[PATCH v4 2/3] rust: pci: impl TryFrom<&Device> for &pci::Device

[Danilo Krummrich <dakr@kernel.org>]

Implement TryFrom<&device::Device> for &Device.

This allows us to get a &pci::Device from a generic &Device in a safe
way; the conversion fails if the device' bus type does not match with
the PCI bus type.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/device.rs |  1 -
 rust/kernel/pci.rs    | 25 +++++++++++++++++++++++--
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 67a2fc46cf4c..0bbc67edd38d 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -66,7 +66,6 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
     }
 
     /// Returns a raw pointer to the device' bus type.
-    #[expect(unused)]
     pub(crate) fn bus_type_raw(&self) -> *const bindings::bus_type {
         // SAFETY:
         // - By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 22a32172b108..8b7fcfe8bffc 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -6,7 +6,7 @@
 
 use crate::{
     alloc::flags::*,
-    bindings, device,
+    bindings, container_of, device,
     device_id::RawDeviceId,
     devres::Devres,
     driver,
@@ -20,7 +20,7 @@
 use core::{
     marker::PhantomData,
     ops::Deref,
-    ptr::{addr_of_mut, NonNull},
+    ptr::{addr_of, addr_of_mut, NonNull},
 };
 use kernel::prelude::*;
 
@@ -466,6 +466,27 @@ fn as_ref(&self) -> &device::Device {
     }
 }
 
+impl TryFrom<&device::Device> for &Device {
+    type Error = kernel::error::Error;
+
+    fn try_from(dev: &device::Device) -> Result<Self, Self::Error> {
+        #[allow(unused_unsafe)]
+        // SAFETY: rustc < 1.82 falsely treats this as unsafe, see:
+        // https://blog.rust-lang.org/2024/10/17/Rust-1.82.0.html#safely-addressing-unsafe-statics
+        if dev.bus_type_raw() != unsafe { addr_of!(bindings::pci_bus_type) } {
+            return Err(EINVAL);
+        }
+
+        // SAFETY: We've just verified that the bus type of `dev` equals `bindings::pci_bus_type`,
+        // hence `dev` must be embedded in a valid `struct pci_dev` as guaranteed by the
+        // corresponding C code.
+        let pdev = unsafe { container_of!(dev.as_raw(), bindings::pci_dev, dev) };
+
+        // SAFETY: `pdev` is a valid pointer to a `struct pci_dev`.
+        Ok(unsafe { &*pdev.cast() })
+    }
+}
+
 // SAFETY: A `Device` is always reference-counted and can be released from any thread.
 unsafe impl Send for Device {}
 
-- 
2.48.1