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Fix UB in ColumnStringBlock::AppendUnsafe when appending empty string_view #489

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Fix UB in ColumnStringBlock::AppendUnsafe when appending empty string_view #489

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9 changes: 7 additions & 2 deletions clickhouse/columns/string.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -136,8 +136,13 @@ struct ColumnString::Block
std::string_view AppendUnsafe(std::string_view str) {
const auto pos = &data_[size];

memcpy(pos, str.data(), str.size());
size += str.size();
// memcpy's source pointer is declared nonnull regardless of the
// size argument, so an empty string_view backed by std::string()
// (where data() may be null) trips UBSan on every empty append.
Comment on lines 137 to +141
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Copilot AI Apr 26, 2026

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const auto pos = &data_[size]; can be undefined behavior when size == capacity (e.g., appending an empty string when the block is full, or when capacity==0), because data_[size] conceptually dereferences one-past-the-end. Prefer computing the pointer via data_.get() + size (and consider applying the same change in the other Block helpers that use &data_[size]), so zero-length operations don’t rely on out-of-bounds operator[] evaluation.

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Comment on lines +140 to +141
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Copilot AI Apr 26, 2026

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The comment suggests an empty std::string may have data() == nullptr, but standard std::string::data() is generally non-null even when empty. The nullptr case is definitely possible for an empty std::string_view (e.g., default-constructed std::string_view{} or a view built from a nullable C string), so it would be clearer/safer to describe the issue as “empty string_view can have null .data()” rather than tying it specifically to std::string().

Suggested change
// size argument, so an empty string_view backed by std::string()
// (where data() may be null) trips UBSan on every empty append.
// size argument, so an empty string_view with a null data() pointer
// trips UBSan on every empty append.

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if (str.size() > 0) {
memcpy(pos, str.data(), str.size());
size += str.size();
}

return std::string_view(pos, str.size());
}
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