ANDROID: ashmem_rust: Add support for querying the size of an ashmem region

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

Bug: 193397560
Change-Id: I24e6a2f7a469d667dc37ec8714e591b911043ee4
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>

drivers/staging/android/ashmem.h

@@ -33,5 +33,6 @@ enum {
 
 bool is_ashmem_file(struct file *file);
 int ashmem_area_name(struct file *file, char *name);
+long ashmem_area_size(struct file *file);
 
 #endif	/* _LINUX_ASHMEM_H */

drivers/staging/android/ashmem_rust.rs

@@ -707,3 +707,20 @@ unsafe extern "C" fn ashmem_area_name(
         Err(err) => err.to_errno() as c_int,
     }
 }
+
+/// # Safety
+///
+/// The caller must ensure that `file` is valid for the duration of this function.
+#[no_mangle]
+unsafe extern "C" fn ashmem_area_size(file: *mut bindings::file) -> isize {
+    // SAFETY: file is valid for the duration of this function.
+    let ashmem = match unsafe { get_ashmem_area(file) } {
+        Ok(area) => area,
+        Err(_err) => return 0,
+    };
+
+    match ashmem.get_size() {
+        Ok(size) => size,
+        Err(_err) => 0,
+    }
+}

drivers/staging/android/ashmem_rust_exports.c

@@ -18,3 +18,4 @@
  */
 EXPORT_SYMBOL_GPL(is_ashmem_file);
 EXPORT_SYMBOL_GPL(ashmem_area_name);
+EXPORT_SYMBOL_GPL(ashmem_area_size);