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Fix UB on empty trampoline template entries #112

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Merged
GJDuck merged 1 commit into from
Jun 28, 2026
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Fix UB on empty trampoline template entries #112

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3 changes: 2 additions & 1 deletion src/e9patch/e9json.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -207,7 +207,8 @@ static Trampoline *dupTrampoline(const std::vector<Entry> &entries)
T->prot = PROT_READ | PROT_EXEC;
T->preload = false;
T->num_entries = num_entries;
memcpy(T->entries, &entries[0], num_entries * sizeof(Entry));
if (num_entries > 0)
memcpy(T->entries, entries.data(), num_entries * sizeof(Entry));

@McSinyx McSinyx Jun 27, 2026

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Hi, just want to note that the branching is not necessary here (and on whether it's desirable, I don't have any informed opinion).

@ehgus607 ehgus607 Jun 28, 2026

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The error message in my description is from libstdc++ debug mode. I also checked with UBSan, and it shows an error message for this exact same access. Accessing index 0 of an empty vector is an Undefined Behavior according to the C++ standard.

I agree that this bug might not be very critical in real situations. However, considering memory safety, I think it is better to keep this branch to prevent the UB explicitly.

@GJDuck GJDuck Jun 29, 2026

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I think @McSinyx was just pointing out that the branch if (num_entries > 0) could be technically removed from the patch, since if num_entries == 0 then entries.data() is still defined and memcpy is a nop. The original code was clearly UB however.


static std::set<Trampoline *, TrampolineCmp> cache;
auto i = cache.insert(T);
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