ok(unsafe { self.0.data.access() })
+ // proves that `self.data` hasn't been revoked and is guaranteed to not be revoked as long
+ // as `dev` lives; `dev` lives at least as long as `self`.
+ Ok(unsafe { self.data().access() })
}
/// [`Devres`] accessor for [`Revocable::try_access`].
pub fn try_access(&self) -> Option<RevocableGuard<'_, T>> {
- self.0.data.try_access()
+ self.data().try_access()
}
/// [`Devres`] accessor for [`Revocable::try_access_with`].
pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
- self.0.data.try_access_with(f)
+ self.data().try_access_with(f)
}
/// [`Devres`] accessor for [`Revocable::try_access_with_guard`].
pub fn try_access_with_guard<'a>(&'a self, guard: &'a rcu::Guard) -> Option<&'a T> {
- self.0.data.try_access_with_guard(guard)
+ self.data().try_access_with_guard(guard)
}
}
-impl<T> Drop for Devres<T> {
- fn drop(&mut self) {
+// SAFETY: `Devres` can be send to any task, if `T: Send`.
+unsafe impl<T: Send> Send for Devres<T> {}
+
+// SAFETY: `Devres` can be shared with any task, if `T: Sync`.
+unsafe impl<T: Sync> Sync for Devres<T> {}
+
+#[pinned_drop]
+impl<T> PinnedDrop for Devres<T> {
+ fn drop(self: Pin<&mut Self>) {
// SAFETY: When `drop` runs, it is guaranteed that nobody is accessing the revocable data
// anymore, hence it is safe not to wait for the grace period to finish.
- if unsafe { self.0.data.revoke_nosync() } {
- // We revoked `self.0.data` before the devres action did, hence try to remove it.
- if !DevresInner::remove_action(&self.0) {
+ if unsafe { self.data().revoke_nosync() } {
+ // We revoked `self.data` before the devres action did, hence try to remove it.
+ if !self.remove_action() {
// We could not remove the devres action, which means that it now runs concurrently,
- // hence signal that `self.0.data` has been revoked successfully.
- self.0.revoke.complete_all();
+ // hence signal that `self.data` has been revoked by us successfully.
+ self.inner().revoke.complete_all();
+
+ // Wait for `Self::devres_callback` to be done using this object.
+ self.inner().devm.wait_for_completion();
}
+ } else {
+ // `Self::devres_callback` revokes `self.data` for us, hence wait for it to be done
+ // using this object.
+ self.inner().devm.wait_for_completion();
}
+
+ // SAFETY: `inner` is valid for dropping.
+ unsafe { core::ptr::drop_in_place(self.inner.get()) };
}
}
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 8435f8132e38..db0eb7eaf9b1 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -5,7 +5,6 @@
//! C header: [`include/linux/pci.h`](srctree/include/linux/pci.h)
use crate::{
- alloc::flags::*,
bindings, container_of, device,
device_id::RawDeviceId,
devres::Devres,
@@ -398,19 +397,20 @@ pub fn resource_len(&self, bar: u32) -> Result<bindings::resource_size_t> {
impl Device<device::Bound> {
/// Mapps an entire PCI-BAR after performing a region-request on it. I/O operation bound checks
/// can be performed on compile time for offsets (plus the requested type size) < SIZE.
- pub fn iomap_region_sized<const SIZE: usize>(
- &self,
+ pub fn iomap_region_sized<'a, const SIZE: usize>(
+ &'a self,
bar: u32,
- name: &CStr,
- ) -> Result<Devres<Bar<SIZE>>> {
- let bar = Bar::<SIZE>::new(self, bar, name)?;
- let devres = Devres::new(self.as_ref(), bar, GFP_KERNEL)?;
-
- Ok(devres)
+ name: &'a CStr,
+ ) -> impl PinInit<Devres<Bar<SIZE>>, Error> + 'a {
+ Devres::new(self.as_ref(), Bar::<SIZE>::new(self, bar, name))
}
/// Mapps an entire PCI-BAR after performing a region-request on it.
- pub fn iomap_region(&self, bar: u32, name: &CStr) -> Result<Devres<Bar>> {
+ pub fn iomap_region<'a>(
+ &'a self,
+ bar: u32,
+ name: &'a CStr,
+ ) -> impl PinInit<Devres<Bar>, Error> + 'a {
self.iomap_region_sized::<0>(bar, name)
}
}
diff --git a/samples/rust/rust_driver_pci.rs b/samples/rust/rust_driver_pci.rs
index 15147e4401b2..5c35f1414172 100644
--- a/samples/rust/rust_driver_pci.rs
+++ b/samples/rust/rust_driver_pci.rs
@@ -25,8 +25,10 @@ impl TestIndex {
const NO_EVENTFD: Self = Self(0);
}
+#[pin_data(PinnedDrop)]
struct SampleDriver {
pdev: ARef<pci::Device>,
+ #[pin]
bar: Devres<Bar0>,
}
@@ -73,13 +75,11 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
pdev.enable_device_mem()?;
pdev.set_master();
- let bar = pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci"))?;
-
- let drvdata = KBox::new(
- Self {
+ let drvdata = KBox::pin_init(
+ try_pin_init!(Self {
pdev: pdev.into(),
- bar,
- },
+ bar <- pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci")),
+ }),
GFP_KERNEL,
)?;
@@ -90,12 +90,13 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
Self::testdev(info, bar)?
);
- Ok(drvdata.into())
+ Ok(drvdata)
}
}
-impl Drop for SampleDriver {
- fn drop(&mut self) {
+#[pinned_drop]
+impl PinnedDrop for SampleDriver {
+ fn drop(self: Pin<&mut Self>) {
dev_dbg!(self.pdev.as_ref(), "Remove Rust PCI driver sample.\n");
}
}
--
2.49.0On Sun, Jun 22, 2025 at 06:40:40PM +0200, Danilo Krummrich wrote:
> So far Devres uses an inner memory allocation and reference count, i.e.
> an inner Arc, in order to ensure that the devres callback can't run into
> a use-after-free in case where the Devres object is dropped while the
> devres callback runs concurrently.
>
> Instead, use a completion in order to avoid a potential UAF: In
> Devres::drop(), if we detect that we can't remove the devres action
> anymore, we wait for the completion that is completed from the devres
> callback. If, in turn, we were able to successfully remove the devres
> action, we can just go ahead.
>
> This, again, allows us to get rid of the internal Arc, and instead let
I like the idea ;-)
> Devres consume an `impl PinInit<T, E>` in order to return an
> `impl PinInit<Devres<T>, E>`, which enables us to get away with less
> memory allocations.
>
> Additionally, having the resulting explicit synchronization in
> Devres::drop() prevents potential subtle undesired side effects of the
> devres callback dropping the final Arc reference asynchronously within
> the devres callback.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
[...]
> +impl<T> Devres<T> {
[...]
>
> #[allow(clippy::missing_safety_doc)]
> unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
> - let ptr = ptr as *mut DevresInner<T>;
> - // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
> - // reference.
> - // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
> - // `DevresInner::new`.
> - let inner = unsafe { Arc::from_raw(ptr) };
> + // SAFETY: In `Self::new` we've passed a valid pointer to `Inner` to `devm_add_action()`,
> + // hence `ptr` must be a valid pointer to `Inner`.
I think you also need to mention that `inner` only remains valid until
`inner.devm.complete_all()` unblocks `Devres::drop()`, because after
`Devres::drop()`'s `devm.wait_for_completion()` returns, `inner` may be
dropped or freed.
Regards,
Boqun
> + let inner = unsafe { &*ptr.cast::<Inner<T>>() };
>
> if !inner.data.revoke() {
> // If `revoke()` returns false, it means that `Devres::drop` already started revoking
> - // `inner.data` for us. Hence we have to wait until `Devres::drop()` signals that it
> - // completed revoking `inner.data`.
> + // `data` for us. Hence we have to wait until `Devres::drop` signals that it
> + // completed revoking `data`.
> inner.revoke.wait_for_completion();
> }
[...]
> + // Signal that we're done using `inner`.
> + inner.devm.complete_all();
> + }
[...]
On Sun, Jun 22, 2025 at 06:54:07PM -0700, Boqun Feng wrote: > I think you also need to mention that `inner` only remains valid until > `inner.devm.complete_all()` unblocks `Devres::drop()`, because after > `Devres::drop()`'s `devm.wait_for_completion()` returns, `inner` may be > dropped or freed. I think of it the other way around: The invariant guarantees that `inner` is *always* valid. The the `drop_in_place(inner)` call has to justify that it upholds this invariant, by ensuring that at the time it is called no other code that accesses `inner` can ever run. Defining it the other way around would make the `inner()` accessor unsafe.
On Tue, Jun 24, 2025 at 05:18:23PM +0200, Danilo Krummrich wrote:
> On Sun, Jun 22, 2025 at 06:54:07PM -0700, Boqun Feng wrote:
> > I think you also need to mention that `inner` only remains valid until
> > `inner.devm.complete_all()` unblocks `Devres::drop()`, because after
> > `Devres::drop()`'s `devm.wait_for_completion()` returns, `inner` may be
> > dropped or freed.
>
> I think of it the other way around: The invariant guarantees that `inner` is
> *always* valid.
>
> The the `drop_in_place(inner)` call has to justify that it upholds this
> invariant, by ensuring that at the time it is called no other code that accesses
> `inner` can ever run.
>
> Defining it the other way around would make the `inner()` accessor unsafe.
Maybe I wasn't clear enough, I meant in the following function:
unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
- let ptr = ptr as *mut DevresInner<T>;
- // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
- // reference.
- // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
- // `DevresInner::new`.
- let inner = unsafe { Arc::from_raw(ptr) };
+ // SAFETY: In `Self::new` we've passed a valid pointer to `Inner` to `devm_add_action()`,
+ // hence `ptr` must be a valid pointer to `Inner`.
+ let inner = unsafe { &*ptr.cast::<Inner<T>>() };
^ this `inner` was constructed by reborrowing from `ptr`, but it should
only be used before the following `inner.devm.complete_all()`...
if !inner.data.revoke() {
// If `revoke()` returns false, it means that `Devres::drop` already started revoking
- // `inner.data` for us. Hence we have to wait until `Devres::drop()` signals that it
- // completed revoking `inner.data`.
+ // `data` for us. Hence we have to wait until `Devres::drop` signals that it
+ // completed revoking `data`.
inner.revoke.wait_for_completion();
[...]
+ // Signal that we're done using `inner`.
+ inner.devm.complete_all();
... because the `DevresInner` might be freed after we signal the
`Devres::drop()`. And for example, doing a:
inner.data.try_access();
after the above line would be unsound.
+ }
And I would prefer we document this or use `ScopeGuard`, does it make
sense?
Regards,
Boqun
On Tue, Jun 24, 2025 at 08:46:53AM -0700, Boqun Feng wrote:
> On Tue, Jun 24, 2025 at 05:18:23PM +0200, Danilo Krummrich wrote:
> > On Sun, Jun 22, 2025 at 06:54:07PM -0700, Boqun Feng wrote:
> > > I think you also need to mention that `inner` only remains valid until
> > > `inner.devm.complete_all()` unblocks `Devres::drop()`, because after
> > > `Devres::drop()`'s `devm.wait_for_completion()` returns, `inner` may be
> > > dropped or freed.
> >
> > I think of it the other way around: The invariant guarantees that `inner` is
> > *always* valid.
> >
> > The the `drop_in_place(inner)` call has to justify that it upholds this
> > invariant, by ensuring that at the time it is called no other code that accesses
> > `inner` can ever run.
> >
> > Defining it the other way around would make the `inner()` accessor unsafe.
>
> Maybe I wasn't clear enough, I meant in the following function:
>
> unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
> - let ptr = ptr as *mut DevresInner<T>;
> - // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
> - // reference.
> - // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
> - // `DevresInner::new`.
> - let inner = unsafe { Arc::from_raw(ptr) };
> + // SAFETY: In `Self::new` we've passed a valid pointer to `Inner` to `devm_add_action()`,
> + // hence `ptr` must be a valid pointer to `Inner`.
> + let inner = unsafe { &*ptr.cast::<Inner<T>>() };
>
> ^ this `inner` was constructed by reborrowing from `ptr`, but it should
> only be used before the following `inner.devm.complete_all()`...
Oh, so you meant adding this to the safety comment. Yes, that makes sense. Maybe
ScopeGuard works too, as you say.
On Tue, Jun 24, 2025 at 06:15:43PM +0200, Danilo Krummrich wrote:
> Oh, so you meant adding this to the safety comment. Yes, that makes sense. Maybe
> ScopeGuard works too, as you say.
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 2591cacecb7b..afd73a8c6012 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -14,7 +14,7 @@
prelude::*,
revocable::{Revocable, RevocableGuard},
sync::{rcu, Completion},
- types::{ARef, ForeignOwnable, Opaque},
+ types::{ARef, ForeignOwnable, ScopeGuard, Opaque},
};
use core::ops::Deref;
@@ -177,15 +177,15 @@ fn data(&self) -> &Revocable<T> {
// hence `ptr` must be a valid pointer to `Inner`.
let inner = unsafe { &*ptr.cast::<Inner<T>>() };
+ // Ensure that `inner` can't be used anymore after we signal completion of this callback.
+ let inner = ScopeGuard::new_with_data(inner, |inner| inner.devm.complete_all());
+
if !inner.data.revoke() {
// If `revoke()` returns false, it means that `Devres::drop` already started revoking
// `data` for us. Hence we have to wait until `Devres::drop` signals that it
// completed revoking `data`.
inner.revoke.wait_for_completion();
}
-
- // Signal that we're done using `inner`.
- inner.devm.complete_all();
}
fn remove_action(&self) -> bool {
Is this what you thought of?
On Tue, Jun 24, 2025 at 06:20:41PM +0200, Danilo Krummrich wrote:
> On Tue, Jun 24, 2025 at 06:15:43PM +0200, Danilo Krummrich wrote:
> > Oh, so you meant adding this to the safety comment. Yes, that makes sense. Maybe
> > ScopeGuard works too, as you say.
>
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 2591cacecb7b..afd73a8c6012 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -14,7 +14,7 @@
> prelude::*,
> revocable::{Revocable, RevocableGuard},
> sync::{rcu, Completion},
> - types::{ARef, ForeignOwnable, Opaque},
> + types::{ARef, ForeignOwnable, ScopeGuard, Opaque},
> };
> use core::ops::Deref;
>
> @@ -177,15 +177,15 @@ fn data(&self) -> &Revocable<T> {
> // hence `ptr` must be a valid pointer to `Inner`.
> let inner = unsafe { &*ptr.cast::<Inner<T>>() };
>
> + // Ensure that `inner` can't be used anymore after we signal completion of this callback.
> + let inner = ScopeGuard::new_with_data(inner, |inner| inner.devm.complete_all());
> +
> if !inner.data.revoke() {
> // If `revoke()` returns false, it means that `Devres::drop` already started revoking
> // `data` for us. Hence we have to wait until `Devres::drop` signals that it
> // completed revoking `data`.
> inner.revoke.wait_for_completion();
> }
> -
> - // Signal that we're done using `inner`.
> - inner.devm.complete_all();
> }
>
> fn remove_action(&self) -> bool {
>
> Is this what you thought of?
Yep, looks good to me. Thanks!
Regards,
Boqun
On Sun Jun 22, 2025 at 6:40 PM CEST, Danilo Krummrich wrote:
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 250073749279..15a0a94e992b 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -13,20 +13,31 @@
> ffi::c_void,
> prelude::*,
> revocable::{Revocable, RevocableGuard},
> - sync::{rcu, Arc, Completion},
> - types::{ARef, ForeignOwnable},
> + sync::{rcu, Completion},
> + types::{ARef, ForeignOwnable, Opaque},
> };
>
> +use pin_init::Wrapper;
> +
> +/// [`Devres`] inner data accessed from [`Devres::callback`].
> #[pin_data]
> -struct DevresInner<T> {
> - dev: ARef<Device>,
> - callback: unsafe extern "C" fn(*mut c_void),
> +struct Inner<T> {
> #[pin]
> data: Revocable<T>,
> + /// Tracks whether [`Devres::callback`] has been completed.
> + #[pin]
> + devm: Completion,
> + /// Tracks whether revoking [`Self::data`] has been completed.
> #[pin]
> revoke: Completion,
> }
>
> +impl<T> Inner<T> {
> + fn as_ptr(&self) -> *const Self {
> + self
> + }
> +}
Instead of creating this function, you can use `ptr::from_ref`.
> +
> /// This abstraction is meant to be used by subsystems to containerize [`Device`] bound resources to
> /// manage their lifetime.
> ///
> @@ -88,100 +99,106 @@ struct DevresInner<T> {
> /// # fn no_run(dev: &Device<Bound>) -> Result<(), Error> {
> /// // SAFETY: Invalid usage for example purposes.
> /// let iomem = unsafe { IoMem::<{ core::mem::size_of::<u32>() }>::new(0xBAAAAAAD)? };
> -/// let devres = Devres::new(dev, iomem, GFP_KERNEL)?;
> +/// let devres = KBox::pin_init(Devres::new(dev, iomem), GFP_KERNEL)?;
> ///
> /// let res = devres.try_access().ok_or(ENXIO)?;
> /// res.write8(0x42, 0x0);
> /// # Ok(())
> /// # }
> /// ```
> -pub struct Devres<T>(Arc<DevresInner<T>>);
> +#[pin_data(PinnedDrop)]
> +pub struct Devres<T> {
> + dev: ARef<Device>,
> + /// Pointer to [`Self::devres_callback`].
> + ///
> + /// Has to be stored, since Rust does not guarantee to always return the same address for a
> + /// function. However, the C API uses the address as a key.
> + callback: unsafe extern "C" fn(*mut c_void),
> + /// Contains all the fields shared with [`Self::callback`].
> + // TODO: Replace with `UnsafePinned`, once available.
> + #[pin]
> + inner: Opaque<Inner<T>>,
> +}
>
> -impl<T> DevresInner<T> {
> - fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
> - let inner = Arc::pin_init(
> - try_pin_init!( DevresInner {
> - dev: dev.into(),
> - callback: Self::devres_callback,
> +impl<T> Devres<T> {
> + /// Creates a new [`Devres`] instance of the given `data`.
> + ///
> + /// The `data` encapsulated within the returned `Devres` instance' `data` will be
> + /// (revoked)[`Revocable`] once the device is detached.
> + pub fn new<'a, E>(
> + dev: &'a Device<Bound>,
> + data: impl PinInit<T, E> + 'a,
> + ) -> impl PinInit<Self, Error> + 'a
> + where
> + T: 'a,
> + Error: From<E>,
> + {
> + let callback = Self::devres_callback;
> +
> + try_pin_init!(&this in Self {
> + inner <- Opaque::pin_init(try_pin_init!(Inner {
> data <- Revocable::new(data),
> + devm <- Completion::new(),
> revoke <- Completion::new(),
> - }),
> - flags,
> - )?;
> -
> - // Convert `Arc<DevresInner>` into a raw pointer and make devres own this reference until
> - // `Self::devres_callback` is called.
> - let data = inner.clone().into_raw();
> + })),
> + callback,
> + dev: {
> + // SAFETY: It is valid to dereference `this` to find the address of `inner`.
// SAFETY: `this` is a valid pointer to uninitialized memory.
> + let inner = unsafe { core::ptr::addr_of_mut!((*this.as_ptr()).inner) };
We can use `raw` instead of `addr_of_mut!`:
let inner = unsafe { &raw mut (*this.as_ptr()).inner };
>
> - // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
> - // detached.
> - let ret =
> - unsafe { bindings::devm_add_action(dev.as_raw(), Some(inner.callback), data as _) };
> -
> - if ret != 0 {
> - // SAFETY: We just created another reference to `inner` in order to pass it to
> - // `bindings::devm_add_action`. If `bindings::devm_add_action` fails, we have to drop
> - // this reference accordingly.
> - let _ = unsafe { Arc::from_raw(data) };
> - return Err(Error::from_errno(ret));
> - }
> + // SAFETY:
> + // - `dev.as_raw()` is a pointer to a valid bound device.
> + // - `inner` is guaranteed to be a valid for the duration of the lifetime of `Self`.
> + // - `devm_add_action()` is guaranteed not to call `callback` until `this` has been
> + // properly initialized, because we require `dev` (i.e. the *bound* device) to
> + // live at least as long as the returned `impl PinInit<Self, Error>`.
Just wanted to highlight that this is a very cool application of the
borrow checker :)
> + to_result(unsafe {
> + bindings::devm_add_action(dev.as_raw(), Some(callback), inner.cast())
> + })?;
>
> - Ok(inner)
> + dev.into()
> + },
> + })
> }
>
> - fn as_ptr(&self) -> *const Self {
> - self as _
> + fn inner(&self) -> &Inner<T> {
> + // SAFETY: `inner` is valid and properly initialized.
We should have an invariant here that `inner` is always accessed
read-only and that it is initialized.
---
Cheers,
Benno
> + unsafe { &*self.inner.get() }
> }
>
> - fn remove_action(this: &Arc<Self>) -> bool {
> - // SAFETY:
> - // - `self.inner.dev` is a valid `Device`,
> - // - the `action` and `data` pointers are the exact same ones as given to devm_add_action()
> - // previously,
> - // - `self` is always valid, even if the action has been released already.
> - let success = unsafe {
> - bindings::devm_remove_action_nowarn(
> - this.dev.as_raw(),
> - Some(this.callback),
> - this.as_ptr() as _,
> - )
> - } == 0;
> -
> - if success {
> - // SAFETY: We leaked an `Arc` reference to devm_add_action() in `DevresInner::new`; if
> - // devm_remove_action_nowarn() was successful we can (and have to) claim back ownership
> - // of this reference.
> - let _ = unsafe { Arc::from_raw(this.as_ptr()) };
> - }
> -
> - success
> + fn data(&self) -> &Revocable<T> {
> + &self.inner().data
> }
On Sun, Jun 22, 2025 at 10:45:02PM +0200, Benno Lossin wrote:
> On Sun Jun 22, 2025 at 6:40 PM CEST, Danilo Krummrich wrote:
> > +impl<T> Inner<T> {
> > + fn as_ptr(&self) -> *const Self {
> > + self
> > + }
> > +}
>
> Instead of creating this function, you can use `ptr::from_ref`.
Yeah, I like this much better.
> > +impl<T> Devres<T> {
> > + /// Creates a new [`Devres`] instance of the given `data`.
> > + ///
> > + /// The `data` encapsulated within the returned `Devres` instance' `data` will be
> > + /// (revoked)[`Revocable`] once the device is detached.
> > + pub fn new<'a, E>(
> > + dev: &'a Device<Bound>,
> > + data: impl PinInit<T, E> + 'a,
> > + ) -> impl PinInit<Self, Error> + 'a
> > + where
> > + T: 'a,
> > + Error: From<E>,
> > + {
> > + let callback = Self::devres_callback;
> > +
> > + try_pin_init!(&this in Self {
> > + inner <- Opaque::pin_init(try_pin_init!(Inner {
> > data <- Revocable::new(data),
> > + devm <- Completion::new(),
> > revoke <- Completion::new(),
> > - }),
> > - flags,
> > - )?;
> > -
> > - // Convert `Arc<DevresInner>` into a raw pointer and make devres own this reference until
> > - // `Self::devres_callback` is called.
> > - let data = inner.clone().into_raw();
> > + })),
> > + callback,
> > + dev: {
> > + // SAFETY: It is valid to dereference `this` to find the address of `inner`.
>
> // SAFETY: `this` is a valid pointer to uninitialized memory.
>
> > + let inner = unsafe { core::ptr::addr_of_mut!((*this.as_ptr()).inner) };
>
> We can use `raw` instead of `addr_of_mut!`:
>
> let inner = unsafe { &raw mut (*this.as_ptr()).inner };
Indeed, forgot we finally have raw_ref_op since v6.15. :)
> >
> > - // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
> > - // detached.
> > - let ret =
> > - unsafe { bindings::devm_add_action(dev.as_raw(), Some(inner.callback), data as _) };
> > -
> > - if ret != 0 {
> > - // SAFETY: We just created another reference to `inner` in order to pass it to
> > - // `bindings::devm_add_action`. If `bindings::devm_add_action` fails, we have to drop
> > - // this reference accordingly.
> > - let _ = unsafe { Arc::from_raw(data) };
> > - return Err(Error::from_errno(ret));
> > - }
> > + // SAFETY:
> > + // - `dev.as_raw()` is a pointer to a valid bound device.
> > + // - `inner` is guaranteed to be a valid for the duration of the lifetime of `Self`.
> > + // - `devm_add_action()` is guaranteed not to call `callback` until `this` has been
> > + // properly initialized, because we require `dev` (i.e. the *bound* device) to
> > + // live at least as long as the returned `impl PinInit<Self, Error>`.
>
> Just wanted to highlight that this is a very cool application of the
> borrow checker :)
It is indeed -- I really like this one!
> > + to_result(unsafe {
> > + bindings::devm_add_action(dev.as_raw(), Some(callback), inner.cast())
> > + })?;
> >
> > - Ok(inner)
> > + dev.into()
> > + },
> > + })
> > }
> >
> > - fn as_ptr(&self) -> *const Self {
> > - self as _
> > + fn inner(&self) -> &Inner<T> {
> > + // SAFETY: `inner` is valid and properly initialized.
>
> We should have an invariant here that `inner` is always accessed
> read-only and that it is initialized.
Makes sense, gonna add it.
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