[bssl] Add no_std compatible BoringSSL wrapper library for AVF

Bug: 301068421
Test: atest rialto_test
Change-Id: I8af77d457f7a956b0bc88ba4a0498483651426b0
diff --git a/libs/bssl/src/cbb.rs b/libs/bssl/src/cbb.rs
new file mode 100644
index 0000000..9b5f7fe
--- /dev/null
+++ b/libs/bssl/src/cbb.rs
@@ -0,0 +1,53 @@
+// Copyright 2023, The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Helpers for using BoringSSL CBB (crypto byte builder) objects.
+
+use bssl_ffi::{CBB_init_fixed, CBB};
+use core::marker::PhantomData;
+use core::mem::MaybeUninit;
+
+/// Wraps a CBB that references a existing fixed-sized buffer; no memory is allocated, but the
+/// buffer cannot grow.
+pub struct CbbFixed<'a> {
+    cbb: CBB,
+    /// The CBB contains a mutable reference to the buffer, disguised as a pointer.
+    /// Make sure the borrow checker knows that.
+    _buffer: PhantomData<&'a mut [u8]>,
+}
+
+impl<'a> CbbFixed<'a> {
+    /// Create a new CBB that writes to the given buffer.
+    pub fn new(buffer: &'a mut [u8]) -> Self {
+        let mut cbb = MaybeUninit::uninit();
+        // SAFETY: `CBB_init_fixed()` is infallible and always returns one.
+        // The buffer remains valid during the lifetime of `cbb`.
+        unsafe { CBB_init_fixed(cbb.as_mut_ptr(), buffer.as_mut_ptr(), buffer.len()) };
+        // SAFETY: `cbb` has just been initialized by `CBB_init_fixed()`.
+        let cbb = unsafe { cbb.assume_init() };
+        Self { cbb, _buffer: PhantomData }
+    }
+}
+
+impl<'a> AsRef<CBB> for CbbFixed<'a> {
+    fn as_ref(&self) -> &CBB {
+        &self.cbb
+    }
+}
+
+impl<'a> AsMut<CBB> for CbbFixed<'a> {
+    fn as_mut(&mut self) -> &mut CBB {
+        &mut self.cbb
+    }
+}
diff --git a/libs/bssl/src/lib.rs b/libs/bssl/src/lib.rs
new file mode 100644
index 0000000..a4e00f0
--- /dev/null
+++ b/libs/bssl/src/lib.rs
@@ -0,0 +1,22 @@
+// Copyright 2023, The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Safe wrappers around the BoringSSL API.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+mod cbb;
+
+pub use bssl_avf_error::{ApiName, Error, Result};
+pub use cbb::CbbFixed;