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/block.rs

@@ -17,7 +17,7 @@ use std::{ffi, io};
 
 use block::async_io::DiskFile;
 use block::raw_sync::RawFileDiskSync;
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use seccompiler::SeccompAction;
 use virtio_devices::{Block, VirtioDevice, VirtioInterrupt, VirtioInterruptType};
 use virtio_queue::{Queue, QueueT};
@@ -40,9 +40,9 @@ const AVAIL_RING_SIZE: u64 = 6_u64 + 2 * QUEUE_SIZE as u64;
 // Guest physical address for used ring (requires to 4-bytes aligned)
 const USED_RING_ADDR: u64 = (AVAIL_RING_ADDR + AVAIL_RING_SIZE + 3) & !3_u64;
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     if bytes.len() < QUEUE_DATA_SIZE || bytes.len() > (QUEUE_DATA_SIZE + MEM_SIZE) {
-        return;
+        return Corpus::Reject;
     }
 
     let queue_data = &bytes[..QUEUE_DATA_SIZE];
@@ -76,7 +76,7 @@ fuzz_target!(|bytes| {
     // Setup the guest memory with the input bytes
     let mem = GuestMemoryMmap::from_ranges(&[(GuestAddress(0), MEM_SIZE)]).unwrap();
     if mem.write_slice(mem_bytes, GuestAddress(0 as u64)).is_err() {
-        return;
+        return Corpus::Reject;
     }
     let guest_memory = GuestMemoryAtomic::new(mem);
 
@@ -96,6 +96,8 @@ fuzz_target!(|bytes| {
 
     // Wait for the events to finish and block device worker thread to return
     block.wait_for_epoll_threads();
+
+    Corpus::Keep
 });
 
 fn memfd_create(name: &ffi::CStr, flags: u32) -> Result<RawFd, io::Error> {