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/linux_loader_cmdline.rs

@@ -8,7 +8,7 @@
 
 #![no_main]
 
-use libfuzzer_sys::fuzz_target;
+use libfuzzer_sys::{fuzz_target, Corpus};
 use vm_memory::bitmap::AtomicBitmap;
 use vm_memory::GuestAddress;
 
@@ -18,7 +18,7 @@ const MEM_SIZE: usize = 256 * 1024 * 1024;
 // From 'arch::x86_64::layout::CMDLINE_START'
 const CMDLINE_START: GuestAddress = GuestAddress(0x20000);
 
-fuzz_target!(|bytes| {
+fuzz_target!(|bytes: &[u8]| -> Corpus {
     let payload_config = vmm::vm_config::PayloadConfig {
         firmware: None,
         kernel: None,
@@ -29,9 +29,11 @@ fuzz_target!(|bytes| {
     };
     let kernel_cmdline = match vmm::vm::Vm::generate_cmdline(&payload_config) {
         Ok(cmdline) => cmdline,
-        _ => return,
+        _ => return Corpus::Reject,
     };
     let guest_memory = GuestMemoryMmap::from_ranges(&[(GuestAddress(0), MEM_SIZE)]).unwrap();
 
     linux_loader::loader::load_cmdline(&guest_memory, CMDLINE_START, &kernel_cmdline).ok();
+
+    Corpus::Keep
 });