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/qcow.rs

@@ -12,14 +12,14 @@ use std::mem::size_of;
 use std::os::unix::io::{FromRawFd, RawFd};
 
 use block::qcow::{QcowFile, RawFile};
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 
 // Take the first 64 bits of data as an address and the next 64 bits as data to
 // store there. The rest of the data is used as a qcow image.
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     if bytes.len() < 16 {
         // Need an address and data, each are 8 bytes.
-        return;
+        return Corpus::Reject;
     }
     let mut disk_image = Cursor::new(bytes);
     let addr = read_u64(&mut disk_image);
@@ -33,6 +33,8 @@ fuzz_target!(|bytes| {
             let _ = qcow.write_all(&value.to_le_bytes());
         }
     }
+
+    Corpus::Keep
 });
 
 fn read_u64<T: Read>(readable: &mut T) -> u64 {