[PATCH v3] rust: block: set GenDisk block_device_operations.owner to THIS_MODULE

GenDiskBuilder left block_device_operations.owner NULL. Set owner from a
const block_device_operations table using this_module::<M>() and update
rnull to pass NullBlkModule.

Add ModuleMetadata::THIS_MODULE and this_module() so the owner pointer is
available in const context. Rebased on Alvins fix-fops-owner series [1].

Link: https://lore.kernel.org/all/[email protected]/

v2:
- Free fops in free_disk instead of GenDisk::drop to fix use-after-free
  when the device stays open after removal. (Sashiko)
- Install the cleanup guard before fops allocation to avoid leaking gendisk
  on -ENOMEM. (Sashiko)
- Link to v1: https://lore.kernel.org/all/[email protected]/

v3:
- Use const fops with this_module::<M>() instead of heap allocation
  (as suggested by Andreas).
- Integrate ModuleMetadata::THIS_MODULE from [1].
- Dismiss recover_data before fallible build steps after the cleanup guard.
  (Sashiko)
- Link to v2: https://lore.kernel.org/all/[email protected]/

[1] https://lore.kernel.org/r/[email protected]

Signed-off-by: Adarsh Das <[email protected]>
---
 drivers/block/rnull/rnull.rs     |  2 +-
 rust/kernel/block/mq.rs          | 13 ++++++--
 rust/kernel/block/mq/gen_disk.rs | 56 +++++++++++++++++---------------
 rust/kernel/lib.rs               |  9 +++++
 rust/macros/module.rs            | 16 +++++++++
 5 files changed, 65 insertions(+), 31 deletions(-)

diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
index 0ca8715febe8..912526be4ec2 100644
--- a/drivers/block/rnull/rnull.rs
+++ b/drivers/block/rnull/rnull.rs
@@ -61,7 +61,7 @@ fn new(
             .logical_block_size(block_size)?
             .physical_block_size(block_size)?
             .rotational(rotational)
-            .build(fmt!("{}", name.to_str()?), tagset, queue_data)
+            .build::<NullBlkModule, Self>(fmt!("{}", name.to_str()?), tagset, queue_data)
     }
 }
 
diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index 1fd0d54dd549..faa9fcbac935 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -8,8 +8,8 @@
 //! - Implement [`Operations`] for a type `T`.
 //! - Create a [`TagSet<T>`].
 //! - Create a [`GenDisk<T>`], via the [`GenDiskBuilder`].
-//! - Add the disk to the system by calling [`GenDiskBuilder::build`] passing in
-//!   the `TagSet` reference.
+//! - Add the disk to the system by calling [`GenDiskBuilder::build`], passing in
+//!   the module type, the disk name, the `TagSet`, and queue data.
 //!
 //! The types available in this module that have direct C counterparts are:
 //!
@@ -86,9 +86,16 @@
 //!
 //! let tagset: Arc<TagSet<MyBlkDevice>> =
 //!     Arc::pin_init(TagSet::new(1, 256, 1), flags::GFP_KERNEL)?;
+//! # struct MyModule;
+//! # impl kernel::ModuleMetadata for MyModule {
+//! #     const NAME: &'static kernel::str::CStr = c"myblk";
+//! #     // SAFETY: Doctest stub; no module is loaded.
+//! #     const THIS_MODULE: ThisModule =
+//! #         unsafe { ThisModule::from_ptr(core::ptr::null_mut()) };
+//! # }
 //! let mut disk = gen_disk::GenDiskBuilder::new()
 //!     .capacity_sectors(4096)
-//!     .build(fmt!("myblk"), tagset, ())?;
+//!     .build::<MyModule, MyBlkDevice>(fmt!("myblk"), tagset, ())?;
 //!
 //! # Ok::<(), kernel::error::Error>(())
 //! ```
diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs
index fc97dd873974..4a503f183cc9 100644
--- a/rust/kernel/block/mq/gen_disk.rs
+++ b/rust/kernel/block/mq/gen_disk.rs
@@ -15,8 +15,34 @@
     str::NullTerminatedFormatter,
     sync::Arc,
     types::{ForeignOwnable, ScopeGuard},
+    ModuleMetadata,
 };
 
+struct BlockFops<M: ModuleMetadata>(core::marker::PhantomData<M>);
+
+impl<M: ModuleMetadata> BlockFops<M> {
+    const TABLE: bindings::block_device_operations = bindings::block_device_operations {
+        submit_bio: None,
+        open: None,
+        release: None,
+        ioctl: None,
+        compat_ioctl: None,
+        check_events: None,
+        unlock_native_capacity: None,
+        getgeo: None,
+        set_read_only: None,
+        swap_slot_free_notify: None,
+        report_zones: None,
+        devnode: None,
+        alternative_gpt_sector: None,
+        get_unique_id: None,
+        owner: crate::this_module::<M>().as_ptr(),
+        pr_ops: core::ptr::null_mut(),
+        free_disk: None,
+        poll_bio: None,
+    };
+}
+
 /// A builder for [`GenDisk`].
 ///
 /// Use this struct to configure and add new [`GenDisk`] to the VFS.
@@ -95,7 +121,7 @@ pub fn capacity_sectors(mut self, capacity: u64) -> Self {
     }
 
     /// Build a new `GenDisk` and add it to the VFS.
-    pub fn build<T: Operations>(
+    pub fn build<M: ModuleMetadata, T: Operations>(
         self,
         name: fmt::Arguments<'_>,
         tagset: Arc<TagSet<T>>,
@@ -125,30 +151,8 @@ pub fn build<T: Operations>(
             )
         })?;
 
-        const TABLE: bindings::block_device_operations = bindings::block_device_operations {
-            submit_bio: None,
-            open: None,
-            release: None,
-            ioctl: None,
-            compat_ioctl: None,
-            check_events: None,
-            unlock_native_capacity: None,
-            getgeo: None,
-            set_read_only: None,
-            swap_slot_free_notify: None,
-            report_zones: None,
-            devnode: None,
-            alternative_gpt_sector: None,
-            get_unique_id: None,
-            // TODO: Set to `THIS_MODULE`.
-            owner: core::ptr::null_mut(),
-            pr_ops: core::ptr::null_mut(),
-            free_disk: None,
-            poll_bio: None,
-        };
-
-        // SAFETY: `gendisk` is a valid pointer as we initialized it above
-        unsafe { (*gendisk).fops = &TABLE };
+        // SAFETY: `gendisk` is a valid pointer as we initialized it above.
+        unsafe { (*gendisk).fops = &BlockFops::<M>::TABLE };
 
         let cleanup_failure = ScopeGuard::new_with_data((gendisk, data), |(gendisk, data)| {
             // SAFETY: `gendisk` came from `__blk_mq_alloc_disk()` above and
@@ -159,8 +163,6 @@ pub fn build<T: Operations>(
             drop(unsafe { T::QueueData::from_foreign(data) });
         });
 
-        // The failure guard now owns both pieces of cleanup; the early guard
-        // must not run on this path anymore.
         recover_data.dismiss();
 
         let mut writer = NullTerminatedFormatter::new(
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9512af7156df..9bbabaf84e20 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -185,6 +185,15 @@ fn init(module: &'static ThisModule) -> impl pin_init::PinInit<Self, error::Erro
 pub trait ModuleMetadata {
     /// The name of the module as specified in the `module!` macro.
     const NAME: &'static crate::str::CStr;
+
+    /// The module's `THIS_MODULE` pointer.
+    const THIS_MODULE: ThisModule;
+}
+
+/// Returns the [`ThisModule`] pointer for the given module type.
+#[inline]
+pub const fn this_module<M: ModuleMetadata>() -> &'static ThisModule {
+    &M::THIS_MODULE
 }
 
 /// Equivalent to `THIS_MODULE` in the C API.
diff --git a/rust/macros/module.rs b/rust/macros/module.rs
index 06c18e207508..8df38718224b 100644
--- a/rust/macros/module.rs
+++ b/rust/macros/module.rs
@@ -519,6 +519,22 @@ pub(crate) fn module(info: ModuleInfo) -> Result<TokenStream> {
 
         impl ::kernel::ModuleMetadata for #type_ {
             const NAME: &'static ::kernel::str::CStr = #name_cstr;
+
+            #[cfg(MODULE)]
+            const THIS_MODULE: ::kernel::ThisModule = {
+                extern "C" {
+                    static __this_module: ::kernel::types::Opaque<::kernel::bindings::module>;
+                }
+
+                // SAFETY: `__this_module` is constructed by the kernel at load time and lives
+                // until the module is unloaded.
+                unsafe { ::kernel::ThisModule::from_ptr(__this_module.get()) }
+            };
+
+            #[cfg(not(MODULE))]
+            const THIS_MODULE: ::kernel::ThisModule = unsafe {
+                ::kernel::ThisModule::from_ptr(::core::ptr::null_mut())
+            };
         }
 
         // Double nested modules, since then nobody can access the public items inside.
-- 
2.55.0