ffi-unwind #19

nikomatsakis · 2020-06-11T19:18:04Z

Summary

Working out the rules for cross-language unwinding as well as longjmp.

Status

This section lists the efforts that we are working towards.

"C-unwind" ABI

The "C-unwind" ABI allows you to invoke foreign functions that may unwind using the native ABI.

Specification: RFC: 'C-unwind' ABI rfcs#2945 has been merged. This RFC introduces the "C-unwind" ABI.
Implementation: We are looking for someone to implement! The tracking issue for the implementation work is Tracking Issue for "C-unwind ABI", RFC 2945 rust#74990.

Interaction with `catch_unwind`

We need to define what happens when foreign exceptions interact with catch_unwind.

longjmp compatibility

We need to define the conditions in which it is legal to longjmp through Rust frames. We've done some initial conversation about this.

Links

Repository
Zulip-stream: #project-ffi-unwind

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nikomatsakis · 2020-06-11T21:58:28Z

2020-04-02: Working towards RFC

nikomatsakis · 2020-06-11T21:58:36Z

2020-04-09: BatmanAod opened a PR with draft of RFC

nikomatsakis · 2020-06-11T21:58:49Z

2020-04-30:
- The only question is whether to make it “defined behavior” if a foreign except propagates across a “C unwind” boundary (and there is no catch_unwind)
- Advantage: we probably want to support this
- Disadvantage: it will require a shim to catch C++ exceptions — this is probably necessary later, unless we reverse the “two ABIs” proposal
- Interesting downside of the “two ABI” design
- If you have a C function but it takes callbacks, then there could potentially be pressure to make callbacks into “C unwind” and the function too (for flexibility)
- but note that this is not a trivial change, necessarily, and indeed many C libraries are not ready for this to happen “just because”
- True, but just a downside of this approach.

nikomatsakis · 2020-06-11T21:58:57Z

2020-06-01:
- Plain Old Frames — for frames that don’t have a pending destructor or catch_unwind

nikomatsakis · 2020-06-11T22:00:30Z

2020-06-11:

We uncovered a few interesting things in discussing the latest draft. The most important is that we are not comfortable saying that longjmp functions can unwind Plain Old Frames (POF) without any further annotation, because that will hinder our efforts to optimize. For the time being, we plan to edit the RFC to leave the details of when longjmp is legal as TBD, but we discussed some possible alternative proposals here rust-lang/project-ffi-unwind#30.

nikomatsakis · 2020-06-22T19:19:36Z

2020-06-22:

RFC 2945 opened
Not just about "C" vs "C unwind" ABIs, but we currently use #[unwind(allowed)] for "system":

    extern "system" {
        #[unwind(allowed)]
        pub fn _CxxThrowException(pExceptionObject: *mut c_void, pThrowInfo: *mut u8) -> !;
    }

Options:
- add "system unwind", "stdcall unwind", etc
- maybe "system" itself is redundant

nikomatsakis · 2020-07-06T19:25:54Z

2020-07-06:

RFC is believed to be ready for FCP, needs someone from lang-team to review and fcp merge.
We clarified that other ABIs (e.g., system) may have unwind variants (e.g., extern "system unwind").
Josh: If there were an ABI that is used in some context where unwinding is always used, it's not useful to have a non-unwind variant of it, in practice there is often a "defacto no-unwind" guarantee. (Niko: but we may want to call it "foo unwind", even if there is no "foo").

nikomatsakis · 2020-07-20T19:23:41Z

2020-07-20:

RFC has just entered FCP.
Bikeshedding now about "C unwind" vs "C-unwind"

nikomatsakis · 2020-07-27T19:09:53Z

2020-07-27:

Bikeshed resolved ABI name to "C-unwind".

nikomatsakis · 2020-07-31T21:16:31Z

2020-07-31:

RFC: 'C-unwind' ABI rfcs#2945 is merged and tracking issue Tracking Issue for "C-unwind ABI", RFC 2945 rust#74990 is opened.

nikomatsakis · 2020-09-14T19:22:05Z

2020-09-14:

Implementation work is ongoing, we may start looking at the longjmp question.

nikomatsakis · 2020-09-21T19:24:56Z

2020-09-21:

No update.

joshtriplett · 2020-09-28T19:24:00Z

Discussion in today's meeting:

New discussion and proposals for handling "target ABI"
- e.g. target for native iOS vs target for iOS simulator, which have different ABIs
- Rust doesn't have a way to express this today

This isn't directly in the current ffi-unwind charter. Rather than expanding it (which we'd do to cover c++/longjmp/etc), we may need to consider chartering an additional project for new FFI issues.

joshtriplett · 2020-09-28T19:24:40Z

(Not sure why this unassigned @BatmanAoD; I didn't change anything there.)

nikomatsakis · 2020-10-08T17:51:29Z

Update:

@BatmanAoD plans to write a blog post setting the direction for work on functions that can be deallocated ("canceled") with longjmp &c.
@katie-martin-fastly hasn't had time to work on Implement RFC 2945: "C-unwind" ABI rust#76570 recently but hopes to get some more time soon =)

nikomatsakis · 2020-11-17T18:19:32Z

2020-11-17:

Previous comment still basically accurate.

nikomatsakis · 2020-12-15T18:11:45Z

Update: PR is ready for review, and we'll have a longjmp blog post going out soon

…945-c-unwind-abi, r=Amanieu Implement RFC 2945: "C-unwind" ABI ## Implement RFC 2945: "C-unwind" ABI This branch implements [RFC 2945]. The tracking issue for this RFC is rust-lang#74990. The feature gate for the issue is `#![feature(c_unwind)]`. This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19. ### Changes Further details will be provided in commit messages, but a high-level overview of the changes follows: * A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`, and `Thiscall` variants, marking whether unwinding across FFI boundaries is acceptable. The cases where each of these variants' `unwind` member is true correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and `thiscall-unwind` ABI strings introduced in RFC 2945 [3]. * This commit adds a `c_unwind` feature gate for the new ABI strings. Tests for this feature gate are included in `src/test/ui/c-unwind/`, which ensure that this feature gate works correctly for each of the new ABIs. A new language features entry in the unstable book is added as well. * We adjust the `rustc_middle::ty::layout::fn_can_unwind` function, used to compute whether or not a `FnAbi` object represents a function that should be able to unwind when `panic=unwind` is in use. * Changes are also made to `rustc_mir_build::build::should_abort_on_panic` so that the function ABI is used to determind whether it should abort, assuming that the `panic=unwind` strategy is being used, and no explicit unwind attribute was provided. [RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

…d-abi, r=Amanieu Implement RFC 2945: "C-unwind" ABI ## Implement RFC 2945: "C-unwind" ABI This branch implements [RFC 2945]. The tracking issue for this RFC is rust-lang#74990. The feature gate for the issue is `#![feature(c_unwind)]`. This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19. ### Changes Further details will be provided in commit messages, but a high-level overview of the changes follows: * A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`, and `Thiscall` variants, marking whether unwinding across FFI boundaries is acceptable. The cases where each of these variants' `unwind` member is true correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and `thiscall-unwind` ABI strings introduced in RFC 2945 [3]. * This commit adds a `c_unwind` feature gate for the new ABI strings. Tests for this feature gate are included in `src/test/ui/c-unwind/`, which ensure that this feature gate works correctly for each of the new ABIs. A new language features entry in the unstable book is added as well. * We adjust the `rustc_middle::ty::layout::fn_can_unwind` function, used to compute whether or not a `FnAbi` object represents a function that should be able to unwind when `panic=unwind` is in use. * Changes are also made to `rustc_mir_build::build::should_abort_on_panic` so that the function ABI is used to determind whether it should abort, assuming that the `panic=unwind` strategy is being used, and no explicit unwind attribute was provided. [RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

…abi, r=Amanieu Implement RFC 2945: "C-unwind" ABI ## Implement RFC 2945: "C-unwind" ABI This branch implements [RFC 2945]. The tracking issue for this RFC is rust-lang#74990. The feature gate for the issue is `#![feature(c_unwind)]`. This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19. ### Changes Further details will be provided in commit messages, but a high-level overview of the changes follows: * A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`, and `Thiscall` variants, marking whether unwinding across FFI boundaries is acceptable. The cases where each of these variants' `unwind` member is true correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and `thiscall-unwind` ABI strings introduced in RFC 2945 [3]. * This commit adds a `c_unwind` feature gate for the new ABI strings. Tests for this feature gate are included in `src/test/ui/c-unwind/`, which ensure that this feature gate works correctly for each of the new ABIs. A new language features entry in the unstable book is added as well. * We adjust the `rustc_middle::ty::layout::fn_can_unwind` function, used to compute whether or not a `FnAbi` object represents a function that should be able to unwind when `panic=unwind` is in use. * Changes are also made to `rustc_mir_build::build::should_abort_on_panic` so that the function ABI is used to determind whether it should abort, assuming that the `panic=unwind` strategy is being used, and no explicit unwind attribute was provided. [RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

…anieu Implement RFC 2945: "C-unwind" ABI ## Implement RFC 2945: "C-unwind" ABI This branch implements [RFC 2945]. The tracking issue for this RFC is #74990. The feature gate for the issue is `#![feature(c_unwind)]`. This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19. ### Changes Further details will be provided in commit messages, but a high-level overview of the changes follows: * A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`, and `Thiscall` variants, marking whether unwinding across FFI boundaries is acceptable. The cases where each of these variants' `unwind` member is true correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and `thiscall-unwind` ABI strings introduced in RFC 2945 [3]. * This commit adds a `c_unwind` feature gate for the new ABI strings. Tests for this feature gate are included in `src/test/ui/c-unwind/`, which ensure that this feature gate works correctly for each of the new ABIs. A new language features entry in the unstable book is added as well. * We adjust the `rustc_middle::ty::layout::fn_can_unwind` function, used to compute whether or not a `FnAbi` object represents a function that should be able to unwind when `panic=unwind` is in use. * Changes are also made to `rustc_mir_build::build::should_abort_on_panic` so that the function ABI is used to determind whether it should abort, assuming that the `panic=unwind` strategy is being used, and no explicit unwind attribute was provided. [RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

BatmanAoD · 2021-04-07T03:56:53Z

2021-04-06:

"C-unwind" PR was merged!
- This accidentally introduced a change to "C" unwind behavior even without the feature flag; we will need to fix this before the next stable release. We have alerted the release team and are working on a fix.
- There are a few more follow-up issues to address, listed here.
a longjmp blog post was published, but we are prioritizing the "C-unwind" issues for now

nikomatsakis · 2021-04-14T17:46:31Z

Lang team planning meeting update:

We should promote this project to "evaluation"
On the topic of longjmp:
- Josh: Fairly unusual to see people using longjmp these days, should not sacrifice optimizablity
- Josh: But it should be possible -- with enough care -- to longjmp without UB
- Amanieu: this also affects functions that call pthread_exit
Would be nice to open a PR and update the charter to include new scope (can link to the blog post)

BatmanAoD · 2021-05-18T20:01:06Z

updated charter
Follow-up issues for "C-unwind" remain our top priority.
- In particular, we need to implement the specified "abort-on-unwind" behavior for "C-unwind" FFI calls in panic=abort mode.
Regarding longjmp, we believe there's an LLVM bug causing incorrect code-gen when strict-aliasing is enabled in a frame that gets discarded by longjmp. We haven't yet discussed how to move forward, though we plan to open a bug report for it.

nikomatsakis · 2021-07-07T17:03:09Z

Update:

@alexcrichton has a PR fixing various bugs and implementing the intended semantics, under review
Plan of record is to
- remove the UB from panics in "C" while adding "C-unwind" as a target
- once "C-unwind" is stabilized -- hopefully soon! -- add back in the UB for panics in "C"

nikomatsakis · 2022-02-01T20:36:35Z

Closing in favor of rust-lang/project-ffi-unwind#36

nikomatsakis added the lang-initiative An active lang team initiative label Jun 11, 2020

nikomatsakis changed the title ~~ffi-unwind project group~~ ffi-unwind Jun 11, 2020

nikomatsakis assigned nikomatsakis and BatmanAoD Jun 11, 2020

nikomatsakis mentioned this issue Jul 31, 2020

Tracking Issue for "C-unwind ABI", RFC 2945 rust-lang/rust#74990

Closed

11 tasks

cratelyn mentioned this issue Sep 10, 2020

Implement RFC 2945: "C-unwind" ABI rust-lang/rust#76570

Merged

joshtriplett unassigned BatmanAoD Sep 28, 2020

nikomatsakis mentioned this issue Oct 7, 2020

[RFC]: Libs Team Governance rust-lang/rfcs#2979

Closed

nikomatsakis added this to Initiatives Feb 1, 2022

nikomatsakis moved this to Development in Initiatives Feb 1, 2022

nikomatsakis mentioned this issue Feb 1, 2022

tracking issue for ffi-unwind rust-lang/project-ffi-unwind#36

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nikomatsakis closed this as completed Feb 1, 2022

Repository owner moved this from Development to Feature Complete in Initiatives Feb 1, 2022

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ffi-unwind #19

ffi-unwind #19

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ffi-unwind #19

ffi-unwind #19

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Summary

Status

"C-unwind" ABI

Interaction with catch_unwind

longjmp compatibility

Links

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Interaction with `catch_unwind`

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