1 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Implementation of Rust stack unwinding
13 //! For background on exception handling and stack unwinding please see
14 //! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
15 //! documents linked from it.
16 //! These are also good reads:
17 //! http://theofilos.cs.columbia.edu/blog/2013/09/22/base_abi/
18 //! http://monoinfinito.wordpress.com/series/exception-handling-in-c/
19 //! http://www.airs.com/blog/index.php?s=exception+frames
21 //! ## A brief summary
23 //! Exception handling happens in two phases: a search phase and a cleanup phase.
25 //! In both phases the unwinder walks stack frames from top to bottom using
26 //! information from the stack frame unwind sections of the current process's
27 //! modules ("module" here refers to an OS module, i.e. an executable or a
30 //! For each stack frame, it invokes the associated "personality routine", whose
31 //! address is also stored in the unwind info section.
33 //! In the search phase, the job of a personality routine is to examine exception
34 //! object being thrown, and to decide whether it should be caught at that stack
35 //! frame. Once the handler frame has been identified, cleanup phase begins.
37 //! In the cleanup phase, personality routines invoke cleanup code associated
38 //! with their stack frames (i.e. destructors). Once stack has been unwound down
39 //! to the handler frame level, unwinding stops and the last personality routine
40 //! transfers control to its catch block.
42 //! ## Frame unwind info registration
44 //! Each module has its own frame unwind info section (usually ".eh_frame"), and
45 //! unwinder needs to know about all of them in order for unwinding to be able to
46 //! cross module boundaries.
48 //! On some platforms, like Linux, this is achieved by dynamically enumerating
49 //! currently loaded modules via the dl_iterate_phdr() API and finding all
50 //! .eh_frame sections.
52 //! Others, like Windows, require modules to actively register their unwind info
53 //! sections by calling __register_frame_info() API at startup. In the latter
54 //! case it is essential that there is only one copy of the unwinder runtime in
55 //! the process. This is usually achieved by linking to the dynamic version of
56 //! the unwind runtime.
58 //! Currently Rust uses unwind runtime provided by libgcc.
62 use alloc::boxed::Box;
63 use collections::string::String;
64 use collections::str::StrAllocating;
65 use collections::vec::Vec;
72 use core::raw::Closure;
85 uwe: uw::_Unwind_Exception,
86 cause: Option<Box<Any + Send>>,
89 pub type Callback = fn(msg: &(Any + Send), file: &'static str, line: uint);
91 // Variables used for invoking callbacks when a task starts to unwind.
93 // For more information, see below.
94 const MAX_CALLBACKS: uint = 16;
95 static CALLBACKS: [atomic::AtomicUint, ..MAX_CALLBACKS] =
96 [atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
97 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
98 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
99 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
100 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
101 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
102 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT,
103 atomic::INIT_ATOMIC_UINT, atomic::INIT_ATOMIC_UINT];
104 static CALLBACK_CNT: atomic::AtomicUint = atomic::INIT_ATOMIC_UINT;
107 pub fn new() -> Unwinder {
113 pub fn unwinding(&self) -> bool {
118 /// Invoke a closure, capturing the cause of panic if one occurs.
120 /// This function will return `None` if the closure did not panic, and will
121 /// return `Some(cause)` if the closure panics. The `cause` returned is the
122 /// object with which panic was originally invoked.
124 /// This function also is unsafe for a variety of reasons:
126 /// * This is not safe to call in a nested fashion. The unwinding
127 /// interface for Rust is designed to have at most one try/catch block per
128 /// task, not multiple. No runtime checking is currently performed to uphold
129 /// this invariant, so this function is not safe. A nested try/catch block
130 /// may result in corruption of the outer try/catch block's state, especially
131 /// if this is used within a task itself.
133 /// * It is not sound to trigger unwinding while already unwinding. Rust tasks
134 /// have runtime checks in place to ensure this invariant, but it is not
135 /// guaranteed that a rust task is in place when invoking this function.
136 /// Unwinding twice can lead to resource leaks where some destructors are not
138 pub unsafe fn try(f: ||) -> ::core::result::Result<(), Box<Any + Send>> {
139 let closure: Closure = mem::transmute(f);
140 let ep = rust_try(try_fn, closure.code as *mut c_void,
141 closure.env as *mut c_void);
142 return if ep.is_null() {
145 let my_ep = ep as *mut Exception;
146 rtdebug!("caught {}", (*my_ep).uwe.exception_class);
147 let cause = (*my_ep).cause.take();
148 uw::_Unwind_DeleteException(ep);
152 extern fn try_fn(code: *mut c_void, env: *mut c_void) {
154 let closure: || = mem::transmute(Closure {
155 code: code as *mut (),
162 #[link(name = "rustrt_native", kind = "static")]
168 // When f(...) returns normally, the return value is null.
169 // When f(...) throws, the return value is a pointer to the caught
171 fn rust_try(f: extern "C" fn(*mut c_void, *mut c_void),
173 data: *mut c_void) -> *mut uw::_Unwind_Exception;
177 // An uninlined, unmangled function upon which to slap yer breakpoints
180 fn rust_panic(cause: Box<Any + Send>) -> ! {
181 rtdebug!("begin_unwind()");
184 let exception = box Exception {
185 uwe: uw::_Unwind_Exception {
186 exception_class: rust_exception_class(),
187 exception_cleanup: exception_cleanup,
188 private: [0, ..uw::unwinder_private_data_size],
192 let error = uw::_Unwind_RaiseException(mem::transmute(exception));
193 rtabort!("Could not unwind stack, error = {}", error as int)
196 extern fn exception_cleanup(_unwind_code: uw::_Unwind_Reason_Code,
197 exception: *mut uw::_Unwind_Exception) {
198 rtdebug!("exception_cleanup()");
200 let _: Box<Exception> = mem::transmute(exception);
205 // Rust's exception class identifier. This is used by personality routines to
206 // determine whether the exception was thrown by their own runtime.
207 fn rust_exception_class() -> uw::_Unwind_Exception_Class {
208 // M O Z \0 R U S T -- vendor, language
212 // We could implement our personality routine in pure Rust, however exception
213 // info decoding is tedious. More importantly, personality routines have to
214 // handle various platform quirks, which are not fun to maintain. For this
215 // reason, we attempt to reuse personality routine of the C language:
216 // __gcc_personality_v0.
218 // Since C does not support exception catching, __gcc_personality_v0 simply
219 // always returns _URC_CONTINUE_UNWIND in search phase, and always returns
220 // _URC_INSTALL_CONTEXT (i.e. "invoke cleanup code") in cleanup phase.
222 // This is pretty close to Rust's exception handling approach, except that Rust
223 // does have a single "catch-all" handler at the bottom of each task's stack.
224 // So we have two versions of the personality routine:
225 // - rust_eh_personality, used by all cleanup landing pads, which never catches,
226 // so the behavior of __gcc_personality_v0 is perfectly adequate there, and
227 // - rust_eh_personality_catch, used only by rust_try(), which always catches.
229 // Note, however, that for implementation simplicity, rust_eh_personality_catch
230 // lacks code to install a landing pad, so in order to obtain exception object
231 // pointer (which it needs to return upstream), rust_try() employs another trick:
232 // it calls into the nested rust_try_inner(), whose landing pad does not resume
233 // unwinds. Instead, it extracts the exception pointer and performs a "normal"
236 // See also: rt/rust_try.ll
238 #[cfg(all(not(target_arch = "arm"),
239 not(all(windows, target_arch = "x86_64")),
247 fn __gcc_personality_v0(version: c_int,
248 actions: uw::_Unwind_Action,
249 exception_class: uw::_Unwind_Exception_Class,
250 ue_header: *mut uw::_Unwind_Exception,
251 context: *mut uw::_Unwind_Context)
252 -> uw::_Unwind_Reason_Code;
255 #[lang="eh_personality"]
256 #[no_mangle] // referenced from rust_try.ll
257 extern fn rust_eh_personality(
259 actions: uw::_Unwind_Action,
260 exception_class: uw::_Unwind_Exception_Class,
261 ue_header: *mut uw::_Unwind_Exception,
262 context: *mut uw::_Unwind_Context
263 ) -> uw::_Unwind_Reason_Code
266 __gcc_personality_v0(version, actions, exception_class, ue_header,
271 #[no_mangle] // referenced from rust_try.ll
272 pub extern "C" fn rust_eh_personality_catch(
274 actions: uw::_Unwind_Action,
275 _exception_class: uw::_Unwind_Exception_Class,
276 _ue_header: *mut uw::_Unwind_Exception,
277 _context: *mut uw::_Unwind_Context
278 ) -> uw::_Unwind_Reason_Code
281 if (actions as c_int & uw::_UA_SEARCH_PHASE as c_int) != 0 { // search phase
282 uw::_URC_HANDLER_FOUND // catch!
284 else { // cleanup phase
285 uw::_URC_INSTALL_CONTEXT
290 // iOS on armv7 is using SjLj exceptions and therefore requires to use
291 // a specialized personality routine: __gcc_personality_sj0
293 #[cfg(all(target_os = "ios", target_arch = "arm", not(test)))]
300 fn __gcc_personality_sj0(version: c_int,
301 actions: uw::_Unwind_Action,
302 exception_class: uw::_Unwind_Exception_Class,
303 ue_header: *mut uw::_Unwind_Exception,
304 context: *mut uw::_Unwind_Context)
305 -> uw::_Unwind_Reason_Code;
308 #[lang="eh_personality"]
309 #[no_mangle] // referenced from rust_try.ll
310 pub extern "C" fn rust_eh_personality(
312 actions: uw::_Unwind_Action,
313 exception_class: uw::_Unwind_Exception_Class,
314 ue_header: *mut uw::_Unwind_Exception,
315 context: *mut uw::_Unwind_Context
316 ) -> uw::_Unwind_Reason_Code
319 __gcc_personality_sj0(version, actions, exception_class, ue_header,
324 #[no_mangle] // referenced from rust_try.ll
325 pub extern "C" fn rust_eh_personality_catch(
327 actions: uw::_Unwind_Action,
328 _exception_class: uw::_Unwind_Exception_Class,
329 _ue_header: *mut uw::_Unwind_Exception,
330 _context: *mut uw::_Unwind_Context
331 ) -> uw::_Unwind_Reason_Code
333 if (actions as c_int & uw::_UA_SEARCH_PHASE as c_int) != 0 { // search phase
334 uw::_URC_HANDLER_FOUND // catch!
336 else { // cleanup phase
338 __gcc_personality_sj0(_version, actions, _exception_class, _ue_header,
346 // ARM EHABI uses a slightly different personality routine signature,
347 // but otherwise works the same.
348 #[cfg(all(target_arch = "arm", not(target_os = "ios"), not(test)))]
355 fn __gcc_personality_v0(state: uw::_Unwind_State,
356 ue_header: *mut uw::_Unwind_Exception,
357 context: *mut uw::_Unwind_Context)
358 -> uw::_Unwind_Reason_Code;
361 #[lang="eh_personality"]
362 #[no_mangle] // referenced from rust_try.ll
363 extern "C" fn rust_eh_personality(
364 state: uw::_Unwind_State,
365 ue_header: *mut uw::_Unwind_Exception,
366 context: *mut uw::_Unwind_Context
367 ) -> uw::_Unwind_Reason_Code
370 __gcc_personality_v0(state, ue_header, context)
374 #[no_mangle] // referenced from rust_try.ll
375 pub extern "C" fn rust_eh_personality_catch(
376 state: uw::_Unwind_State,
377 _ue_header: *mut uw::_Unwind_Exception,
378 _context: *mut uw::_Unwind_Context
379 ) -> uw::_Unwind_Reason_Code
381 if (state as c_int & uw::_US_ACTION_MASK as c_int)
382 == uw::_US_VIRTUAL_UNWIND_FRAME as c_int { // search phase
383 uw::_URC_HANDLER_FOUND // catch!
385 else { // cleanup phase
386 uw::_URC_INSTALL_CONTEXT
391 // Win64 SEH (see http://msdn.microsoft.com/en-us/library/1eyas8tf.aspx)
393 // This looks a bit convoluted because rather than implementing a native SEH handler,
394 // GCC reuses the same personality routine as for the other architectures by wrapping it
395 // with an "API translator" layer (_GCC_specific_handler).
397 #[cfg(all(windows, target_arch = "x86_64", not(test)))]
399 #[allow(non_camel_case_types, non_snake_case)]
401 pub use self::EXCEPTION_DISPOSITION::*;
403 use libc::{c_void, c_int};
406 pub struct EXCEPTION_RECORD;
410 pub struct DISPATCHER_CONTEXT;
413 pub enum EXCEPTION_DISPOSITION {
414 ExceptionContinueExecution,
415 ExceptionContinueSearch,
416 ExceptionNestedException,
417 ExceptionCollidedUnwind
420 type _Unwind_Personality_Fn =
423 actions: uw::_Unwind_Action,
424 exception_class: uw::_Unwind_Exception_Class,
425 ue_header: *mut uw::_Unwind_Exception,
426 context: *mut uw::_Unwind_Context
427 ) -> uw::_Unwind_Reason_Code;
430 fn __gcc_personality_seh0(
431 exceptionRecord: *mut EXCEPTION_RECORD,
432 establisherFrame: *mut c_void,
433 contextRecord: *mut CONTEXT,
434 dispatcherContext: *mut DISPATCHER_CONTEXT
435 ) -> EXCEPTION_DISPOSITION;
437 fn _GCC_specific_handler(
438 exceptionRecord: *mut EXCEPTION_RECORD,
439 establisherFrame: *mut c_void,
440 contextRecord: *mut CONTEXT,
441 dispatcherContext: *mut DISPATCHER_CONTEXT,
442 personality: _Unwind_Personality_Fn
443 ) -> EXCEPTION_DISPOSITION;
446 #[lang="eh_personality"]
447 #[no_mangle] // referenced from rust_try.ll
448 extern "C" fn rust_eh_personality(
449 exceptionRecord: *mut EXCEPTION_RECORD,
450 establisherFrame: *mut c_void,
451 contextRecord: *mut CONTEXT,
452 dispatcherContext: *mut DISPATCHER_CONTEXT
453 ) -> EXCEPTION_DISPOSITION
456 __gcc_personality_seh0(exceptionRecord, establisherFrame,
457 contextRecord, dispatcherContext)
461 #[no_mangle] // referenced from rust_try.ll
462 pub extern "C" fn rust_eh_personality_catch(
463 exceptionRecord: *mut EXCEPTION_RECORD,
464 establisherFrame: *mut c_void,
465 contextRecord: *mut CONTEXT,
466 dispatcherContext: *mut DISPATCHER_CONTEXT
467 ) -> EXCEPTION_DISPOSITION
471 actions: uw::_Unwind_Action,
472 _exception_class: uw::_Unwind_Exception_Class,
473 _ue_header: *mut uw::_Unwind_Exception,
474 _context: *mut uw::_Unwind_Context
475 ) -> uw::_Unwind_Reason_Code
477 if (actions as c_int & uw::_UA_SEARCH_PHASE as c_int) != 0 { // search phase
478 uw::_URC_HANDLER_FOUND // catch!
480 else { // cleanup phase
481 uw::_URC_INSTALL_CONTEXT
486 _GCC_specific_handler(exceptionRecord, establisherFrame,
487 contextRecord, dispatcherContext,
493 // Entry point of panic from the libcore crate
495 #[lang = "panic_fmt"]
496 pub extern fn rust_begin_unwind(msg: &fmt::Arguments,
497 file: &'static str, line: uint) -> ! {
498 begin_unwind_fmt(msg, &(file, line))
501 /// The entry point for unwinding with a formatted message.
503 /// This is designed to reduce the amount of code required at the call
504 /// site as much as possible (so that `panic!()` has as low an impact
505 /// on (e.g.) the inlining of other functions as possible), by moving
506 /// the actual formatting into this shared place.
507 #[inline(never)] #[cold]
508 pub fn begin_unwind_fmt(msg: &fmt::Arguments, file_line: &(&'static str, uint)) -> ! {
509 use core::fmt::FormatWriter;
511 // We do two allocations here, unfortunately. But (a) they're
512 // required with the current scheme, and (b) we don't handle
513 // panic + OOM properly anyway (see comment in begin_unwind
516 struct VecWriter<'a> { v: &'a mut Vec<u8> }
518 impl<'a> fmt::FormatWriter for VecWriter<'a> {
519 fn write(&mut self, buf: &[u8]) -> fmt::Result {
520 self.v.push_all(buf);
525 let mut v = Vec::new();
526 let _ = write!(&mut VecWriter { v: &mut v }, "{}", msg);
528 let msg = box String::from_utf8_lossy(v.as_slice()).into_string();
529 begin_unwind_inner(msg, file_line)
532 /// This is the entry point of unwinding for panic!() and assert!().
533 #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
534 pub fn begin_unwind<M: Any + Send>(msg: M, file_line: &(&'static str, uint)) -> ! {
535 // Note that this should be the only allocation performed in this code path.
536 // Currently this means that panic!() on OOM will invoke this code path,
537 // but then again we're not really ready for panic on OOM anyway. If
538 // we do start doing this, then we should propagate this allocation to
539 // be performed in the parent of this task instead of the task that's
542 // see below for why we do the `Any` coercion here.
543 begin_unwind_inner(box msg, file_line)
546 /// The core of the unwinding.
548 /// This is non-generic to avoid instantiation bloat in other crates
549 /// (which makes compilation of small crates noticeably slower). (Note:
550 /// we need the `Any` object anyway, we're not just creating it to
551 /// avoid being generic.)
553 /// Do this split took the LLVM IR line counts of `fn main() { panic!()
554 /// }` from ~1900/3700 (-O/no opts) to 180/590.
555 #[inline(never)] #[cold] // this is the slow path, please never inline this
556 fn begin_unwind_inner(msg: Box<Any + Send>, file_line: &(&'static str, uint)) -> ! {
557 // First, invoke call the user-defined callbacks triggered on task panic.
559 // By the time that we see a callback has been registered (by reading
560 // MAX_CALLBACKS), the actual callback itself may have not been stored yet,
561 // so we just chalk it up to a race condition and move on to the next
562 // callback. Additionally, CALLBACK_CNT may briefly be higher than
563 // MAX_CALLBACKS, so we're sure to clamp it as necessary.
565 let amt = CALLBACK_CNT.load(atomic::SeqCst);
566 CALLBACKS[..cmp::min(amt, MAX_CALLBACKS)]
568 for cb in callbacks.iter() {
569 match cb.load(atomic::SeqCst) {
572 let f: Callback = unsafe { mem::transmute(n) };
573 let (file, line) = *file_line;
574 f(&*msg, file, line);
579 // Now that we've run all the necessary unwind callbacks, we actually
580 // perform the unwinding. If we don't have a task, then it's time to die
581 // (hopefully someone printed something about this).
582 let mut task: Box<Task> = match Local::try_take() {
584 None => rust_panic(msg),
587 if task.unwinder.unwinding {
588 // If a task panics while it's already unwinding then we
589 // have limited options. Currently our preference is to
590 // just abort. In the future we may consider resuming
591 // unwinding or otherwise exiting the task cleanly.
592 rterrln!("task failed during unwinding. aborting.");
593 unsafe { intrinsics::abort() }
595 task.unwinder.unwinding = true;
597 // Put the task back in TLS because the unwinding process may run code which
598 // requires the task. We need a handle to its unwinder, however, so after
599 // this we unsafely extract it and continue along.
604 /// Register a callback to be invoked when a task unwinds.
606 /// This is an unsafe and experimental API which allows for an arbitrary
607 /// callback to be invoked when a task panics. This callback is invoked on both
608 /// the initial unwinding and a double unwinding if one occurs. Additionally,
609 /// the local `Task` will be in place for the duration of the callback, and
610 /// the callback must ensure that it remains in place once the callback returns.
612 /// Only a limited number of callbacks can be registered, and this function
613 /// returns whether the callback was successfully registered or not. It is not
614 /// currently possible to unregister a callback once it has been registered.
616 pub unsafe fn register(f: Callback) -> bool {
617 match CALLBACK_CNT.fetch_add(1, atomic::SeqCst) {
618 // The invocation code has knowledge of this window where the count has
619 // been incremented, but the callback has not been stored. We're
620 // guaranteed that the slot we're storing into is 0.
621 n if n < MAX_CALLBACKS => {
622 let prev = CALLBACKS[n].swap(mem::transmute(f), atomic::SeqCst);
623 rtassert!(prev == 0);
626 // If we accidentally bumped the count too high, pull it back.
628 CALLBACK_CNT.store(MAX_CALLBACKS, atomic::SeqCst);