[PATCH RFC 04/11] rust: siphash: Add a simple siphash abstraction

[Asahi Lina]

This simple wrapper allows Rust code to use the Hasher interface with
the kernel siphash implementation. No fancy features supported for now,
just basic bag-of-bytes hashing. No guarantee that hash outputs will
remain stable in the future either.

Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/helpers.c                  |  8 ++++++++
 rust/kernel/lib.rs              |  1 +
 rust/kernel/siphash.rs          | 39 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 49 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 3e601ce2548d..52f32e423b04 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -10,6 +10,7 @@
 #include <linux/slab.h>
 #include <linux/refcount.h>
 #include <linux/wait.h>
+#include <linux/siphash.h>
 #include <linux/sched.h>
 
 /* `bindgen` gets confused at certain things. */
diff --git a/rust/helpers.c b/rust/helpers.c
index bb594da56137..1ed71315d1eb 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -24,6 +24,7 @@
 #include <linux/errname.h>
 #include <linux/refcount.h>
 #include <linux/mutex.h>
+#include <linux/siphash.h>
 #include <linux/spinlock.h>
 #include <linux/sched/signal.h>
 #include <linux/wait.h>
@@ -135,6 +136,13 @@ void rust_helper_put_task_struct(struct task_struct *t)
 }
 EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
 
+u64 rust_helper_siphash(const void *data, size_t len,
+			const siphash_key_t *key)
+{
+	return siphash(data, len, key);
+}
+EXPORT_SYMBOL_GPL(rust_helper_siphash);
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 85b261209977..8fb39078b85c 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -36,6 +36,7 @@
 pub mod ioctl;
 pub mod prelude;
 pub mod print;
+pub mod siphash;
 mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
diff --git a/rust/kernel/siphash.rs b/rust/kernel/siphash.rs
new file mode 100644
index 000000000000..e13a17cd5a93
--- /dev/null
+++ b/rust/kernel/siphash.rs
@@ -0,0 +1,39 @@
+// 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],
+        };
+
+        self.state = unsafe { bindings::siphash(bytes.as_ptr() as *const _, bytes.len(), &key) };
+    }
+}

-- 
2.40.1