ANDROID: ashmem_rust: Add is_ashmem_file()

The minidump module iterates over open files and VMAs for each process
and reports information that is useful for debugging. A useful piece of
information is what ashmem files are currently referenced either via
a userspace memory mapping or a file descriptor. Therefore, add
is_ashmem_file() to aid in identifying those references.

Unfortunately, the logic that exports Rust symbols to other kernel
modules only considers the crates under the rust/ directory at the root
of the kernel repository. Since there's no support to include symbols
from crates defined outside of that repository, add
ashmem_rust_exports.c to export is_ashmem_file().

Changes from older versions of this patch:

- Reworded commit message for clarity.
- Implemented is_ashmem_file() in Rust, since android16-6.12+ will use
  Rust for ashmem.

Bug: 193397560
Change-Id: I5b7816ad4775e5cf2c4f41c28b1c8dacc2c85b7e
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>

drivers/staging/android/Makefile

@@ -3,3 +3,4 @@ ccflags-y += -I$(src)			# needed for trace events
 
 obj-$(CONFIG_ASHMEM_C)			+= ashmem.o
 obj-$(CONFIG_ASHMEM_RUST)		+= ashmem_rust.o
+ashmem_rust-objs += ashmem_rust.o ashmem_rust_exports.o

drivers/staging/android/ashmem.h

@@ -31,4 +31,6 @@ enum {
 };
 #endif
 
+bool is_ashmem_file(struct file *file);
+
 #endif	/* _LINUX_ASHMEM_H */

drivers/staging/android/ashmem_rust.rs

@@ -12,7 +12,8 @@
 
 use core::{
     pin::Pin,
-    sync::atomic::{AtomicBool, AtomicUsize, Ordering},
+    ptr::null_mut,
+    sync::atomic::{AtomicBool, AtomicPtr, AtomicUsize, Ordering},
 };
 use kernel::{
     bindings::{self, ASHMEM_GET_PIN_STATUS, ASHMEM_PIN, ASHMEM_UNPIN},
@@ -71,6 +72,7 @@ fn has_cap_sys_admin() -> bool {
 static NUM_PIN_IOCTLS_WAITING: AtomicUsize = AtomicUsize::new(0);
 static IGNORE_UNSET_PROT_READ: AtomicBool = AtomicBool::new(false);
 static IGNORE_UNSET_PROT_EXEC: AtomicBool = AtomicBool::new(false);
+static ASHMEM_FOPS_PTR: AtomicPtr<bindings::file_operations> = AtomicPtr::new(null_mut());
 
 fn shrinker_should_stop() -> bool {
     NUM_PIN_IOCTLS_WAITING.load(Ordering::Relaxed) > 0
@@ -104,13 +106,20 @@ impl kernel::Module for AshmemModule {
 
         ashmem_range::set_shrinker_enabled(true)?;
 
+        let ashmem_miscdevice_registration = KBox::pin_init(
+            MiscDeviceRegistration::register(MiscDeviceOptions {
+                name: c_str!("ashmem"),
+            }),
+            GFP_KERNEL,
+        )?;
+        let ashmem_miscdevice_ptr = ashmem_miscdevice_registration.as_raw();
+        // SAFETY: ashmem_miscdevice_registration is pinned and is never destroyed, so reading
+        // and storing the fops pointer this way should be fine.
+        let fops_ptr = unsafe { (*ashmem_miscdevice_ptr).fops };
+        ASHMEM_FOPS_PTR.store(fops_ptr.cast_mut(), Ordering::Relaxed);
+
         Ok(Self {
-            _misc: KBox::pin_init(
-                MiscDeviceRegistration::register(MiscDeviceOptions {
-                    name: c_str!("ashmem"),
-                }),
-                GFP_KERNEL,
-            )?,
+            _misc: ashmem_miscdevice_registration,
             _toggle_unpin: AshmemToggleMisc::<AshmemToggleShrinker>::new()?,
             _toggle_read: AshmemToggleMisc::<AshmemToggleRead>::new()?,
             _toggle_exec: AshmemToggleMisc::<AshmemToggleExec>::new()?,
@@ -638,3 +647,18 @@ fn ashmem_memfd_ioctl_inner(file: &File, cmd: u32, arg: usize) -> Result<isize>
         _ => Err(EINVAL),
     }
 }
+
+/// # Safety
+///
+/// The caller must ensure that `file` is valid for the duration of this function.
+#[no_mangle]
+unsafe extern "C" fn is_ashmem_file(file: *mut bindings::file) -> bool {
+    let ashmem_fops_ptr = ASHMEM_FOPS_PTR.load(Ordering::Relaxed);
+    if file.is_null() || ashmem_fops_ptr.is_null() {
+        return false;
+    }
+
+    // SAFETY: Accessing the f_op field of a non-NULL file structure is always okay.
+    let fops_ptr = unsafe { (*file).f_op };
+    fops_ptr == ashmem_fops_ptr
+}

drivers/staging/android/ashmem_rust_exports.c

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Symbols exported from the Ashmem Rust driver for loadable kernel modules to use.
+ *
+ * Copyright (c) 2025, Google LLC.
+ */
+
+#include <linux/export.h>
+
+#include "ashmem.h"
+
+/*
+ * List symbols that need to be exported to loadable kernel modules below. This is needed because
+ * the logic that exports symbols from Rust crates only considers the crates under the rust/
+ * directory at the root of the kernel repo. It currently does not support exporting symbols from
+ * other crates.
+ */
+EXPORT_SYMBOL_GPL(is_ashmem_file);