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/cmos.rs

@@ -5,14 +5,14 @@
 #![no_main]
 use devices::legacy::Cmos;
 use libc::EFD_NONBLOCK;
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use vm_device::BusDevice;
 use vmm_sys_util::eventfd::EventFd;
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     // Need at least 16 bytes for the test
     if bytes.len() < 16 {
-        return;
+        return Corpus::Reject;
     }
 
     let mut below_4g = [0u8; 8];
@@ -46,4 +46,6 @@ fuzz_target!(|bytes| {
             cmos.write(0, offset, &data);
         }
     }
+
+    Corpus::Keep
 });