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/http_api.rs

@@ -8,7 +8,7 @@ use std::path::PathBuf;
 use std::sync::mpsc::{channel, Receiver};
 use std::thread;
 
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use micro_http::Request;
 use once_cell::sync::Lazy;
 use vm_migration::MigratableError;
@@ -27,9 +27,9 @@ use vmm_sys_util::eventfd::EventFd;
 static ROUTES: Lazy<Vec<&Box<dyn EndpointHandler + Sync + Send>>> =
     Lazy::new(|| HTTP_ROUTES.routes.values().collect());
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     if bytes.len() < 2 {
-        return;
+        return Corpus::Reject;
     }
 
     let route = ROUTES[bytes[0] as usize % ROUTES.len()];
@@ -53,6 +53,8 @@ fuzz_target!(|bytes| {
         exit_evt.write(1).ok();
         http_receiver_thread.join().unwrap();
     };
+
+    Corpus::Keep
 });
 
 fn generate_request(bytes: &[u8]) -> Option<Request> {