Unwinding through FFI after Rust 1.33

@gnzlbg in retrospect this makes sense, but also seems to indicate that a no_std dynamic library would potentially be in quite a bit of trouble.

Just trying to understand the possible cases here, it seems like:

  • std:
    • The panic happens
    • The panic_hook is executed
    • The panic runtime is executed, which is either an unwind or an abort
  • no_std:
    • The panic happens
    • The program immediately jumps to the panic_handler function and must never return.

Is this correct?

Assuming so, here’s two annoying questiond that Rust probably needs an answer to because it can happen:

  • What happens if std rust code calls into a dynamic lib with no_std and a panic_handler? My guess, the lib uses its panic_handler and never returns
  • What happens if std Rust calls a std dynamic lib? will the right panic_hook get used? even across compiler versions? Guess: it’s probably totally broken if that happens?

Lots of things that can’t be checked need to align for it to work (e.g. what happens if the two Rust crates linked use the same panic implementation but a different memory allocator due to a different toolchain version / linking issue, and one crates tries to free the panic payload of the other crate, etc.).

All of the EH ABIs specify that the exception record contains some field that is used to destroy the exception record. You could check the exact function pointers to figure out if the Rust exception record could have a mismatched memory allocator issue.

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If this function is exactly a call to free, this dynamic check can work.

Otherwise, what happens if this function is statically linked into each dynamic library ? Two copies of the function will have different function pointers, yet they might be the exact same function. I’d guess we could maybe force these to always be dynamically linked, not sure.

The lang team is requesting users to participate in forming the solution:


The fact that there’s no well-defined way to trigger SEH on Windows, which is what libjpeg and libpng (the libraries originally under discussion) need to do, contradicts your assertion that “this problem has always had a solution that works correctly and reliably 100% of the time”.

How come? What works 100% of the time is using an error code in FFI, converting from/to each language error handling mechanism. On functions exposed from Rust to FFI you catch panics and return an error code, and in FFI functions called from rust you raise the error code as a panic. On the other side of FFI, e.g., if it is C++, you catch all exceptions in exposed functions and return error codes, and when calling FFI you raise the error codes as exceptions.

Rust does not need to be able to raise / catch C++ exceptions using this method.

In the libjpeg example, where you have Rust -> C -> Rust, you need your extern "C" Rust functions to return an error code, then write a C wrapper for it that raises a SEH, then write a C wrapper that catches the C in the code that calls libjpeg, and then call that wrapper from Rust, so you end up with: Rust -> C libjpeg wrapper -> libpeg -> C wrapper over Rust callback -> Rust callback.

You don’t need to be able to trigger SEH from Rust for that to work, and that always works, because you only use the native error reporting mechanism of each language, and via FFI only error codes are propagated.

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Okay, I actually did not think of this. That does seem like it should be legal and solve the problem, though I’d be curious about the performance impact.

Having to write multiple C wrappers adds a substantial amount of pain, here. I still think we need a mechanism to handle this in Rust.

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Perhaps; but in the interim, the C wrappers provide a stable mechanism to achieve the desired effect. Thus, I think it would be reasonable to change to abort now and importantly to close the existing soundness hole in the language.


We had this argument, and I don’t particularly want to have it again. The conclusion in the last round of the argument was “let’s time-bound this to the next couple of releases, and within that time work with the actual developers of code using this to find a solution, and when we have a reasonable solution we can push this again”.

So, let’s actually work with the developers of this code and try to find a solution. (Also, let’s not split that discussion between multiple places.)

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I’m drafting an RFC for #[unwind(Rust)] as suggested in the tracking issue linked above.

Here is what I have so far; suggestions are welcome: https://github.com/rust-lang/rfcs/blob/annotate-unwind-rust/text/XXX-annotate-unwind-rust.md

I’d like to rewrite my RFC to be more strictly directed towards the FFI interoperability use case, and I’ve written up my current thoughts in comments on the GitHub discussion, but I believe I need some constructive feedback before I can move forward productively. @gnzlbg, @kornel, @jcranmer, @josh, @mjbshaw, any thoughts?

The RFC link, again: https://github.com/rust-lang/rfcs/blob/annotate-unwind-rust/text/XXX-annotate-unwind-rust.md

Based on some of the feedback so far:

  1. I’d suggest changing it to unwind(system).
  2. You need a feature name; perhaps unwind_system.
  3. You need to explicitly mention the LLVM tagging of functions as nounwind, and that this attribute will make a function not have nounwind. The current text doesn’t clearly make that point. The goal is to be able to mark extern functions as nounwind, which allows additional optimizations, and then not mark functions as nounwind if they have this new attribute. This should appear as a clarification in the motivation, and as a replacement for the current reference-level explanation. (You can state in the abstract that functions with this attribute are allowed to unwind, and that concretely on LLVM this means not tagging them nounwind.)
  4. Drop the point that Using this annotation **does not** provide any guarantees about the unwinding implementation, in favor of explaining that this is designed for and only supported for the use case of Rust unwinding through C (or similar code without any destructors or other things to unwind) to Rust code, and catching (in Rust) a panic raised in Rust. In particular, it isn’t guaranteed to call C/C++ destructors, and you can’t expect to catch C++ exceptions or Windows SEH exceptions with it. But it should be designed to work with the C case.

That should address some of the major objections I’ve seen.


Thanks for the feedback on the existing RFC, but I am still leaning towards rewriting it entirely. Currently, I am planning torrecommend stabilization of #[unwind(allowed)] rather than introducing a new annotation, and creating some kind of association between that annotation and the panic implementation selected.

I am trying to investigate how this can be done and what it would mean from an implementation perspective before I actually tackle rewriting the RFC, though.


@kornel @josh I have finished the major rewrite; please take a look!

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Do you have a link to the latest revision? The previous links all 404.

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My mistake, those are to the actual file blob in my branch. Here’s the pull request: https://github.com/rust-lang/rfcs/pull/2699