Wasmtime GitHub notifications bot (Apr 01 2026 at 14:26): Wasmtime GitHub notifications bot (Apr 01 2026 at 14:26):alexcrichton opened PR #12928 from alexcrichton:fiber-less-asan-false-positive to bytecodealliance:main:
This commit is aimed at fixing the ASAN false positives in #12899. Initially the fix there was to invoke some
__asan_*intrinsics, and I ended up finding a sort of smaller set of__asan_*intrinsics to call as well. In the end what's happening though is that fibers, upon terminating, have a few frames of Rust code on the stack before switching off. To ASAN these frames never returned so when a stack is subsequently reused ASAN is tricked into thinking this is buffer overflow or use-after-free since it's stomping on frames that haven't returned.The fix in this commit is to avoid this style of function which doesn't returns. Functions which don't return in Rust are easy to leak memory from and are a hazard from a safety perspective as well (e.g. it's unsafe to skip running destructors of stack variables). I feel we've had better success over time with "all Rust functions always return" and so what's what was applied here. Unlike #12899 or my thoughts on that PR this does not have any new
__asan_*intrinsic calls. Instead what this does is it shuffles around responsibility for what exact piece of the infrastructure is responsible for what. Specificallyfiber_startfunctions now actually return, meaning thewasmtime_fiber_startnaked function actually resumes execution, unlike before. Thewasmtime_fiber_startthen delegates towasmtime_fiber_switchimmediately to perform the final switch.Effectively there's now only two function frames that never return, and both of these frames are handwritten inline assembly. This means that ASAN gets to see that all normal functions return and updates all of its metadata accordingly. The end result is that the original issue from #12899 is fixed and this I feel is in general more robust as well.
One caveat is that the handwritten
wasmtime_fiber_startassembly needs to invoke a siblingwasmtime_fiber_switch_function. In lieu of trying to figure out how to get PIC-vs-not calls working (e.g. static calls) I've opted to use indirect function calls and pointers instead. This mirrors historical changes in our fiber implementation too.<!--
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Wasmtime GitHub notifications bot (Apr 01 2026 at 14:26):alexcrichton requested pchickey for a review on PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 14:26):alexcrichton requested wasmtime-core-reviewers for a review on PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 16:58):pchickey submitted PR review.
Wasmtime GitHub notifications bot (Apr 01 2026 at 17:35):alexcrichton added PR #12928 Update fibers to avoid no-return functions to the merge queue.
Wasmtime GitHub notifications bot (Apr 01 2026 at 18:03):github-merge-queue[bot] removed PR #12928 Update fibers to avoid no-return functions from the merge queue.
Wasmtime GitHub notifications bot (Apr 01 2026 at 18:19):alexcrichton updated PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 18:19):alexcrichton has enabled auto merge for PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 18:33):alexcrichton added PR #12928 Update fibers to avoid no-return functions to the merge queue.
Wasmtime GitHub notifications bot (Apr 01 2026 at 18:57):github-merge-queue[bot] removed PR #12928 Update fibers to avoid no-return functions from the merge queue.
Wasmtime GitHub notifications bot (Apr 01 2026 at 20:09):alexcrichton updated PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 20:09):alexcrichton has enabled auto merge for PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 20:22):alexcrichton added PR #12928 Update fibers to avoid no-return functions to the merge queue.
Wasmtime GitHub notifications bot (Apr 01 2026 at 20:56):alexcrichton merged PR #12928.
Wasmtime GitHub notifications bot (Apr 01 2026 at 20:56):alexcrichton removed PR #12928 Update fibers to avoid no-return functions from the merge queue.
Last updated: Apr 12 2026 at 23:10 UTC