ANDROID: ashmem_rust: Add support for retrieving an ashmem area's vmfile

Given a pointer to a file structure that represents an ashmem region,
return the shmem vmfile that backs the region. This should only be
used for dumping information about ashmem regions in debug scenarios.

Bug: 193397560
Change-Id: I6ecd4ed0e02caac29a8bff9c52052066219a5a8c
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>

drivers/staging/android/ashmem.h

@@ -34,5 +34,6 @@ enum {
 bool is_ashmem_file(struct file *file);
 int ashmem_area_name(struct file *file, char *name);
 long ashmem_area_size(struct file *file);
+struct file *ashmem_area_vmfile(struct file *file);
 
 #endif	/* _LINUX_ASHMEM_H */

drivers/staging/android/ashmem_rust.rs

@@ -724,3 +724,31 @@ unsafe extern "C" fn ashmem_area_size(file: *mut bindings::file) -> isize {
         Err(_err) => 0,
     }
 }
+
+/// # Safety
+///
+/// The caller must ensure that `file` is valid for the duration of this function.
+///
+/// If this function returns a non-NULL pointer to a file structure, the refcount for that
+/// file will be incremented by 1. It is the caller's responsibility to decrement the refcount
+/// when the file is no longer needed.
+#[no_mangle]
+unsafe extern "C" fn ashmem_area_vmfile(file: *mut bindings::file) -> *mut bindings::file {
+    // SAFETY: file is valid for the duration of this function.
+    let ashmem = match unsafe { get_ashmem_area(file) } {
+        Ok(area) => area,
+        Err(_err) => return null_mut(),
+    };
+
+    let asma = &mut *ashmem.inner.lock();
+    match asma.file.as_ref() {
+        Some(shmem_file) => {
+            let shmem_file_ptr = shmem_file.file().as_ptr();
+            // SAFETY: file is valid for the duration of the function, which means shmem file is
+            // also valid at this point.
+            unsafe { bindings::get_file(shmem_file_ptr) };
+            shmem_file_ptr
+        }
+        None => null_mut(),
+    }
+}

drivers/staging/android/ashmem_rust_exports.c

@@ -19,3 +19,4 @@
 EXPORT_SYMBOL_GPL(is_ashmem_file);
 EXPORT_SYMBOL_GPL(ashmem_area_name);
 EXPORT_SYMBOL_GPL(ashmem_area_size);
+EXPORT_SYMBOL_GPL(ashmem_area_vmfile);