fix(vm): treat unallocated tables as empty instead of panicking - #175
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fix(vm): treat unallocated tables as empty instead of panicking#175dmitry123 wants to merge 1 commit into
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Criterion results (vs baseline)Heads-up: runner perf is noisy; treat deltas as a smoke check. |
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Fixes FLU-1097.
Problem
A verified rwasm module can reference a table index it never grows, and every table op then panicked the interpreter instead of trapping:
Tables enter
RwasmStore::tablesonly when the module executesTableGrow— the only op that used.entry(idx).or_default(). Verification accepts any index< N_MAX_TABLESand cannot do better, since table indices carry no declaration. Nine call sites acrosstable.rsandcontrol_flow.rsdid.expect("rwasm: unresolved table segment")/.expect("rwasm: missing table"), so in a node a bad contract became a process crash — mid-execution, leaving the store with a dirty value stack, call stack, andlast_signature.Fix
All table lookups go through a single
resolve_tablehelper that materializes a missing table as an empty one, so an ungrown table behaves as a zero-length table. This matches Wasm semantics more closely than trapping outright:table.sizereturns 0,table.fill/table.copywithn == 0succeed, and every element access is bound-checked intoTrapCode::TableOutOfBoundsbyTableEntity. Behaviour for translator-produced modules is unchanged —emit_table_segmentemits aTableGrowper declared table, so the tables always exist.Unresolved syscall index (4a)
Left as-is per the discussion on the ticket: an index the import linker can't resolve is a fatal error, not a trap. The params/results counts come from the linker, so an unresolved index leaves no way to know how many values to pop, and any guess would desynchronize the value stack — zkVM proving needs the same trace on every run. Restricting the reachable syscall set is the linker's job, and
always_failing_syscall_handleris how a declared-but-forbidden syscall is rejected.invoke_syscallnow carries a doc comment explaining this.Tests
New
tests/tables.rs— 12 cases, each building a module that passesnew_verifiedand then executing it:table.sizeof an ungrown table returns 0table.get/table.set/table.fill/table.copy(across tables and within one) trap withTableOutOfBoundstable.fill/table.copyon ungrown tables succeed, per Wasm semanticstable.initinto an ungrown table trapscall_indirect/return_call_indirectthrough an ungrown table trapcargo testandcargo clippy --all-targets --all-featuresare clean. The two--all-featuresfailures intests/wasmtime.rs(test_wasmtime_disabled_f32_sqrt,test_wasmtime_disabled_f64_div) are pre-existing ondeveland unrelated — they were verified to fail on a clean checkout too.