[PATCH v3] rust: drm: Drop the use of Opaque for ioctl arguments

[PATCH v3] rust: drm: Drop the use of Opaque for ioctl arguments

[Beata Michalska]

With the Opaque<T>, the expectations are that Rust should not
make any assumptions on the layout or invariants of the wrapped
C types. That runs rather counter to ioctl arguments, which must
adhere to certain data-layout constraits. By using Opaque<T>,
ioctl handlers are forced to use unsafe code where non is acually
needed. This adds needless complexity and maintenance overhead,
brining no safety benefits.
Drop the use of Opaque for ioctl arguments as that is not the best
fit here.

Signed-off-by: Beata Michalska <beata.michalska@arm.com>
[ nova-drm changes: Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 drivers/gpu/drm/nova/file.rs | 23 ++++++--------
 drivers/gpu/drm/nova/nova.rs |  1 -
 drivers/gpu/drm/nova/uapi.rs | 61 ------------------------------------
 rust/kernel/drm/ioctl.rs     | 11 ++++---
 4 files changed, 16 insertions(+), 80 deletions(-)
 delete mode 100644 drivers/gpu/drm/nova/uapi.rs

diff --git a/drivers/gpu/drm/nova/file.rs b/drivers/gpu/drm/nova/file.rs
index 7e59a34b830d..7e7d4e2de2fb 100644
--- a/drivers/gpu/drm/nova/file.rs
+++ b/drivers/gpu/drm/nova/file.rs
@@ -2,13 +2,11 @@
 
 use crate::driver::{NovaDevice, NovaDriver};
 use crate::gem::NovaObject;
-use crate::uapi::{GemCreate, GemInfo, Getparam};
 use kernel::{
     alloc::flags::*,
     drm::{self, gem::BaseObject},
     pci,
     prelude::*,
-    types::Opaque,
     uapi,
 };
 
@@ -26,20 +24,19 @@ impl File {
     /// IOCTL: get_param: Query GPU / driver metadata.
     pub(crate) fn get_param(
         dev: &NovaDevice,
-        getparam: &Opaque<uapi::drm_nova_getparam>,
+        getparam: &mut uapi::drm_nova_getparam,
         _file: &drm::File<File>,
     ) -> Result<u32> {
         let adev = &dev.adev;
         let parent = adev.parent().ok_or(ENOENT)?;
         let pdev: &pci::Device = parent.try_into()?;
-        let getparam: &Getparam = getparam.into();
 
-        let value = match getparam.param() as u32 {
+        let value = match getparam.param as u32 {
             uapi::NOVA_GETPARAM_VRAM_BAR_SIZE => pdev.resource_len(1)?,
             _ => return Err(EINVAL),
         };
 
-        getparam.set_value(value);
+        getparam.value = value;
 
         Ok(0)
     }
@@ -47,13 +44,12 @@ pub(crate) fn get_param(
     /// IOCTL: gem_create: Create a new DRM GEM object.
     pub(crate) fn gem_create(
         dev: &NovaDevice,
-        req: &Opaque<uapi::drm_nova_gem_create>,
+        req: &mut uapi::drm_nova_gem_create,
         file: &drm::File<File>,
     ) -> Result<u32> {
-        let req: &GemCreate = req.into();
-        let obj = NovaObject::new(dev, req.size().try_into()?)?;
+        let obj = NovaObject::new(dev, req.size.try_into()?)?;
 
-        req.set_handle(obj.create_handle(file)?);
+        req.handle = obj.create_handle(file)?;
 
         Ok(0)
     }
@@ -61,13 +57,12 @@ pub(crate) fn gem_create(
     /// IOCTL: gem_info: Query GEM metadata.
     pub(crate) fn gem_info(
         _dev: &NovaDevice,
-        req: &Opaque<uapi::drm_nova_gem_info>,
+        req: &mut uapi::drm_nova_gem_info,
         file: &drm::File<File>,
     ) -> Result<u32> {
-        let req: &GemInfo = req.into();
-        let bo = NovaObject::lookup_handle(file, req.handle())?;
+        let bo = NovaObject::lookup_handle(file, req.handle)?;
 
-        req.set_size(bo.size().try_into()?);
+        req.size = bo.size().try_into()?;
 
         Ok(0)
     }
diff --git a/drivers/gpu/drm/nova/nova.rs b/drivers/gpu/drm/nova/nova.rs
index 902876aa14d1..730598defe04 100644
--- a/drivers/gpu/drm/nova/nova.rs
+++ b/drivers/gpu/drm/nova/nova.rs
@@ -5,7 +5,6 @@
 mod driver;
 mod file;
 mod gem;
-mod uapi;
 
 use crate::driver::NovaDriver;
 
diff --git a/drivers/gpu/drm/nova/uapi.rs b/drivers/gpu/drm/nova/uapi.rs
deleted file mode 100644
index eb228a58d423..000000000000
--- a/drivers/gpu/drm/nova/uapi.rs
+++ /dev/null
@@ -1,61 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-
-use kernel::uapi;
-
-// TODO Work out some common infrastructure to avoid boilerplate code for uAPI abstractions.
-
-macro_rules! define_uapi_abstraction {
-    ($name:ident <= $inner:ty) => {
-        #[repr(transparent)]
-        pub struct $name(::kernel::types::Opaque<$inner>);
-
-        impl ::core::convert::From<&::kernel::types::Opaque<$inner>> for &$name {
-            fn from(value: &::kernel::types::Opaque<$inner>) -> Self {
-                // SAFETY: `Self` is a transparent wrapper of `$inner`.
-                unsafe { ::core::mem::transmute(value) }
-            }
-        }
-    };
-}
-
-define_uapi_abstraction!(Getparam <= uapi::drm_nova_getparam);
-
-impl Getparam {
-    pub fn param(&self) -> u64 {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_getparam`.
-        unsafe { (*self.0.get()).param }
-    }
-
-    pub fn set_value(&self, v: u64) {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_getparam`.
-        unsafe { (*self.0.get()).value = v };
-    }
-}
-
-define_uapi_abstraction!(GemCreate <= uapi::drm_nova_gem_create);
-
-impl GemCreate {
-    pub fn size(&self) -> u64 {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_gem_create`.
-        unsafe { (*self.0.get()).size }
-    }
-
-    pub fn set_handle(&self, handle: u32) {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_gem_create`.
-        unsafe { (*self.0.get()).handle = handle };
-    }
-}
-
-define_uapi_abstraction!(GemInfo <= uapi::drm_nova_gem_info);
-
-impl GemInfo {
-    pub fn handle(&self) -> u32 {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_gem_info`.
-        unsafe { (*self.0.get()).handle }
-    }
-
-    pub fn set_size(&self, size: u64) {
-        // SAFETY: `self.get()` is a valid pointer to a `struct drm_nova_gem_info`.
-        unsafe { (*self.0.get()).size = size };
-    }
-}
diff --git a/rust/kernel/drm/ioctl.rs b/rust/kernel/drm/ioctl.rs
index 445639404fb7..3425a835f9cd 100644
--- a/rust/kernel/drm/ioctl.rs
+++ b/rust/kernel/drm/ioctl.rs
@@ -83,7 +83,7 @@ pub mod internal {
 ///
 /// ```ignore
 /// fn foo(device: &kernel::drm::Device<Self>,
-///        data: &Opaque<uapi::argument_type>,
+///        data: &mut uapi::argument_type,
 ///        file: &kernel::drm::File<Self::File>,
 /// ) -> Result<u32>
 /// ```
@@ -138,9 +138,12 @@ pub mod internal {
                             // SAFETY: The ioctl argument has size `_IOC_SIZE(cmd)`, which we
                             // asserted above matches the size of this type, and all bit patterns of
                             // UAPI structs must be valid.
-                            let data = unsafe {
-                                &*(raw_data as *const $crate::types::Opaque<$crate::uapi::$struct>)
-                            };
+                            // The `ioctl` argument is exclusively owned by the handler
+                            // and guaranteed by the C implementation (`drm_ioctl()`) to remain
+                            // valid for the entire lifetime of the reference taken here.
+                            // There is no concurrent access or aliasing; no other references
+                            // to this object exist during this call.
+                            let data = unsafe { &mut *(raw_data as *mut $crate::uapi::$struct) };
                             // SAFETY: This is just the DRM file structure
                             let file = unsafe { $crate::drm::File::as_ref(raw_file) };
 
-- 
2.25.1

Re: [PATCH v3] rust: drm: Drop the use of Opaque for ioctl arguments

[Danilo Krummrich]

On Tue, Jun 24, 2025 at 11:22:25PM +0200, Beata Michalska wrote:
> With the Opaque<T>, the expectations are that Rust should not
> make any assumptions on the layout or invariants of the wrapped
> C types. That runs rather counter to ioctl arguments, which must
> adhere to certain data-layout constraits. By using Opaque<T>,
> ioctl handlers are forced to use unsafe code where non is acually

s/non/none/

> needed. This adds needless complexity and maintenance overhead,
> brining no safety benefits.
> Drop the use of Opaque for ioctl arguments as that is not the best
> fit here.
> 
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> [ nova-drm changes: Danilo ]
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

It's your patch, it doesn't need my 'Signed-off-by:', but feel free to convert
it into:

	Acked-by: Danilo Krummrich <dakr@kernel.org>