miri: enforce proper types for c-variadic arguments in shims by RalfJung · Pull Request #161734 · rust-lang/rust

RalfJung · 2026-08-25T09:41:05Z

This adds proper type checks for c-variadic argument handling in Miri shims. So far we would have ICEd for many type mismatches since the innards of the interpreter generally assume that things are well-typed. The checks we are using are the same as for next_arg.

This uncovered some interesting points:

We should accept &mut as equivalent to *mut. We don't allow &mut as a type in next_arg but we do allow it for c-variadic calls, so this change also affects a bit more code in const-eval (if the caller passes an &mut and the callee expects a *mut). Even CFI is okay with treating &mut and *mut as equivalent so this should be fine. Cc @folkertdev
Some places in the standard library futex handling used types for their variadic arguments that are not quite correct according to these rules. I fixed those. Cc @m-ou-se
One variadic function takes pointers to libc::timespec. A program with a libc dependency has two copies of libc (one in the sysroot, and the normal dependency). When Miri looks up the expected argument type, it picks one of them, leading to Miri seeing a type mismatch when there is a call using the other copy. C generally considers two copies of the same type to be identical, so I implemented logic that does the same: two repr(C) types are "identical" if the names of the types, and the names and types of all fields, are identical. I then also used that logic for the normal ABI checks, where it will also help since we had similar trouble there in the past (that we had to find other work-arounds for).

rustbot · 2026-08-25T09:41:08Z

Some changes occurred to the CTFE / Miri interpreter

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These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

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miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

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Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @oli-obk, @lcnr

rustbot · 2026-08-25T09:41:11Z

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RalfJung · 2026-08-25T09:41:24Z

r? @saethlin

RalfJung · 2026-08-25T12:25:33Z

@bors try jobs=aux

miri: enforce proper types for c-variadic arguments in shims try-job: *aux*

rust-bors · 2026-08-25T14:53:58Z

☀️ Try build successful (CI)
Build commit: 7980ab1 (7980ab1bb9a58e8bf9c45898865099a8d7564457)
Base parent: cc05892 (cc05892c8346313865afd91ca12ee1fde6d3603c)

rustbot · 2026-09-03T12:13:37Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

RalfJung · 2026-09-07T10:32:49Z

r? @oli-obk

rustbot · 2026-09-07T10:32:54Z

oli-obk is not on the review rotation at the moment.
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oli-obk · 2026-09-09T11:20:06Z

@bors r+ rollup

rust-bors · 2026-09-09T11:20:08Z

📌 Commit 158f6f9 has been approved by oli-obk

It is now in the queue for this repository.

@folkertdev

miri: enforce proper types for c-variadic arguments in shims This adds proper type checks for c-variadic argument handling in Miri shims. So far we would have ICEd for many type mismatches since the innards of the interpreter generally assume that things are well-typed. The checks we are using are the same as for [`next_arg`](https://doc.rust-lang.org/nightly/std/ffi/struct.VaList.html#method.next_arg). This uncovered some interesting points: - We should accept `&mut` as equivalent to `*mut`. We don't allow `&mut` as a type in `next_arg` but we do allow it for c-variadic calls, so this change also affects a bit more code in const-eval (if the caller passes an `&mut` and the callee expects a `*mut`). Even CFI is okay with treating `&mut` and `*mut` as equivalent so this should be fine. Cc @folkertdev - Some places in the standard library futex handling used types for their variadic arguments that are not quite correct according to these rules. I fixed those. Cc @m-ou-se - One variadic function takes pointers to `libc::timespec`. A program with a libc dependency has two copies of libc (one in the sysroot, and the normal dependency). When Miri looks up the expected argument type, it picks one of them, leading to Miri seeing a type mismatch when there is a call using the other copy. C generally considers two copies of the same type to be identical, so I implemented logic that does the same: two `repr(C)` types are "identical" if the names of the types, and the names and types of all fields, are identical. I then also used that logic for the normal ABI checks, where it will also help since we had similar trouble there in the past (that we had to find other work-arounds for).

…uwer Rollup of 5 pull requests Successful merges: - #162470 (Subtree sync for rustc_codegen_cranelift) - #161734 (miri: enforce proper types for c-variadic arguments in shims) - #161821 (Add missing option to `-Zself-profile-event` help message as well as information about what the default options are) - #162453 (Minimize `DiagCtxt` methods) - #162528 (Add a mention to docs about promoting and demoting platform support)

@folkertdev

Rollup merge of #161734 - RalfJung:c-variadic-types, r=oli-obk miri: enforce proper types for c-variadic arguments in shims This adds proper type checks for c-variadic argument handling in Miri shims. So far we would have ICEd for many type mismatches since the innards of the interpreter generally assume that things are well-typed. The checks we are using are the same as for [`next_arg`](https://doc.rust-lang.org/nightly/std/ffi/struct.VaList.html#method.next_arg). This uncovered some interesting points: - We should accept `&mut` as equivalent to `*mut`. We don't allow `&mut` as a type in `next_arg` but we do allow it for c-variadic calls, so this change also affects a bit more code in const-eval (if the caller passes an `&mut` and the callee expects a `*mut`). Even CFI is okay with treating `&mut` and `*mut` as equivalent so this should be fine. Cc @folkertdev - Some places in the standard library futex handling used types for their variadic arguments that are not quite correct according to these rules. I fixed those. Cc @m-ou-se - One variadic function takes pointers to `libc::timespec`. A program with a libc dependency has two copies of libc (one in the sysroot, and the normal dependency). When Miri looks up the expected argument type, it picks one of them, leading to Miri seeing a type mismatch when there is a call using the other copy. C generally considers two copies of the same type to be identical, so I implemented logic that does the same: two `repr(C)` types are "identical" if the names of the types, and the names and types of all fields, are identical. I then also used that logic for the normal ABI checks, where it will also help since we had similar trouble there in the past (that we had to find other work-arounds for).

…uwer Rollup of 5 pull requests Successful merges: - rust-lang/rust#162470 (Subtree sync for rustc_codegen_cranelift) - rust-lang/rust#161734 (miri: enforce proper types for c-variadic arguments in shims) - rust-lang/rust#161821 (Add missing option to `-Zself-profile-event` help message as well as information about what the default options are) - rust-lang/rust#162453 (Minimize `DiagCtxt` methods) - rust-lang/rust#162528 (Add a mention to docs about promoting and demoting platform support)

The nightly Miri from 2026-09-09 (rust-lang/rust#161734) type-checks c-variadic arguments in its shims, and rustix 1.1.4's libc-backend futex wrapper passes the lock word as `*const AtomicU32` where the `syscall` shim expects `*mut u32`. That fails the `c_abi_miri` test whenever an engine mutex is contended. Real Linux builds keep the futex path; Miri takes the yield-based park that every other Unix already uses. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 25, 2026

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rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 25, 2026

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RalfJung changed the title ~~C variadic types~~ miri: enforce proper types for c-variadic arguments in shims Aug 25, 2026

RalfJung force-pushed the c-variadic-types branch 2 times, most recently from 8cec627 to 888931c Compare August 25, 2026 10:17

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RalfJung force-pushed the c-variadic-types branch from 3a365e2 to 1b22718 Compare August 25, 2026 12:06

rust-bors Bot pushed a commit that referenced this pull request Aug 25, 2026

Auto merge of #161734 - RalfJung:c-variadic-types, r=<try>

7980ab1

miri: enforce proper types for c-variadic arguments in shims try-job: *aux*

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RalfJung force-pushed the c-variadic-types branch from 1b22718 to 329d83d Compare August 25, 2026 20:03

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rustbot removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. has-merge-commits PR has merge commits, merge with caution. labels Aug 25, 2026

miri: enforce proper types for c-variadic arguments in shims

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RalfJung force-pushed the c-variadic-types branch from 329d83d to 16e8469 Compare September 3, 2026 12:13

share identical-c-type logic with regular ABI checks

158f6f9

RalfJung force-pushed the c-variadic-types branch from 16e8469 to 158f6f9 Compare September 3, 2026 12:18

RalfJung mentioned this pull request Sep 4, 2026

Refactor syscall shim with a macro rust-lang/miri#5303

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rustbot assigned oli-obk and unassigned saethlin Sep 7, 2026

oli-obk approved these changes Sep 9, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 9, 2026

Zalathar mentioned this pull request Sep 9, 2026

Rollup of 3 pull requests #162527

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JonathanBrouwer mentioned this pull request Sep 9, 2026

Rollup of 5 pull requests #162532

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rust-bors Bot merged commit 8b9c125 into rust-lang:main Sep 9, 2026
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rustbot added this to the 1.100.0 milestone Sep 9, 2026

DRMacIver mentioned this pull request Sep 10, 2026

Spin instead of futex-parking under Miri on Linux hegeldev/hegel-rust#484

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