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Auto merge of #74225 - poliorcetics:std-thread-unsafe-op-in-unsafe-fn, r=joshtriplett

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Std/thread: deny unsafe op in unsafe fn

Partial fix of #73904.

This encloses `unsafe` operations in `unsafe fn` in `libstd/thread`.
`@rustbot` modify labels: F-unsafe-block-in-unsafe-fn
This commit is contained in:
bors 2020-09-26 03:54:00 +00:00
commit 9e1c436178
2 changed files with 126 additions and 43 deletions
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141
library/std/src/thread/local.rs
View file

@ -289,15 +289,23 @@ mod lazy {
}
pub unsafe fn get(&self) -> Option<&'static T> {
(*self.inner.get()).as_ref()
// SAFETY: The caller must ensure no reference is ever handed out to
// the inner cell nor mutable reference to the Option<T> inside said
// cell. This make it safe to hand a reference, though the lifetime
// of 'static is itself unsafe, making the get method unsafe.
unsafe { (*self.inner.get()).as_ref() }
}
/// The caller must ensure that no reference is active: this method
/// needs unique access.
pub unsafe fn initialize<F: FnOnce() -> T>(&self, init: F) -> &'static T {
// Execute the initialization up front, *then* move it into our slot,
// just in case initialization fails.
let value = init();
let ptr = self.inner.get();
// SAFETY:
//
// note that this can in theory just be `*ptr = Some(value)`, but due to
// the compiler will currently codegen that pattern with something like:
//
@ -310,22 +318,36 @@ mod lazy {
// value (an aliasing violation). To avoid setting the "I'm running a
// destructor" flag we just use `mem::replace` which should sequence the
// operations a little differently and make this safe to call.
let _ = mem::replace(&mut *ptr, Some(value));
//
// The precondition also ensures that we are the only one accessing
// `self` at the moment so replacing is fine.
unsafe {
let _ = mem::replace(&mut *ptr, Some(value));
}
// After storing `Some` we want to get a reference to the contents of
// what we just stored. While we could use `unwrap` here and it should
// always work it empirically doesn't seem to always get optimized away,
// which means that using something like `try_with` can pull in
// panicking code and cause a large size bloat.
match *ptr {
Some(ref x) => x,
None => hint::unreachable_unchecked(),
// SAFETY: With the call to `mem::replace` it is guaranteed there is
// a `Some` behind `ptr`, not a `None` so `unreachable_unchecked`
// will never be reached.
unsafe {
// After storing `Some` we want to get a reference to the contents of
// what we just stored. While we could use `unwrap` here and it should
// always work it empirically doesn't seem to always get optimized away,
// which means that using something like `try_with` can pull in
// panicking code and cause a large size bloat.
match *ptr {
Some(ref x) => x,
None => hint::unreachable_unchecked(),
}
}
}
/// The other methods hand out references while taking &self.
/// As such, callers of this method must ensure no `&` and `&mut` are
/// available and used at the same time.
#[allow(unused)]
pub unsafe fn take(&mut self) -> Option<T> {
(*self.inner.get()).take()
// SAFETY: See doc comment for this method.
unsafe { (*self.inner.get()).take() }
}
}
}
@ -356,10 +378,17 @@ pub mod statik {
}
pub unsafe fn get(&self, init: fn() -> T) -> Option<&'static T> {
let value = match self.inner.get() {
Some(ref value) => value,
None => self.inner.initialize(init),
// SAFETY: The caller must ensure no reference is ever handed out to
// the inner cell nor mutable reference to the Option<T> inside said
// cell. This make it safe to hand a reference, though the lifetime
// of 'static is itself unsafe, making the get method unsafe.
let value = unsafe {
match self.inner.get() {
Some(ref value) => value,
None => self.inner.initialize(init),
}
};
Some(value)
}
}
@ -414,9 +443,18 @@ pub mod fast {
}
pub unsafe fn get<F: FnOnce() -> T>(&self, init: F) -> Option<&'static T> {
match self.inner.get() {
Some(val) => Some(val),
None => self.try_initialize(init),
// SAFETY: See the definitions of `LazyKeyInner::get` and
// `try_initialize` for more informations.
//
// The caller must ensure no mutable references are ever active to
// the inner cell or the inner T when this is called.
// The `try_initialize` is dependant on the passed `init` function
// for this.
unsafe {
match self.inner.get() {
Some(val) => Some(val),
None => self.try_initialize(init),
}
}
}
@ -429,8 +467,10 @@ pub mod fast {
// LLVM issue: https://bugs.llvm.org/show_bug.cgi?id=41722
#[inline(never)]
unsafe fn try_initialize<F: FnOnce() -> T>(&self, init: F) -> Option<&'static T> {
if !mem::needs_drop::<T>() || self.try_register_dtor() {
Some(self.inner.initialize(init))
// SAFETY: See comment above (this function doc).
if !mem::needs_drop::<T>() || unsafe { self.try_register_dtor() } {
// SAFETY: See comment above (his function doc).
Some(unsafe { self.inner.initialize(init) })
} else {
None
}
@ -442,8 +482,12 @@ pub mod fast {
unsafe fn try_register_dtor(&self) -> bool {
match self.dtor_state.get() {
DtorState::Unregistered => {
// dtor registration happens before initialization.
register_dtor(self as *const _ as *mut u8, destroy_value::<T>);
// SAFETY: dtor registration happens before initialization.
// Passing `self` as a pointer while using `destroy_value<T>`
// is safe because the function will build a pointer to a
// Key<T>, which is the type of self and so find the correct
// size.
unsafe { register_dtor(self as *const _ as *mut u8, destroy_value::<T>) };
self.dtor_state.set(DtorState::Registered);
true
}
@ -459,13 +503,21 @@ pub mod fast {
unsafe extern "C" fn destroy_value<T>(ptr: *mut u8) {
let ptr = ptr as *mut Key<T>;
// SAFETY:
//
// The pointer `ptr` has been built just above and comes from
// `try_register_dtor` where it is originally a Key<T> coming from `self`,
// making it non-NUL and of the correct type.
//
// Right before we run the user destructor be sure to set the
// `Option<T>` to `None`, and `dtor_state` to `RunningOrHasRun`. This
// causes future calls to `get` to run `try_initialize_drop` again,
// which will now fail, and return `None`.
let value = (*ptr).inner.take();
(*ptr).dtor_state.set(DtorState::RunningOrHasRun);
drop(value);
unsafe {
let value = (*ptr).inner.take();
(*ptr).dtor_state.set(DtorState::RunningOrHasRun);
drop(value);
}
}
}
@ -503,21 +555,30 @@ pub mod os {
Key { os: OsStaticKey::new(Some(destroy_value::<T>)), marker: marker::PhantomData }
}
/// It is a requirement for the caller to ensure that no mutable
/// reference is active when this method is called.
pub unsafe fn get(&'static self, init: fn() -> T) -> Option<&'static T> {
let ptr = self.os.get() as *mut Value<T>;
// SAFETY: See the documentation for this method.
let ptr = unsafe { self.os.get() as *mut Value<T> };
if ptr as usize > 1 {
if let Some(ref value) = (*ptr).inner.get() {
// SAFETY: the check ensured the pointer is safe (its destructor
// is not running) + it is coming from a trusted source (self).
if let Some(ref value) = unsafe { (*ptr).inner.get() } {
return Some(value);
}
}
self.try_initialize(init)
// SAFETY: At this point we are sure we have no value and so
// initializing (or trying to) is safe.
unsafe { self.try_initialize(init) }
}
// `try_initialize` is only called once per os thread local variable,
// except in corner cases where thread_local dtors reference other
// thread_local's, or it is being recursively initialized.
unsafe fn try_initialize(&'static self, init: fn() -> T) -> Option<&'static T> {
let ptr = self.os.get() as *mut Value<T>;
// SAFETY: No mutable references are ever handed out meaning getting
// the value is ok.
let ptr = unsafe { self.os.get() as *mut Value<T> };
if ptr as usize == 1 {
// destructor is running
return None;
@ -528,18 +589,26 @@ pub mod os {
// local copy, so do that now.
let ptr: Box<Value<T>> = box Value { inner: LazyKeyInner::new(), key: self };
let ptr = Box::into_raw(ptr);
self.os.set(ptr as *mut u8);
// SAFETY: At this point we are sure there is no value inside
// ptr so setting it will not affect anyone else.
unsafe {
self.os.set(ptr as *mut u8);
}
ptr
} else {
// recursive initialization
ptr
};
Some((*ptr).inner.initialize(init))
// SAFETY: ptr has been ensured as non-NUL just above an so can be
// dereferenced safely.
unsafe { Some((*ptr).inner.initialize(init)) }
}
}
unsafe extern "C" fn destroy_value<T: 'static>(ptr: *mut u8) {
// SAFETY:
//
// The OS TLS ensures that this key contains a NULL value when this
// destructor starts to run. We set it back to a sentinel value of 1 to
// ensure that any future calls to `get` for this thread will return
@ -547,10 +616,12 @@ pub mod os {
//
// Note that to prevent an infinite loop we reset it back to null right
// before we return from the destructor ourselves.
let ptr = Box::from_raw(ptr as *mut Value<T>);
let key = ptr.key;
key.os.set(1 as *mut u8);
drop(ptr);
key.os.set(ptr::null_mut());
unsafe {
let ptr = Box::from_raw(ptr as *mut Value<T>);
let key = ptr.key;
key.os.set(1 as *mut u8);
drop(ptr);
key.os.set(ptr::null_mut());
}
}
}

28
library/std/src/thread/mod.rs
View file

@ -144,6 +144,7 @@
//! [`with`]: LocalKey::with
#![stable(feature = "rust1", since = "1.0.0")]
#![deny(unsafe_op_in_unsafe_fn)]
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
@ -456,14 +457,23 @@ impl Builder {
imp::Thread::set_name(name);
}
thread_info::set(imp::guard::current(), their_thread);
// SAFETY: the stack guard passed is the one for the current thread.
// This means the current thread's stack and the new thread's stack
// are properly set and protected from each other.
thread_info::set(unsafe { imp::guard::current() }, their_thread);
let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
crate::sys_common::backtrace::__rust_begin_short_backtrace(f)
}));
*their_packet.get() = Some(try_result);
// SAFETY: `their_packet` as been built just above and moved by the
// closure (it is an Arc<...>) and `my_packet` will be stored in the
// same `JoinInner` as this closure meaning the mutation will be
// safe (not modify it and affect a value far away).
unsafe { *their_packet.get() = Some(try_result) };
};
Ok(JoinHandle(JoinInner {
// SAFETY:
//
// `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
// through FFI or otherwise used with low-level threading primitives that have no
// notion of or way to enforce lifetimes.
@ -475,12 +485,14 @@ impl Builder {
// Similarly, the `sys` implementation must guarantee that no references to the closure
// exist after the thread has terminated, which is signaled by `Thread::join`
// returning.
native: Some(imp::Thread::new(
stack_size,
mem::transmute::<Box<dyn FnOnce() + 'a>, Box<dyn FnOnce() + 'static>>(Box::new(
main,
)),
)?),
native: unsafe {
Some(imp::Thread::new(
stack_size,
mem::transmute::<Box<dyn FnOnce() + 'a>, Box<dyn FnOnce() + 'static>>(
Box::new(main),
),
)?)
},
thread: my_thread,
packet: Packet(my_packet),
}))