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/mem.rs

@@ -7,7 +7,7 @@
 use std::os::unix::io::{AsRawFd, FromRawFd};
 use std::sync::{Arc, Mutex};
 
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use seccompiler::SeccompAction;
 use virtio_devices::{BlocksState, Mem, VirtioDevice, VirtioInterrupt, VirtioInterruptType};
 use virtio_queue::{Queue, QueueT};
@@ -57,11 +57,11 @@ 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() < VIRTIO_MEM_DATA_SIZE + QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE
         || bytes.len() > (VIRTIO_MEM_DATA_SIZE + QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE + MEM_SIZE)
     {
-        return;
+        return Corpus::Reject;
     }
 
     let virtio_mem_data = &bytes[..VIRTIO_MEM_DATA_SIZE];
@@ -86,7 +86,7 @@ fuzz_target!(|bytes| {
         .write_slice(queue_bytes, GuestAddress(DESC_TABLE_ADDR))
         .is_err()
     {
-        return;
+        return Corpus::Reject;
     }
     // Add the memory region for the virtio-mem device
     let mem = mem.insert_region(virtio_mem_region).unwrap();
@@ -94,7 +94,7 @@ fuzz_target!(|bytes| {
         .write_slice(mem_bytes, GuestAddress(VIRTIO_MEM_REGION_ADDRESS))
         .is_err()
     {
-        return;
+        return Corpus::Reject;
     }
     let guest_memory = GuestMemoryAtomic::new(mem);
 
@@ -114,6 +114,8 @@ fuzz_target!(|bytes| {
 
     // Wait for the events to finish and virtio-mem device worker thread to return
     virtio_mem.wait_for_epoll_threads();
+
+    return Corpus::Keep;
 });
 
 pub struct NoopVirtioInterrupt {}