ANDROID: ashmem_rust: add ASHMEM_PURGE_ALL_CACHES

Add the simplest way of freeing pages that are unpinned: the
ASHMEM_PURGE_ALL_CACHES ioctl. It will free every single unpinned page.

Bug: 370906207
Change-Id: Ic1411148c036dec6528aa99f32d47a5b2e145c86
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/android/ashmem_rust.rs

@@ -45,7 +45,7 @@ mod ashmem_shrinker;
 use ashmem_shrinker::{ShrinkerBuilder, ShrinkerRegistration};
 
 mod ashmem_range;
-use ashmem_range::{Area, AshmemGuard, NewRange, ASHMEM_MUTEX};
+use ashmem_range::{Area, AshmemGuard, NewRange, ASHMEM_MUTEX, LRU_COUNT};
 
 mod shmem;
 use shmem::ShmemFile;
@@ -57,6 +57,12 @@ fn read_implies_exec(task: &Task) -> bool {
     (personality & bindings::READ_IMPLIES_EXEC) != 0
 }
 
+/// Calls `capable(CAP_SYS_ADMIN)`.
+fn has_cap_sys_admin() -> bool {
+    use kernel::bindings::CAP_SYS_ADMIN;
+    unsafe { bindings::capable(CAP_SYS_ADMIN as c_int) }
+}
+
 static NUM_PIN_IOCTLS_WAITING: AtomicUsize = AtomicUsize::new(0);
 
 fn shrinker_should_stop() -> bool {
@@ -239,6 +245,7 @@ impl MiscDevice for Ashmem {
             ASHMEM_PIN | ASHMEM_UNPIN | ASHMEM_GET_PIN_STATUS => {
                 me.pin_unpin(cmd, UserSlice::new(arg, size).reader())
             }
+            bindings::ASHMEM_PURGE_ALL_CACHES => me.purge_all_caches(),
             _ => Err(EINVAL),
         }
     }
@@ -430,6 +437,17 @@ impl Ashmem {
             _ => unreachable!(),
         }
     }
+
+    fn purge_all_caches(&self) -> Result<isize> {
+        if !has_cap_sys_admin() {
+            return Err(EPERM);
+        }
+        let mut guard = AshmemGuard(ASHMEM_MUTEX.lock());
+        let total_num_pages = LRU_COUNT.load(Ordering::Relaxed);
+        let _num_freed = guard.free_lru(usize::MAX);
+        // ASHMEM_PURGE_ALL_CACHES returns the total number of pages even if we stopped early.
+        Ok(isize::try_from(total_num_pages).unwrap_or(isize::MAX))
+    }
 }
 
 impl AshmemInner {