1 //! Implementation of panics backed by libgcc/libunwind (in some form).
3 //! For background on exception handling and stack unwinding please see
4 //! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
5 //! documents linked from it.
6 //! These are also good reads:
7 //! * <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html>
8 //! * <https://monoinfinito.wordpress.com/series/exception-handling-in-c/>
9 //! * <https://www.airs.com/blog/index.php?s=exception+frames>
11 //! ## A brief summary
13 //! Exception handling happens in two phases: a search phase and a cleanup
16 //! In both phases the unwinder walks stack frames from top to bottom using
17 //! information from the stack frame unwind sections of the current process's
18 //! modules ("module" here refers to an OS module, i.e., an executable or a
21 //! For each stack frame, it invokes the associated "personality routine", whose
22 //! address is also stored in the unwind info section.
24 //! In the search phase, the job of a personality routine is to examine
25 //! exception object being thrown, and to decide whether it should be caught at
26 //! that stack frame. Once the handler frame has been identified, cleanup phase
29 //! In the cleanup phase, the unwinder invokes each personality routine again.
30 //! This time it decides which (if any) cleanup code needs to be run for
31 //! the current stack frame. If so, the control is transferred to a special
32 //! branch in the function body, the "landing pad", which invokes destructors,
33 //! frees memory, etc. At the end of the landing pad, control is transferred
34 //! back to the unwinder and unwinding resumes.
36 //! Once stack has been unwound down to the handler frame level, unwinding stops
37 //! and the last personality routine transfers control to the catch block.
39 use alloc::boxed::Box;
45 // In case where multiple copies of std is compiled into a single binary,
46 // we use address of this static variable to distinguish an exception raised by
47 // this copy and some other copy (which needs to be treated as foreign exception).
48 static CANARY: u8 = 0;
51 // Once `c_unwind` feature is stabilized, there will be ABI stability requirement
52 // on this struct. The first two field must be `_Unwind_Exception` and `canary`,
53 // as it may be accessed by a different version of the std with a different compiler.
56 _uwe: uw::_Unwind_Exception,
58 cause: Box<dyn Any + Send>,
61 pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
62 let exception = Box::new(Exception {
63 _uwe: uw::_Unwind_Exception {
64 exception_class: rust_exception_class(),
66 private: [0; uw::unwinder_private_data_size],
71 let exception_param = Box::into_raw(exception) as *mut uw::_Unwind_Exception;
72 return uw::_Unwind_RaiseException(exception_param) as u32;
74 extern "C" fn exception_cleanup(
75 _unwind_code: uw::_Unwind_Reason_Code,
76 exception: *mut uw::_Unwind_Exception,
79 let _: Box<Exception> = Box::from_raw(exception as *mut Exception);
80 super::__rust_drop_panic();
85 pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
86 let exception = ptr as *mut uw::_Unwind_Exception;
87 if (*exception).exception_class != rust_exception_class() {
88 uw::_Unwind_DeleteException(exception);
89 super::__rust_foreign_exception();
92 let exception = exception.cast::<Exception>();
93 // Just access the canary field, avoid accessing the entire `Exception` as
94 // it can be a foreign Rust exception.
95 let canary = ptr::addr_of!((*exception).canary).read();
96 if !ptr::eq(canary, &CANARY) {
97 // A foreign Rust exception, treat it slightly differently from other
98 // foreign exceptions, because call into `_Unwind_DeleteException` will
99 // call into `__rust_drop_panic` which produces a confusing
100 // "Rust panic must be rethrown" message.
101 super::__rust_foreign_exception();
104 let exception = Box::from_raw(exception as *mut Exception);
108 // Rust's exception class identifier. This is used by personality routines to
109 // determine whether the exception was thrown by their own runtime.
110 fn rust_exception_class() -> uw::_Unwind_Exception_Class {
111 // M O Z \0 R U S T -- vendor, language