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Report a host limit for memories too large for the shell to allocate #8898

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Merged
kripken merged 2 commits into from
Jul 13, 2026
+31 −1
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Report a host limit for memories too large for the shell to allocate #8898

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9f250c8
Report a host limit for memories too large for the shell to allocate
JPL11 Jul 13, 2026
0433046
Use fuzz-exec-before in the test, which runs the module once
JPL11 Jul 13, 2026
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18 changes: 17 additions & 1 deletion src/shell-interface.h
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Original file line number Diff line number Diff line change
Expand Up @@ -90,8 +90,24 @@ struct ShellExternalInterface : ModuleRunner::ExternalInterface {

void init(Module& wasm, ModuleRunner& instance) override {
ModuleUtils::iterDefinedMemories(wasm, [&](wasm::Memory* memory) {
// Compute the initial size in bytes carefully: the byte size of a
// maximal memory64 (2^48 pages of 64KiB) does not fit in 64 bits. If
// we let it wrap, instantiation would "succeed" with an empty buffer
// and every later access would spuriously trap as out of bounds.
uint64_t bytes = uint64_t(memory->initial) << memory->pageSizeLog2;
if ((bytes >> memory->pageSizeLog2) != uint64_t(memory->initial)) {
// The byte size is not even representable; we certainly cannot
// allocate it, just like a real VM could not.
hostLimit("memory too large");
}
Comment on lines +97 to +102

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@JPL11 JPL11 Jul 13, 2026

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That works for validated modules: as we established on #8882, under the validator's bound the only value the shift can wrap to is exactly 0. The round trip form additionally stays correct for modules that skipped validation (--no-validation), where an out of range initial can wrap to a nonzero value that !bytes would silently accept as a real size. Since both compile to the same couple of instructions I kept the self contained one, but happy to switch if @kripken prefers the shorter form.

@kripken kripken Jul 13, 2026

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We can assume that modules being executed are valid. The only place a module might be invalid is during parsing, basically. But I think the current code is clearer as it stands, as it is self-contained - the short form would need a longer comment, in my opinion.

@kripken kripken Jul 13, 2026

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Also, the validation limit might change if the spec changes, in theory...

auto shellMemory = Memory();
shellMemory.resize(memory->initial << memory->pageSizeLog2);
try {
shellMemory.resize(bytes);
} catch (const std::exception&) {
// bad_alloc or length_error from the underlying vector: the memory
// is representable but too large to actually allocate.
hostLimit("memory too large");
}
memories[memory->name] = shellMemory;
});
}
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14 changes: 14 additions & 0 deletions test/lit/exec/memory64-max-size.wast
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@@ -0,0 +1,14 @@
;; NOTE: Assertions have been generated by update_lit_checks.py --output=fuzz-exec and should not be edited.

;; RUN: wasm-opt %s -all --fuzz-exec-before -q -o /dev/null 2>&1 | filecheck %s

;; A maximal memory64 declares 2^48 pages, exactly 2^64 bytes, which cannot be
;; allocated (and whose byte size does not even fit in 64 bits). Instantiation
;; must fail as a host limit, not wrap the size to zero and then spuriously
;; trap on the active segment (which optimizations may legitimately remove,
;; changing the spurious behavior).
(module
;; CHECK: [host limit memory too large]
(memory $0 i64 281474976710656 281474976710656 shared)
(data $0 (i64.const 0) "\00")
)
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