[PATCH v4] rust: Add support for feeding entropy to randomness pool

[Matthew Maurer <mmaurer@google.com>]

Adds just enough support to allow device drivers to feed entropy to the
central pool.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
Changes in v4:
- Adjusted documentation phrasing.
- Renamed module from `rand` to `random` to be more in line with the
  header name.
- Link to v3: https://lore.kernel.org/r/20251226-add-entropy-v3-1-a3440823a07d@google.com

Changes in v3:
- Fixed doclink to be srctree-based instead of relative.
- Switched to prelude import instead of fine-grained imports.
- Link to v2: https://lore.kernel.org/r/20251216-add-entropy-v2-1-4d866f251474@google.com

Changes in v2:
- Added more details in the docs about the API, specifically about it
  not crediting entropy and when it ought to be used.
- Link to v1: https://lore.kernel.org/r/20251212-add-entropy-v1-1-e70ad1bc9c65@google.com
---
 rust/kernel/lib.rs    |  1 +
 rust/kernel/random.rs | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f812cf12004286962985a068665443dc22c389a2..c82e6747043e2472d6b5e4d9043fa8bc6d3c8434 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -130,6 +130,7 @@
 pub mod ptr;
 #[cfg(CONFIG_RUST_PWM_ABSTRACTIONS)]
 pub mod pwm;
+pub mod random;
 pub mod rbtree;
 pub mod regulator;
 pub mod revocable;
diff --git a/rust/kernel/random.rs b/rust/kernel/random.rs
new file mode 100644
index 0000000000000000000000000000000000000000..a9b19698e09665696fef77b4486c4a1a20d0a42e
--- /dev/null
+++ b/rust/kernel/random.rs
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Randomness.
+//!
+//! C header: [`include/linux/random.h`](srctree/include/linux/random.h)
+
+use crate::prelude::*;
+
+/// Adds the given buffer to the entropy pool, but does not credit any entropy.
+///
+/// This function mixes in data that is likely to differ between devices or boots, but may
+/// otherwise be predictable. Examples include MAC addresses or RTC values. This slightly improves
+/// randomness in entropy-constrained environments (especially common for embedded devices).
+pub fn add_device_randomness(buf: &[u8]) {
+    // SAFETY: We just need the pointer to be valid for the length, which a slice provides.
+    unsafe { bindings::add_device_randomness(buf.as_ptr().cast::<c_void>(), buf.len()) };
+}

---
base-commit: 008d3547aae5bc86fac3eda317489169c3fda112
change-id: 20251029-add-entropy-f57e12ebe110

Best regards,
-- 
Matthew Maurer <mmaurer@google.com>