fuzz: explicitly keep or reject fuzzer corpus

When the main fuzzer function returns (), it is equivalent to
returning Corpus::Keep.

In some of the return paths, we want to reject the input so that the
libfuzzer won't spend more time mutating them.

The should make fuzzing more efficient. No functional change intended.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

fuzz/fuzz_targets/rng.rs

@@ -7,7 +7,7 @@
 use std::os::unix::io::{AsRawFd, FromRawFd};
 use std::sync::Arc;
 
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use seccompiler::SeccompAction;
 use virtio_devices::{VirtioDevice, VirtioInterrupt, VirtioInterruptType};
 use virtio_queue::{Queue, QueueT};
@@ -52,11 +52,11 @@ const USED_RING_ADDR: u64 = align!(AVAIL_RING_ADDR + AVAIL_RING_SIZE, USED_RING_
 // Virtio-queue size in bytes
 const QUEUE_BYTES_SIZE: usize = (USED_RING_ADDR + USED_RING_SIZE - DESC_TABLE_ADDR) as usize;
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     if bytes.len() < (QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE)
         || bytes.len() > (QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE + MEM_SIZE)
     {
-        return;
+        return Corpus::Reject;
     }
 
     let mut rng = virtio_devices::Rng::new(
@@ -86,10 +86,10 @@ fuzz_target!(|bytes| {
         .write_slice(queue_bytes, GuestAddress(DESC_TABLE_ADDR))
         .is_err()
     {
-        return;
+        return Corpus::Reject;
     }
     if mem.write_slice(mem_bytes, GuestAddress(0 as u64)).is_err() {
-        return;
+        return Corpus::Reject;
     }
     let guest_memory = GuestMemoryAtomic::new(mem);
 
@@ -108,6 +108,8 @@ fuzz_target!(|bytes| {
 
     // Wait for the events to finish and rng device worker thread to return
     rng.wait_for_epoll_threads();
+
+    Corpus::Keep
 });
 
 pub struct NoopVirtioInterrupt {}