Enable the uffd feature by default, but filter on OS

Notably, this should get us building and running uffd in Linux CI.

It turns out to be a tremendous pain to enable a feature flag for just one crate within a
workspace. The situation is [being addressed][1], but in the meantime I believe the best route
forward is to just have uffd on by default for Linux.

[1]: rust-lang/cargo#5364

lucet-runtime/Cargo.toml

@@ -49,7 +49,7 @@ assets = [
 ]
 
 [features]
-default-features = []
+default = ["uffd"]
 uffd = ["lucet-runtime-internals/uffd"]
 
 [package.metadata.docs.rs]

lucet-runtime/lucet-runtime-internals/Cargo.toml

@@ -27,6 +27,8 @@ num-traits = "0.2"
 raw-cpuid = "6.0.0"
 thiserror = "1.0.4"
 tracing = "0.1.12"
+
+[target.'cfg(target_os = "linux")'.dependencies]
 userfaultfd = { version = "0.1.0", optional = true }
 
 # This is only a dependency to ensure that other crates don't pick a newer version as a transitive
@@ -43,7 +45,7 @@ byteorder = "1.2"
 cc = "1.0"
 
 [features]
-default-features = []
+default = ["uffd"]
 uffd = ["userfaultfd"]
 
 [package.metadata.docs.rs]

lucet-runtime/lucet-runtime-internals/src/region.rs

@@ -1,6 +1,6 @@
 pub mod mmap;
 
-#[cfg(feature = "uffd")]
+#[cfg(all(target_os = "linux", feature = "uffd"))]
 pub mod uffd;
 
 use crate::alloc::{Alloc, Limits, Slot};

lucet-runtime/src/c_api.rs

@@ -1,4 +1,4 @@
-#[cfg(feature = "uffd")]
+#[cfg(all(target_os = "linux", feature = "uffd"))]
 use crate::UffdRegion;
 use crate::{DlModule, Instance, Limits, MmapRegion, Module, Region};
 use libc::{c_char, c_int, c_void};
@@ -103,7 +103,7 @@ pub unsafe extern "C" fn lucet_uffd_region_create(
     region_out: *mut *mut lucet_region,
 ) -> lucet_error {
     cfg_if::cfg_if! {
-        if #[cfg(feature = "uffd")] {
+        if #[cfg(all(target_os = "linux", feature = "uffd"))] {
             assert_nonnull!(region_out);
             let limits = limits
                 .as_ref()

lucet-runtime/src/lib.rs

@@ -386,7 +386,14 @@
 //! `Instance`, reducing startup time. Instance stack pages can also be lazily initialized, reducing
 //! the memory footprint of instances that only use a small portion of their available stack space.
 //!
-//! To use `UffdRegion`, enable the `uffd` Cargo feature, which is off by default.
+//! `UffdRegion` is enabled by default on Linux platforms, but can be disabled by disabling default
+//! features for this crate and `lucet-runtime-internals`:
+//!
+//! ```toml
+//! [dependencies]
+//! lucet-runtime = { version = "0.6.1", default-features = false }
+//! lucet-runtime-internals = { version = "0.6.1", default-features = false }
+//! ```
 
 #![deny(bare_trait_objects)]
 
@@ -410,7 +417,7 @@ pub use lucet_runtime_internals::instance::{
 pub use lucet_runtime_internals::lucet_hostcalls;
 pub use lucet_runtime_internals::module::{DlModule, Module};
 pub use lucet_runtime_internals::region::mmap::MmapRegion;
-#[cfg(feature = "uffd")]
+#[cfg(all(target_os = "linux", feature = "uffd"))]
 pub use lucet_runtime_internals::region::uffd::UffdRegion;
 pub use lucet_runtime_internals::region::{InstanceBuilder, Region, RegionCreate};
 pub use lucet_runtime_internals::val::{UntypedRetVal, Val};

lucet-runtime/tests/entrypoint.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::entrypoint_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         entrypoint_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/globals.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::globals_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         globals_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/guest_fault.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::guest_fault_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         guest_fault_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/host.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::host_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         host_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/memory.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::memory_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         memory_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/stack.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::stack_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         stack_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/start.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::start_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         start_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/strcmp.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::strcmp_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         strcmp_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion

lucet-runtime/tests/timeout.rs

@@ -1,7 +1,7 @@
 use lucet_runtime_tests::timeout_tests;
 
 cfg_if::cfg_if! {
-    if #[cfg(feature = "uffd")] {
+    if #[cfg(all(target_os = "linux", feature = "uffd"))] {
         timeout_tests!(
             mmap => lucet_runtime::MmapRegion,
             uffd => lucet_runtime::UffdRegion