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//! Implementation of panics backed by libgcc/libunwind (in some form).
//!
//! For background on exception handling and stack unwinding please see
//! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
//! documents linked from it.
//! These are also good reads:
//! * <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html>
//! * <https://monoinfinito.wordpress.com/series/exception-handling-in-c/>
//! * <https://www.airs.com/blog/index.php?s=exception+frames>
//!
//! ## A brief summary
//!
//! Exception handling happens in two phases: a search phase and a cleanup
//! phase.
//!
//! In both phases the unwinder walks stack frames from top to bottom using
//! information from the stack frame unwind sections of the current process's
//! modules ("module" here refers to an OS module, i.e., an executable or a
//! dynamic library).
//!
//! For each stack frame, it invokes the associated "personality routine", whose
//! address is also stored in the unwind info section.
//!
//! In the search phase, the job of a personality routine is to examine
//! exception object being thrown, and to decide whether it should be caught at
//! that stack frame. Once the handler frame has been identified, cleanup phase
//! begins.
//!
//! In the cleanup phase, the unwinder invokes each personality routine again.
//! This time it decides which (if any) cleanup code needs to be run for
//! the current stack frame. If so, the control is transferred to a special
//! branch in the function body, the "landing pad", which invokes destructors,
//! frees memory, etc. At the end of the landing pad, control is transferred
//! back to the unwinder and unwinding resumes.
//!
//! Once stack has been unwound down to the handler frame level, unwinding stops
//! and the last personality routine transfers control to the catch block.
use alloc::boxed::Box;
use core::any::Any;
use unwind as uw;
#[repr(C)]
struct Exception {
_uwe: uw::_Unwind_Exception,
cause: Box<dyn Any + Send>,
}
pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
let exception = Box::new(Exception {
_uwe: uw::_Unwind_Exception {
exception_class: rust_exception_class(),
exception_cleanup,
private: [0; uw::unwinder_private_data_size],
},
cause: data,
});
let exception_param = Box::into_raw(exception) as *mut uw::_Unwind_Exception;
return uw::_Unwind_RaiseException(exception_param) as u32;
extern "C" fn exception_cleanup(
_unwind_code: uw::_Unwind_Reason_Code,
exception: *mut uw::_Unwind_Exception,
) {
unsafe {
let _: Box<Exception> = Box::from_raw(exception as *mut Exception);
super::__rust_drop_panic();
}
}
}
pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
let exception = ptr as *mut uw::_Unwind_Exception;
if (*exception).exception_class != rust_exception_class() {
uw::_Unwind_DeleteException(exception);
super::__rust_foreign_exception();
} else {
let exception = Box::from_raw(exception as *mut Exception);
exception.cause
}
}
// Rust's exception class identifier. This is used by personality routines to
// determine whether the exception was thrown by their own runtime.
fn rust_exception_class() -> uw::_Unwind_Exception_Class {
// M O Z \0 R U S T -- vendor, language
0x4d4f5a_00_52555354
}
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