From d6fadf5ac1967469d38e16105d37a1dc52e28610 Mon Sep 17 00:00:00 2001 From: joboet Date: Fri, 28 Jun 2024 10:44:26 +0200 Subject: [PATCH] std: add safety comments --- std/src/sys/thread_local/os.rs | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/std/src/sys/thread_local/os.rs b/std/src/sys/thread_local/os.rs index 625943bb25512..e06185f00690b 100644 --- a/std/src/sys/thread_local/os.rs +++ b/std/src/sys/thread_local/os.rs @@ -50,6 +50,7 @@ unsafe impl Sync for Storage {} struct Value { value: T, + // INVARIANT: if this value is stored under a TLS key, `key` must be that `key`. key: Key, } @@ -73,10 +74,14 @@ impl Storage { // is not running) + it is coming from a trusted source (self). unsafe { &(*ptr).value } } else { + // SAFETY: trivially correct. unsafe { Self::try_initialize(key, ptr, i, f) } } } + /// # Safety + /// * `key` must be the result of calling `self.key.force()` + /// * `ptr` must be the current value associated with `key`. unsafe fn try_initialize( key: Key, ptr: *mut Value, @@ -91,11 +96,16 @@ impl Storage { let value = Box::new(Value { value: i.and_then(Option::take).unwrap_or_else(f), key }); let ptr = Box::into_raw(value); - let old = unsafe { get(key) as *mut Value }; - // SAFETY: `ptr` is a correct pointer that can be destroyed by the key destructor. - unsafe { + // SAFETY: + // * key came from a `LazyKey` and is thus correct. + // * `ptr` is a correct pointer that can be destroyed by the key destructor. + // * the value is stored under the key that it contains. + let old = unsafe { + let old = get(key) as *mut Value; set(key, ptr as *mut u8); - } + old + }; + if !old.is_null() { // If the variable was recursively initialized, drop the old value. // SAFETY: We cannot be inside a `LocalKey::with` scope, as the @@ -123,8 +133,10 @@ unsafe extern "C" fn destroy_value(ptr: *mut u8) { abort_on_dtor_unwind(|| { let ptr = unsafe { Box::from_raw(ptr as *mut Value) }; let key = ptr.key; + // SAFETY: `key` is the TLS key `ptr` was stored under. unsafe { set(key, ptr::without_provenance_mut(1)) }; drop(ptr); + // SAFETY: `key` is the TLS key `ptr` was stored under. unsafe { set(key, ptr::null_mut()) }; }); }