[PATCH 3/9] rust: random: add a safe get_random_bytes wrapper

[Mike Lothian <mike@fireburn.co.uk>]

Expose a slice-based wrapper around get_random_bytes() so Rust callers
can fill key, nonce, and other random buffers without touching raw
pointers.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
---
 rust/kernel/lib.rs    |  1 +
 rust/kernel/random.rs | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 rust/kernel/random.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 08e5730753eb..f39648246271 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -116,6 +116,7 @@
 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 000000000000..5f2288969dbf
--- /dev/null
+++ b/rust/kernel/random.rs
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Random number generation.
+//!
+//! C header: [`include/linux/random.h`](srctree/include/linux/random.h)
+
+use crate::bindings;
+
+/// Fills `buf` with cryptographically secure random bytes from the kernel's CSPRNG.
+///
+/// This is the in-kernel equivalent of reading from `/dev/urandom`, and is suitable for generating
+/// keys, nonces and other secrets. It never blocks: once the CSPRNG has been seeded during boot it
+/// stays seeded, and callers running that early should use
+/// [`wait_for_random_bytes()`] instead of assuming otherwise.
+///
+/// [`wait_for_random_bytes()`]: srctree/include/linux/random.h
+///
+/// # Examples
+///
+/// ```
+/// use kernel::random;
+///
+/// let mut key = [0u8; 16];
+/// random::fill_bytes(&mut key);
+///
+/// // A zero-length request is valid and does nothing.
+/// random::fill_bytes(&mut []);
+/// ```
+#[inline]
+pub fn fill_bytes(buf: &mut [u8]) {
+    // SAFETY: `buf` is a valid slice, so its pointer is valid for writes of `buf.len()` bytes, and
+    // `get_random_bytes()` writes exactly that many.
+    unsafe { bindings::get_random_bytes(buf.as_mut_ptr().cast(), buf.len()) };
+}