fix(bindings): make Context borrow immutable

bindings/rust/extended/s2n-tls/src/callbacks.rs

@@ -48,12 +48,12 @@ pub use pkey::*;
 /// callbacks were configured through the Rust bindings.
 pub(crate) unsafe fn with_context<F, T>(conn_ptr: *mut s2n_connection, action: F) -> T
 where
-    F: FnOnce(&mut Connection, &mut Context) -> T,
+    F: FnOnce(&mut Connection, &Context) -> T,
 {
     let raw = NonNull::new(conn_ptr).expect("connection should not be null");
     let mut conn = Connection::from_raw(raw);
-    let mut config = conn.config().expect("config should not be null");
-    let context = config.context_mut();
+    let config = conn.config().expect("config should not be null");
+    let context = config.context();
     let r = action(&mut conn, context);
     // Since this is a callback, it receives a pointer to the connection
     // but doesn't own that connection or control its lifecycle.

bindings/rust/extended/s2n-tls/src/config.rs

@@ -93,14 +93,16 @@ impl Config {
     /// Retrieve a mutable reference to the [`Context`] stored on the config.
     ///
     /// Corresponds to [s2n_config_get_ctx].
-    pub(crate) fn context_mut(&mut self) -> &mut Context {
+    ///
+    /// SAFETY: There must only ever by mutable reference to `Context` alive at
+    ///         any time. Configs can be shared across threads, so this method is
+    ///         likely not correct for your usecase.
+    unsafe fn context_mut(&mut self) -> &mut Context {
         let mut ctx = core::ptr::null_mut();
-        unsafe {
-            s2n_config_get_ctx(self.as_mut_ptr(), &mut ctx)
-                .into_result()
-                .unwrap();
-            &mut *(ctx as *mut Context)
-        }
+        s2n_config_get_ctx(self.as_mut_ptr(), &mut ctx)
+            .into_result()
+            .unwrap();
+        &mut *(ctx as *mut Context)
     }
 
     #[cfg(test)]
@@ -135,7 +137,7 @@ impl Clone for Config {
 impl Drop for Config {
     /// Corresponds to [s2n_config_free].
     fn drop(&mut self) {
-        let context = self.context_mut();
+        let context = self.context();
         let count = context.refcount.fetch_sub(1, Ordering::Release);
         debug_assert!(count > 0, "refcount should not drop below 1 instance");
 
@@ -158,13 +160,15 @@ impl Drop for Config {
         // https://github.com/rust-lang/rust/blob/e012a191d768adeda1ee36a99ef8b92d51920154/library/alloc/src/sync.rs#L1637
         std::sync::atomic::fence(Ordering::Acquire);
 
-        unsafe {
-            // This is the last instance so free the context.
-            let context = Box::from_raw(context);
-            drop(context);
+        // This is the last instance so free the context.
+        let context = unsafe {
+            // SAFETY: The reference count is verified to be 1, so this is the
+            // last instance of the config, and the only reference to the context.
+            Box::from_raw(self.context_mut())
+        };
+        drop(context);
 
-            let _ = s2n_config_free(self.0.as_ptr()).into_result();
-        }
+        let _ = unsafe { s2n_config_free(self.0.as_ptr()).into_result() };
     }
 }
 
@@ -334,7 +338,12 @@ impl Builder {
             )
             .into_result()
         };
-        self.context_mut().application_owned_certs.push(chain);
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because the
+            // Builder owns the only reference to the config.
+            self.config.context_mut()
+        };
+        context.application_owned_certs.push(chain);
         result?;
 
         Ok(self)
@@ -373,9 +382,12 @@ impl Builder {
 
         let collected_chains = chain_arrays.into_iter().take(cert_chain_count).flatten();
 
-        self.context_mut()
-            .application_owned_certs
-            .extend(collected_chains);
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because the
+            // Builder owns the only reference to the config.
+            self.config.context_mut()
+        };
+        context.application_owned_certs.extend(collected_chains);
 
         unsafe {
             s2n_config_set_cert_chain_and_key_defaults(
@@ -547,12 +559,17 @@ impl Builder {
             verify_host(host_name, host_name_len, handler)
         }
 
-        self.context_mut().verify_host_callback = Some(Box::new(handler));
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because the
+            // Builder owns the only reference to the config.
+            self.config.context_mut()
+        };
+        context.verify_host_callback = Some(Box::new(handler));
         unsafe {
             s2n_config_set_verify_host_callback(
                 self.as_mut_ptr(),
                 Some(verify_host_cb_fn),
-                self.context_mut() as *mut Context as *mut c_void,
+                self.config.context() as *const Context as *mut c_void,
             )
             .into_result()?;
         }
@@ -607,7 +624,11 @@ impl Builder {
         }
 
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.client_hello_callback = Some(handler);
 
         unsafe {
@@ -628,7 +649,11 @@ impl Builder {
     ) -> Result<&mut Self, Error> {
         // Store callback in config context
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.connection_initializer = Some(handler);
         Ok(self)
     }
@@ -659,14 +684,18 @@ impl Builder {
 
         // Store callback in context
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.session_ticket_callback = Some(handler);
 
         unsafe {
             s2n_config_set_session_ticket_cb(
                 self.as_mut_ptr(),
                 Some(session_ticket_cb),
-                self.context_mut() as *mut Context as *mut c_void,
+                self.config.context() as *const Context as *mut c_void,
             )
             .into_result()
         }?;
@@ -698,7 +727,11 @@ impl Builder {
         }
 
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.private_key_callback = Some(handler);
 
         unsafe {
@@ -734,13 +767,17 @@ impl Builder {
         }
 
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.wall_clock = Some(handler);
         unsafe {
             s2n_config_set_wall_clock(
                 self.as_mut_ptr(),
                 Some(clock_cb),
-                self.context_mut() as *mut _ as *mut c_void,
+                self.config.context() as *const _ as *mut c_void,
             )
             .into_result()?;
         }
@@ -773,13 +810,17 @@ impl Builder {
         }
 
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.monotonic_clock = Some(handler);
         unsafe {
             s2n_config_set_monotonic_clock(
                 self.as_mut_ptr(),
                 Some(clock_cb),
-                self.context_mut() as *mut _ as *mut c_void,
+                self.config.context() as *const _ as *mut c_void,
             )
             .into_result()?;
         }
@@ -913,17 +954,6 @@ impl Builder {
     pub(crate) fn as_mut_ptr(&mut self) -> *mut s2n_config {
         self.config.as_mut_ptr()
     }
-
-    /// Retrieve a mutable reference to the [`Context`] stored on the config.
-    pub(crate) fn context_mut(&mut self) -> &mut Context {
-        let mut ctx = core::ptr::null_mut();
-        unsafe {
-            s2n_config_get_ctx(self.as_mut_ptr(), &mut ctx)
-                .into_result()
-                .unwrap();
-            &mut *(ctx as *mut Context)
-        }
-    }
 }
 
 #[cfg(feature = "quic")]

bindings/rust/extended/s2n-tls/src/renegotiate.rs

@@ -41,7 +41,7 @@
 //!
 //! impl RenegotiateCallback for Callback {
 //!     fn on_renegotiate_request(
-//!         &mut self,
+//!         &self,
 //!         conn: &mut Connection,
 //!     ) -> Option<RenegotiateResponse> {
 //!         let response = match conn.server_name() {
@@ -51,7 +51,7 @@
 //!         Some(response)
 //!     }
 //!
-//!     fn on_renegotiate_wipe(&mut self, conn: &mut Connection) -> Result<(), Error> {
+//!     fn on_renegotiate_wipe(&self, conn: &mut Connection) -> Result<(), Error> {
 //!         conn.set_application_protocol_preference(Some("http"))?;
 //!         Ok(())
 //!     }
@@ -155,23 +155,20 @@ pub trait RenegotiateCallback: 'static + Send + Sync {
     //
     // This method returns Option instead of Result because the callback has no mechanism
     // for surfacing errors to the application, so Result would be somewhat deceptive.
-    fn on_renegotiate_request(
-        &mut self,
-        connection: &mut Connection,
-    ) -> Option<RenegotiateResponse>;
+    fn on_renegotiate_request(&self, connection: &mut Connection) -> Option<RenegotiateResponse>;
 
     /// A callback that triggers after the connection is wiped for renegotiation.
     ///
     /// Because renegotiation requires wiping the connection, connection-level
     /// configuration will need to be set again via this callback.
     /// See [`Connection::wipe_for_renegotiate()`] for more information.
-    fn on_renegotiate_wipe(&mut self, _connection: &mut Connection) -> Result<(), Error> {
+    fn on_renegotiate_wipe(&self, _connection: &mut Connection) -> Result<(), Error> {
         Ok(())
     }
 }
 
 impl RenegotiateCallback for RenegotiateResponse {
-    fn on_renegotiate_request(&mut self, _conn: &mut Connection) -> Option<RenegotiateResponse> {
+    fn on_renegotiate_request(&self, _conn: &mut Connection) -> Option<RenegotiateResponse> {
         Some(*self)
     }
 }
@@ -390,7 +387,7 @@ impl config::Builder {
             response: *mut s2n_renegotiate_response::Type,
         ) -> libc::c_int {
             with_context(conn_ptr, |conn, context| {
-                let callback = context.renegotiate.as_mut();
+                let callback = context.renegotiate.as_ref();
                 if let Some(callback) = callback {
                     if let Some(result) = callback.on_renegotiate_request(conn) {
                         // If the callback indicates renegotiation, schedule it.
@@ -408,7 +405,11 @@ impl config::Builder {
         }
 
         let handler = Box::new(handler);
-        let context = self.context_mut();
+        let context = unsafe {
+            // SAFETY: usage of context_mut is safe in the builder, because while
+            // it is being built, the Builder is the only reference to the config.
+            self.config.context_mut()
+        };
         context.renegotiate = Some(handler);
         unsafe {
             s2n_config_set_renegotiate_request_cb(
@@ -717,10 +718,7 @@ mod tests {
     fn error_callback() -> Result<(), Box<dyn Error>> {
         struct ErrorRenegotiateCallback {}
         impl RenegotiateCallback for ErrorRenegotiateCallback {
-            fn on_renegotiate_request(
-                &mut self,
-                _: &mut Connection,
-            ) -> Option<RenegotiateResponse> {
+            fn on_renegotiate_request(&self, _: &mut Connection) -> Option<RenegotiateResponse> {
                 None
             }
         }