drm::Device is allocated through __drm_dev_alloc() (which uses
kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
initialized in-place.
Due to the order of fields in drm::Device
pub struct Device<T: drm::Driver> {
dev: Opaque<bindings::drm_device>,
data: T::Data,
}
even with an arbitrary large alignment requirement of T::Data it can't
happen that the size of Device is smaller than its alignment requirement.
However, let's not rely on this subtle circumstance and create a proper
kmalloc() compatible Layout.
Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/kernel/drm/device.rs | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 3bb7c83966cf..d19410deaf6c 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -5,6 +5,7 @@
//! C header: [`include/linux/drm/drm_device.h`](srctree/include/linux/drm/drm_device.h)
use crate::{
+ alloc::allocator::Kmalloc,
bindings, device, drm,
drm::driver::AllocImpl,
error::from_err_ptr,
@@ -12,7 +13,7 @@
prelude::*,
types::{ARef, AlwaysRefCounted, Opaque},
};
-use core::{mem, ops::Deref, ptr, ptr::NonNull};
+use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
#[cfg(CONFIG_DRM_LEGACY)]
macro_rules! drm_legacy_fields {
@@ -96,6 +97,10 @@ impl<T: drm::Driver> Device<T> {
/// Create a new `drm::Device` for a `drm::Driver`.
pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
+ // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
+ // compatible `Layout`.
+ let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
+
// SAFETY:
// - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
// - `dev` is valid by its type invarants,
@@ -103,7 +108,7 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
bindings::__drm_dev_alloc(
dev.as_raw(),
&Self::VTABLE,
- mem::size_of::<Self>(),
+ layout.size(),
mem::offset_of!(Self, dev),
)
}
--
2.50.0On Thu, Jul 31, 2025 at 05:48:07PM +0200, Danilo Krummrich wrote:
> drm::Device is allocated through __drm_dev_alloc() (which uses
> kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
> initialized in-place.
>
> Due to the order of fields in drm::Device
>
> pub struct Device<T: drm::Driver> {
> dev: Opaque<bindings::drm_device>,
> data: T::Data,
> }
I'm not convinced this patch is right.
Imagine this scenario: T::Data has size and alignment both equal to 16,
and lets say that drm_device has a size that is a multiple of 8 but not
16 such as 72. In that case, you will allocate 72+16=88 bytes for
Device, but actually the size of Device is 96 because there is 8 bytes
of padding between dev and data.
Alice
> even with an arbitrary large alignment requirement of T::Data it can't
> happen that the size of Device is smaller than its alignment requirement.
>
> However, let's not rely on this subtle circumstance and create a proper
> kmalloc() compatible Layout.
>
> Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
> rust/kernel/drm/device.rs | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 3bb7c83966cf..d19410deaf6c 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -5,6 +5,7 @@
> //! C header: [`include/linux/drm/drm_device.h`](srctree/include/linux/drm/drm_device.h)
>
> use crate::{
> + alloc::allocator::Kmalloc,
> bindings, device, drm,
> drm::driver::AllocImpl,
> error::from_err_ptr,
> @@ -12,7 +13,7 @@
> prelude::*,
> types::{ARef, AlwaysRefCounted, Opaque},
> };
> -use core::{mem, ops::Deref, ptr, ptr::NonNull};
> +use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
>
> #[cfg(CONFIG_DRM_LEGACY)]
> macro_rules! drm_legacy_fields {
> @@ -96,6 +97,10 @@ impl<T: drm::Driver> Device<T> {
>
> /// Create a new `drm::Device` for a `drm::Driver`.
> pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
> + // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
> + // compatible `Layout`.
> + let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
> +
> // SAFETY:
> // - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
> // - `dev` is valid by its type invarants,
> @@ -103,7 +108,7 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
> bindings::__drm_dev_alloc(
> dev.as_raw(),
> &Self::VTABLE,
> - mem::size_of::<Self>(),
> + layout.size(),
> mem::offset_of!(Self, dev),
> )
> }
> --
> 2.50.0
>
On Fri Aug 1, 2025 at 11:18 AM CEST, Alice Ryhl wrote:
> On Thu, Jul 31, 2025 at 05:48:07PM +0200, Danilo Krummrich wrote:
>> drm::Device is allocated through __drm_dev_alloc() (which uses
>> kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
>> initialized in-place.
>>
>> Due to the order of fields in drm::Device
>>
>> pub struct Device<T: drm::Driver> {
>> dev: Opaque<bindings::drm_device>,
>> data: T::Data,
>> }
>
> I'm not convinced this patch is right.
>
> Imagine this scenario: T::Data has size and alignment both equal to 16,
> and lets say that drm_device has a size that is a multiple of 8 but not
> 16 such as 72. In that case, you will allocate 72+16=88 bytes for
> Device, but actually the size of Device is 96 because there is 8 bytes
> of padding between dev and data.
Are you saying that there is an issue with
(1) the existing implementation with uses mem::size_of::<Self>() or
(2) the proper one that uses Kmalloc::aligned_layout(Layout::new::<Self>())?
I think neither has, because we're not allocating
size_of::<Opaque<bindings::drm_device>>() + size_of::<T::Data>() as you seem to
assume above, but size_of::<Device<T>>().
>> even with an arbitrary large alignment requirement of T::Data it can't
>> happen that the size of Device is smaller than its alignment requirement.
>>
>> However, let's not rely on this subtle circumstance and create a proper
>> kmalloc() compatible Layout.
>>
>> Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>> ---
>> rust/kernel/drm/device.rs | 9 +++++++--
>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
>> index 3bb7c83966cf..d19410deaf6c 100644
>> --- a/rust/kernel/drm/device.rs
>> +++ b/rust/kernel/drm/device.rs
>> @@ -5,6 +5,7 @@
>> //! C header: [`include/linux/drm/drm_device.h`](srctree/include/linux/drm/drm_device.h)
>>
>> use crate::{
>> + alloc::allocator::Kmalloc,
>> bindings, device, drm,
>> drm::driver::AllocImpl,
>> error::from_err_ptr,
>> @@ -12,7 +13,7 @@
>> prelude::*,
>> types::{ARef, AlwaysRefCounted, Opaque},
>> };
>> -use core::{mem, ops::Deref, ptr, ptr::NonNull};
>> +use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
>>
>> #[cfg(CONFIG_DRM_LEGACY)]
>> macro_rules! drm_legacy_fields {
>> @@ -96,6 +97,10 @@ impl<T: drm::Driver> Device<T> {
>>
>> /// Create a new `drm::Device` for a `drm::Driver`.
>> pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
>> + // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
>> + // compatible `Layout`.
>> + let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
>> +
>> // SAFETY:
>> // - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
>> // - `dev` is valid by its type invarants,
>> @@ -103,7 +108,7 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
>> bindings::__drm_dev_alloc(
>> dev.as_raw(),
>> &Self::VTABLE,
>> - mem::size_of::<Self>(),
>> + layout.size(),
>> mem::offset_of!(Self, dev),
>> )
>> }
>> --
>> 2.50.0
>>
On Fri, Aug 1, 2025 at 11:29 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> On Fri Aug 1, 2025 at 11:18 AM CEST, Alice Ryhl wrote:
> > On Thu, Jul 31, 2025 at 05:48:07PM +0200, Danilo Krummrich wrote:
> >> drm::Device is allocated through __drm_dev_alloc() (which uses
> >> kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
> >> initialized in-place.
> >>
> >> Due to the order of fields in drm::Device
> >>
> >> pub struct Device<T: drm::Driver> {
> >> dev: Opaque<bindings::drm_device>,
> >> data: T::Data,
> >> }
> >
> > I'm not convinced this patch is right.
> >
> > Imagine this scenario: T::Data has size and alignment both equal to 16,
> > and lets say that drm_device has a size that is a multiple of 8 but not
> > 16 such as 72. In that case, you will allocate 72+16=88 bytes for
> > Device, but actually the size of Device is 96 because there is 8 bytes
> > of padding between dev and data.
>
> Are you saying that there is an issue with
>
> (1) the existing implementation with uses mem::size_of::<Self>() or
>
> (2) the proper one that uses Kmalloc::aligned_layout(Layout::new::<Self>())?
>
> I think neither has, because we're not allocating
> size_of::<Opaque<bindings::drm_device>>() + size_of::<T::Data>() as you seem to
> assume above, but size_of::<Device<T>>().
Yes, you're right. I misunderstood how __drm_dev_alloc works.
Alice
On Thu, Jul 31, 2025 at 5:49 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> drm::Device is allocated through __drm_dev_alloc() (which uses
> kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
> initialized in-place.
>
> Due to the order of fields in drm::Device
>
> pub struct Device<T: drm::Driver> {
> dev: Opaque<bindings::drm_device>,
> data: T::Data,
> }
>
> even with an arbitrary large alignment requirement of T::Data it can't
> happen that the size of Device is smaller than its alignment requirement.
>
> However, let's not rely on this subtle circumstance and create a proper
> kmalloc() compatible Layout.
>
> Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
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