Add disable_urandom_fallback crate feature

CHANGELOG.md

@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+### Added
+- `disable_urandom_fallback` crate feature to disable `/dev/urandom`-based fallback on Linux targets.
+  Enabling this feature bumps minimum supported Linux kernel version to 4.17. [#396]
+
 ### Changed
 - Disable `/dev/urandom`-based fallback for all Android targets and for Linux targets
   outside of the following `target_arch`es: `x86`, `x86_64`, `arm`, `armv7`, `i586`, `i686`,

Cargo.toml

@@ -32,6 +32,8 @@ wasm-bindgen-test = "0.3.18"
 [features]
 # Implement std-only traits for getrandom::Error
 std = []
+# Disable `/dev/urandom` fallback for Linux targets (requires 4.17+ kernel)
+disable_urandom_fallback = []
 # Feature to enable fallback RDRAND-based implementation on x86/x86_64
 rdrand = []
 # Feature to enable JavaScript bindings on wasm*-unknown-unknown

src/lib.rs

@@ -220,17 +220,21 @@ cfg_if! {
     if #[cfg(any(target_os = "haiku", target_os = "redox", target_os = "nto", target_os = "aix"))] {
         mod util_libc;
         #[path = "use_file.rs"] mod imp;
-    } else if #[cfg(all(target_os = "linux", any(
-        target_arch = "i586",
-        target_arch = "i686",
-        target_arch = "x86",
-        target_arch = "x86_64",
-        target_arch = "arm",
-        target_arch = "armv7",
-        target_arch = "powerpc",
-        target_arch = "powerpc64le",
-        target_arch = "s390x",
-    )))] {
+    } else if #[cfg(all(
+        not(feature = "disable_urandom_fallback"),
+        target_os = "linux",
+        any(
+            target_arch = "i586",
+            target_arch = "i686",
+            target_arch = "x86",
+            target_arch = "x86_64",
+            target_arch = "arm",
+            target_arch = "armv7",
+            target_arch = "powerpc",
+            target_arch = "powerpc64le",
+            target_arch = "s390x",
+        ),
+    ))] {
         mod util_libc;
         mod use_file;
         mod lazy;