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library: Compute Rust exception class from its string repr #130381

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Sep 15, 2024
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library: Compute Rust exception class from its string repr #130381

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9 changes: 3 additions & 6 deletions library/panic_unwind/src/gcc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -61,7 +61,7 @@ struct Exception {
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_class: RUST_EXCEPTION_CLASS,
exception_cleanup: Some(exception_cleanup),
private: [core::ptr::null(); uw::unwinder_private_data_size],
},
Expand All @@ -84,7 +84,7 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {

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() {
if (*exception).exception_class != RUST_EXCEPTION_CLASS {
uw::_Unwind_DeleteException(exception);
super::__rust_foreign_exception();
}
Expand All @@ -107,7 +107,4 @@ pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {

// 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
}
const RUST_EXCEPTION_CLASS: uw::_Unwind_Exception_Class = u64::from_be_bytes(*b"MOZ\0RUST");
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Pre-existing, but should we really reverse the stored order between little-endian and big-endian systems? We currently show MOZ\0RUST on big-endian systems and TSUR\0ZOM on little-endian systems when interpreted as string/doing a hexdump.

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I noticed that when I poked at it. 👀 It is funny to me. All the other runtimes make the same mistake, so I initially decided not to change it. I think I wanted to actually look at the data through a hex dump before doing so.

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Maybe a relevant datapoint: windows-drivers-rs' wdk-alloc specifically chose u32::from_ne_bytes(*b"rust") for their allocator tag: https://github.com/microsoft/windows-drivers-rs/blob/c3b7c4aa06e43e928f27eb704f76932688802921/crates/wdk-alloc/src/lib.rs#L50

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@RalfJung RalfJung Sep 23, 2024 •
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Maybe it's not a mistake, maybe the type of this should be thought of as [u8; 8] rather than u64 -- with bytes always being interpreted left-to-right no matter the endianess.

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The current impl is a mistake if it should be interpreted as [u8; 8] as currently little endian systems put the string in reverse order, while big endian systems put it in the correct order.

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Oh right, that would have to use from_ne_bytes... then transmuting back to a u8 array would always give the same results.

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i assume it was just someone filling out the u64 literal in left to right order, as the previous comment described.

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@workingjubilee workingjubilee Sep 26, 2024 •
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cool, I checked the hexdump emitted and yeah, it is indeed backwards on x86-64. silly Rust.

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