Go: fix CFG bug - implicit field selections with depth 0 - #22670
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🟢 Approval recommended
The depth guard matches CFG indexing semantics, and the regression test covers the reported false positive.
Review effort: Balanced
Findings: None
What changed in this PR
Fixes Go CFG generation so direct field and method selections do not create invalid depth-zero implicit-field nodes, preventing false unreachable-statement alerts.
Changes:
- Restricts implicit selections to positive depths.
- Adds a regression test using recursive embedding.
| File | Description |
|---|---|
go/ql/lib/semmle/go/controlflow/ControlFlowGraphImpl.qll |
Excludes depth-zero implicit selections. |
go/ql/test/query-tests/RedundantCode/UnreachableStatement/main.go |
Tests reachability for a direct field on a recursively embedded type. |
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michaelnebel
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Looks plausible to me.
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This fixes a bug where CFG nodes are made for implicit field selections with depth 0 when there is a recursive struct. The intention was that they should only exist for depth >= 1. This bug was introduced when migrating Go to the new shared CFG library. It caused FPs in go/unreachable-statement. A test has been added to demonstrate the bug and show that it is fixed.
Here is a bit of context about implicit field selections. Go doesn't have inheritance of struct types. Instead of having B extend A, you can put an unnamed field in B with type A, and then you can access any fields of A from B as if they were fields of A. So, if A has a field named f, then you can access it explicitly as
b.(A).f, but you would normally just writeb.f. To model this correctly in QL we add in synthetic nodes which desugarb.ftob.(A).f. As part of the machinery of this, when we seex.yandyisn't a direct field ofX, we calculate how many implicit field selections there have to be before we can find a field calledy. It was never intended that this depth could be 0, as this is only possible with a recursive struct, which we didn't think to test.This does not need a change note as it is fixing an FP that hasn't been in any release.