[PATCH RFC 2/4] samples: rust: Add sample demonstrating C linked list iteration

[Joel Fernandes <joelagnelf@nvidia.com>]

Demonstrates usage of the clist module for iterating over
C-managed linked lists. C code creates and populates the list,
Rust code performs safe iteration using the clist abstraction.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 samples/rust/Kconfig            |  11 +++
 samples/rust/Makefile           |   2 +
 samples/rust/rust_clist_c.c     |  54 +++++++++++++
 samples/rust/rust_clist_main.rs | 138 ++++++++++++++++++++++++++++++++
 4 files changed, 205 insertions(+)
 create mode 100644 samples/rust/rust_clist_c.c
 create mode 100644 samples/rust/rust_clist_main.rs

diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
index c1cc787a9add..b45631e2593c 100644
--- a/samples/rust/Kconfig
+++ b/samples/rust/Kconfig
@@ -10,6 +10,17 @@ menuconfig SAMPLES_RUST
 
 if SAMPLES_RUST
 
+config SAMPLE_RUST_CLIST
+	tristate "C Linked List sample"
+	help
+	  This option builds the Rust CList sample demonstrating
+	  the clist module for working with C list_head structures.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_clist.
+
+	  If unsure, say N.
+
 config SAMPLE_RUST_CONFIGFS
 	tristate "Configfs sample"
 	depends on CONFIGFS_FS
diff --git a/samples/rust/Makefile b/samples/rust/Makefile
index cf8422f8f219..f8899c0df762 100644
--- a/samples/rust/Makefile
+++ b/samples/rust/Makefile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 ccflags-y += -I$(src)				# needed for trace events
 
+obj-$(CONFIG_SAMPLE_RUST_CLIST)			+= rust_clist.o
 obj-$(CONFIG_SAMPLE_RUST_MINIMAL)		+= rust_minimal.o
 obj-$(CONFIG_SAMPLE_RUST_MISC_DEVICE)		+= rust_misc_device.o
 obj-$(CONFIG_SAMPLE_RUST_PRINT)			+= rust_print.o
@@ -14,6 +15,7 @@ obj-$(CONFIG_SAMPLE_RUST_DRIVER_FAUX)		+= rust_driver_faux.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_AUXILIARY)	+= rust_driver_auxiliary.o
 obj-$(CONFIG_SAMPLE_RUST_CONFIGFS)		+= rust_configfs.o
 
+rust_clist-y := rust_clist_main.o rust_clist_c.o
 rust_print-y := rust_print_main.o rust_print_events.o
 
 subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs
diff --git a/samples/rust/rust_clist_c.c b/samples/rust/rust_clist_c.c
new file mode 100644
index 000000000000..7a8f5e6c642a
--- /dev/null
+++ b/samples/rust/rust_clist_c.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/list.h>
+#include <linux/slab.h>
+
+/* Sample item with embedded C list_head */
+struct sample_item_c {
+	int value;
+	struct list_head link;
+};
+
+/* Create a list_head and populate it with items */
+struct list_head *clist_sample_create(int count)
+{
+	struct list_head *head;
+	int i;
+
+	head = kmalloc(sizeof(*head), GFP_KERNEL);
+	if (!head)
+		return NULL;
+
+	INIT_LIST_HEAD(head);
+
+	/* Populate with items */
+	for (i = 0; i < count; i++) {
+		struct sample_item_c *item = kmalloc(sizeof(*item), GFP_KERNEL);
+
+		if (!item)
+			continue;
+
+		item->value = i * 10;
+		INIT_LIST_HEAD(&item->link);
+		list_add_tail(&item->link, head);
+	}
+
+	return head;
+}
+
+/* Free the list_head and all items */
+void clist_sample_free(struct list_head *head)
+{
+	struct sample_item_c *item, *tmp;
+
+	if (!head)
+		return;
+
+	/* Free all items in the list */
+	list_for_each_entry_safe(item, tmp, head, link) {
+		list_del(&item->link);
+		kfree(item);
+	}
+
+	kfree(head);
+}
diff --git a/samples/rust/rust_clist_main.rs b/samples/rust/rust_clist_main.rs
new file mode 100644
index 000000000000..6600b0c79558
--- /dev/null
+++ b/samples/rust/rust_clist_main.rs
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Sample for Rust code interfacing with C linked lists (list_head).
+//!
+//! This sample demonstrates iteration of a C-managed linked list using the [`clist`] module.
+//! C code creates and populates the list, Rust code performs iteration.
+
+use core::ptr::NonNull;
+use kernel::{
+    bindings,
+    clist,
+    container_of,
+    prelude::*, //
+};
+
+module! {
+    type: RustClistModule,
+    name: "rust_clist",
+    authors: ["Joel Fernandes"],
+    description: "Rust Clist sample",
+    license: "GPL",
+}
+
+// FFI declarations for C functions
+extern "C" {
+    fn clist_sample_create(count: i32) -> *mut bindings::list_head;
+    fn clist_sample_free(head: *mut bindings::list_head);
+}
+
+/// Sample item with embedded C list_head (This would typically be a C struct).
+#[repr(C)]
+struct SampleItemC {
+    value: i32,
+    link: bindings::list_head,
+}
+
+/// Rust wrapper for SampleItemC object pointer allocated on the C side.
+///
+/// # Invariants
+///
+/// `ptr` points to a valid [`SampleItemC`] with an initialized embedded `list_head`.
+struct SampleItem {
+    ptr: NonNull<SampleItemC>,
+}
+
+impl clist::FromListHead for SampleItem {
+    unsafe fn from_list_head(link: *const bindings::list_head) -> Self {
+        // SAFETY: Caller ensures link points to a valid, initialized list_head.
+        unsafe {
+            let item_ptr = container_of!(link, SampleItemC, link) as *mut _;
+            SampleItem {
+                ptr: NonNull::new_unchecked(item_ptr),
+            }
+        }
+    }
+}
+
+impl SampleItem {
+    /// Get the value from the sample item.
+    fn value(&self) -> i32 {
+        // SAFETY: self.ptr is valid as per the SampleItem invariants.
+        unsafe { (*self.ptr.as_ptr()).value }
+    }
+}
+
+/// Clist struct - holds the C list_head and manages its lifecycle.
+#[repr(C)]
+struct RustClist {
+    list_head: NonNull<bindings::list_head>,
+}
+
+// SAFETY: RustClist can be sent between threads since it only holds a pointer
+// which can be accessed from any thread with proper synchronization.
+unsafe impl Send for RustClist {}
+
+impl RustClist {
+    /// Create a container for a pointer to a C-allocated list_head.
+    fn new(list_head: *mut bindings::list_head) -> Self {
+        // SAFETY: Caller ensures list_head is a valid, non-null pointer.
+        Self {
+            list_head: unsafe { NonNull::new_unchecked(list_head) },
+        }
+    }
+
+    /// Demonstrate iteration over the list.
+    fn do_iteration(&self) {
+        // Wrap the C list_head with a Rust [`Clist`].
+        // SAFETY: list_head is a valid, initialized, populated list_head.
+        let list = unsafe { clist::Clist::<SampleItem>::new(self.list_head.as_ptr()) };
+        pr_info!("Created C list with items, is_empty: {}\n", list.is_empty());
+
+        // Iterate over the list.
+        pr_info!("Iterating over C list from Rust:\n");
+        for item in list.iter() {
+            pr_info!("  Item value: {}\n", item.value());
+        }
+
+        pr_info!("Iteration complete\n");
+    }
+}
+
+impl Drop for RustClist {
+    fn drop(&mut self) {
+        // Free the list and all items using C FFI.
+        // SAFETY: list_head was allocated by clist_sample_create.
+        // C side handles freeing both the list_head and all items.
+        unsafe {
+            clist_sample_free(self.list_head.as_ptr());
+        }
+    }
+}
+
+struct RustClistModule;
+
+impl kernel::Module for RustClistModule {
+    fn init(_module: &'static ThisModule) -> Result<Self> {
+        pr_info!("Rust Clist sample (init)\n");
+
+        // C creates and populates a list_head with items.
+        // SAFETY: clist_sample_create allocates, initializes, and populates a list.
+        let c_list_head = unsafe { clist_sample_create(6) };
+        if c_list_head.is_null() {
+            pr_err!("Failed to create and populate C list\n");
+            return Err(ENOMEM);
+        }
+
+        // Create the list container (separate from module).
+        let rust_clist = RustClist::new(c_list_head);
+
+        // Demonstrate list operations.
+        rust_clist.do_iteration();
+
+        // rust_clist is dropped here, which frees the C list via Drop impl.
+        pr_info!("Rust Clist sample (exit)\n");
+
+        Ok(RustClistModule {})
+    }
+}
-- 
2.34.1