Configure async task context on realloc calls#13949
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alexcrichton merged 3 commits intoJul 24, 2026
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This commit is an implementation of WebAssembly/component-model#680 which is a semantic change for invocations of the `realloc` canonical ABI option. Previously when this function was invoked the configured context-storage was whatever happened to run last in the store and in general wasn't well-defined. Additionally the context of the thread being run in was whatever happened to run last. The goal of the upstream spec change, and this commit, is to fully define what happens in this situation. Specifically: * Invocations of `realloc` always start with `context` slots set to 0. * The `thread.index` intrinsics, part of the component-model-threading proposal, is now no longer exempt from may-leave checks. These changes combined mean that it's not actually possible for `realloc` to witness anything about its thread identity. This means that we don't actually have to allocate anything, all that's necessary is to save/restore context around invocation of `cabi_realloc`. While this is a breaking change it's not expected to be observable in the ecosystem. This only affects `context` slots which are primarily only used for the WASIp3 ABI as the stack pointer and TLS base. There is no shipping WASIp3 target anywhere right now, so this breakage should not be observable.
cfallin
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Jul 23, 2026
| bail!("realloc return: beyond end of memory") | ||
| } | ||
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| *self.store.0.vm_store_context_mut().component_context_mut() = orig_context; |
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Do we need a drop-guard to restore the context if an error is returned (e.g. the bail! above), or are those always trap-cases I guess?
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We should be good in this case because once a component traps everything internally is inaccessible and no longer able to be seen by a guest. There's a fair number of locations in concurrent.rs that do this as well (don't restore on trap/panic) as well. I've added a comment now to that effect though for future readers
alexcrichton
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Heh well @dicej maybe your Windows-specific failure wasn't Windows-specific after all (failing test on macOS) |
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* Update some required features for tests * Flag `memory64.wast` as now-passing * Temporarily allow `post-return.wast` to fail while bytecodealliance#13949 and WebAssembly/component-model#680 are being aligned. * Fix a debug assert tripping erroneously in an async `subtask.cancel` intrinsic.
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This commit is an update to the `cabi_realloc` export configured by wasi-libc, specifically for wasip3 targets. The change here is to change the entrypoint from a C function to a small assembly wrapper which configures the stack pointer and TLS base before calling into C. This is to account for the spec change in WebAssembly/component-model#680 where `realloc` hooks are now going to be invoked with context slots set to 0. This means that `cabi_realloc` runs with no stack, and this would quickly abort in wasi-libc otherwise. The change here is to, when the ABI is using `context` slots for the stack pointer, have an assembly wrapper that configures a known-good stack and TLS block for `cabi_realloc` to use. Once configured the normal C-based `cabi_realloc` routine runs. This is tested against bytecodealliance/wasmtime#13949 to ensure it works on the p3 target with/without coop threading and before/after that Wasmtime change.
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* Configure async task context on `realloc` calls This commit is an implementation of WebAssembly/component-model#680 which is a semantic change for invocations of the `realloc` canonical ABI option. Previously when this function was invoked the configured context-storage was whatever happened to run last in the store and in general wasn't well-defined. Additionally the context of the thread being run in was whatever happened to run last. The goal of the upstream spec change, and this commit, is to fully define what happens in this situation. Specifically: * Invocations of `realloc` always start with `context` slots set to 0. * The `thread.index` intrinsics, part of the component-model-threading proposal, is now no longer exempt from may-leave checks. These changes combined mean that it's not actually possible for `realloc` to witness anything about its thread identity. This means that we don't actually have to allocate anything, all that's necessary is to save/restore context around invocation of `cabi_realloc`. While this is a breaking change it's not expected to be observable in the ecosystem. This only affects `context` slots which are primarily only used for the WASIp3 ABI as the stack pointer and TLS base. There is no shipping WASIp3 target anywhere right now, so this breakage should not be observable. * Fix conditional builds * Add a note about restoration
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* Update some required features for tests * Flag `memory64.wast` as now-passing * Temporarily allow `post-return.wast` to fail while bytecodealliance#13949 and WebAssembly/component-model#680 are being aligned. * Fix a debug assert tripping erroneously in an async `subtask.cancel` intrinsic.
alexcrichton
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This commit is an update to the `cabi_realloc` export configured by wasi-libc, specifically for wasip3 targets. The change here is to change the entrypoint from a C function to a small assembly wrapper which configures the stack pointer and TLS base before calling into C. This is to account for the spec change in WebAssembly/component-model#680 where `realloc` hooks are now going to be invoked with context slots set to 0. This means that `cabi_realloc` runs with no stack, and this would quickly abort in wasi-libc otherwise. The change here is to, when the ABI is using `context` slots for the stack pointer, have an assembly wrapper that configures a known-good stack and TLS block for `cabi_realloc` to use. Once configured the normal C-based `cabi_realloc` routine runs. This is tested against bytecodealliance/wasmtime#13949 to ensure it works on the p3 target with/without coop threading and before/after that Wasmtime change.
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This commit is an implementation of WebAssembly/component-model#680 which is a semantic change for invocations of the
realloccanonical ABI option. Previously when this function was invoked the configured context-storage was whatever happened to run last in the store and in general wasn't well-defined. Additionally the context of the thread being run in was whatever happened to run last. The goal of the upstream spec change, and this commit, is to fully define what happens in this situation. Specifically:reallocalways start withcontextslots set to 0.thread.indexintrinsics, part of the component-model-threading proposal, is now no longer exempt from may-leave checks.These changes combined mean that it's not actually possible for
reallocto witness anything about its thread identity. This means that we don't actually have to allocate anything, all that's necessary is to save/restore context around invocation ofcabi_realloc.While this is a breaking change it's not expected to be observable in the ecosystem. This only affects
contextslots which are primarily only used for the WASIp3 ABI as the stack pointer and TLS base. There is no shipping WASIp3 target anywhere right now, so this breakage should not be observable.