Secret memory with memfd_secret (#321)

Implements:
- An additional allocator to use memfd_secret(2) and guard pages using mmap(2), implemented in quininer/memsec#16
- An allocator that abstracts away underlying allocators, and uses specified allocator set by rosenpass_secret_memory::policy functions (or a function that sets rosenpass_secret_memory::alloc::ALLOC_INIT
- Updates to tests- integration, fuzz, bench: some tests use procspawn to spawn multiple processes with different allocator policies

secret-memory/src/policy.rs

@@ -0,0 +1,82 @@
+pub fn secret_policy_try_use_memfd_secrets() {
+    let alloc_type = {
+        #[cfg(target_os = "linux")]
+        {
+            if crate::alloc::memsec::memfdsec::memfdsec_box_try(0u8).is_ok() {
+                crate::alloc::SecretAllocType::MemsecMemfdSec
+            } else {
+                crate::alloc::SecretAllocType::MemsecMalloc
+            }
+        }
+
+        #[cfg(not(target_os = "linux"))]
+        {
+            crate::alloc::SecretAllocType::MemsecMalloc
+        }
+    };
+    assert_eq!(
+        alloc_type,
+        crate::alloc::get_or_init_secret_alloc_type(alloc_type)
+    );
+
+    log::info!("Secrets will be allocated using {:?}", alloc_type);
+}
+
+#[cfg(target_os = "linux")]
+pub fn secret_policy_use_only_memfd_secrets() {
+    let alloc_type = crate::alloc::SecretAllocType::MemsecMemfdSec;
+
+    assert_eq!(
+        alloc_type,
+        crate::alloc::get_or_init_secret_alloc_type(alloc_type)
+    );
+
+    log::info!("Secrets will be allocated using {:?}", alloc_type);
+}
+
+pub fn secret_policy_use_only_malloc_secrets() {
+    let alloc_type = crate::alloc::SecretAllocType::MemsecMalloc;
+    assert_eq!(
+        alloc_type,
+        crate::alloc::get_or_init_secret_alloc_type(alloc_type)
+    );
+
+    log::info!("Secrets will be allocated using {:?}", alloc_type);
+}
+
+pub mod test {
+    #[macro_export]
+    macro_rules! test_spawn_process_with_policies {
+        ($body:block, $($f: expr),*) => {
+            $(
+                let handle = procspawn::spawn((), |_| {
+
+                $f();
+
+                $body
+
+                });
+                handle.join().unwrap();
+            )*
+            };
+        }
+
+    #[macro_export]
+    macro_rules! test_spawn_process_provided_policies {
+        ($body: block) => {
+            $crate::test_spawn_process_with_policies!(
+                $body,
+                $crate::policy::secret_policy_try_use_memfd_secrets,
+                $crate::secret_policy_use_only_malloc_secrets
+            );
+
+            #[cfg(target_os = "linux")]
+            {
+                $crate::test_spawn_process_with_policies!(
+                    $body,
+                    $crate::policy::secret_policy_use_only_memfd_secrets
+                );
+            }
+        };
+    }
+}