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/net.rs

@@ -9,7 +9,7 @@ use std::io::{Read, Write};
 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};
@@ -55,12 +55,12 @@ const QUEUE_BYTES_SIZE: usize = align!(
     DESC_TABLE_ALIGN_SIZE
 ) as usize;
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     if bytes.len() < TAP_INPUT_SIZE + (QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE) * QUEUE_NUM
         || bytes.len()
             > TAP_INPUT_SIZE + (QUEUE_DATA_SIZE + QUEUE_BYTES_SIZE) * QUEUE_NUM + MEM_SIZE
     {
-        return;
+        return Corpus::Reject;
     }
 
     let (dummy_tap_frontend, dummy_tap_backend) = create_socketpair().unwrap();
@@ -111,10 +111,10 @@ fuzz_target!(|bytes| {
         .write_slice(queue_bytes, GuestAddress(BASE_VIRT_QUEUE_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);
 
@@ -155,6 +155,8 @@ fuzz_target!(|bytes| {
     // Terminate the thread for the dummy tap backend
     exit_evt.write(1).ok();
     tap_backend_thread.join().unwrap();
+
+    return Corpus::Keep;
 });
 
 pub struct NoopVirtioInterrupt {}