rust: kernel: Add simple siphash abstraction

This allows Rust code to use the Hasher interface with the kernel
siphash implementation.

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

rust/bindings/bindings_helper.h

@@ -31,6 +31,7 @@
 #include <linux/poll.h>
 #include <linux/property.h>
 #include <linux/refcount.h>
+#include <linux/siphash.h>
 #include <linux/sched.h>
 #include <linux/security.h>
 #include <linux/slab.h>

rust/helpers/helpers.c

@@ -29,6 +29,7 @@
 #include "refcount.c"
 #include "security.c"
 #include "signal.c"
+#include "siphash.c"
 #include "slab.c"
 #include "spinlock.c"
 #include "task.c"

rust/helpers/siphash.c

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/siphash.h>
+
+u64 rust_helper_siphash(const void *data, size_t len,
+			const siphash_key_t *key)
+{
+	    return siphash(data, len, key);
+}

rust/kernel/lib.rs

@@ -77,6 +77,7 @@ pub mod rbtree;
 pub mod revocable;
 pub mod security;
 pub mod seq_file;
+pub mod siphash;
 pub mod sizes;
 mod static_assert;
 #[doc(hidden)]

rust/kernel/siphash.rs

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A core::hash::Hasher wrapper for the kernel siphash implementation.
+//!
+//! This module allows Rust code to use the kernel's siphash implementation
+//! to hash Rust objects.
+
+use core::hash::Hasher;
+
+/// A Hasher implementation that uses the kernel siphash implementation.
+#[derive(Default)]
+pub struct SipHasher {
+    // SipHash state is 4xu64, but the Linux implementation
+    // doesn't expose incremental hashing so let's just chain
+    // individual SipHash calls for now, which return a u64
+    // hash.
+    state: u64,
+}
+
+impl SipHasher {
+    /// Create a new SipHasher with zeroed state.
+    pub fn new() -> Self {
+        SipHasher { state: 0 }
+    }
+}
+
+impl Hasher for SipHasher {
+    fn finish(&self) -> u64 {
+        self.state
+    }
+
+    fn write(&mut self, bytes: &[u8]) {
+        let key = bindings::siphash_key_t {
+            key: [self.state, 0],
+        };
+
+        // SAFETY: Safe to call on a valid slice
+        self.state = unsafe { bindings::siphash(bytes.as_ptr() as *const _, bytes.len(), &key) };
+    }
+}