syn: fix stack-use-after-return in constant fold sliceDff, revised#10771
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syn: fix stack-use-after-return in constant fold sliceDff, revised#10771povik wants to merge 3 commits into
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This pull request updates the return type of the Graph::add function from Bundle to BundleView in both its declaration and definition. The review feedback suggests updating the corresponding documentation comment to reflect this change. Additionally, it recommends optimizing the Bundle::Bundle(BundleView) constructor to prevent unnecessary heap allocations when callers assign the returned BundleView to a Bundle, particularly since Graph::add returns consecutive nets.
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The LLM has good suggestions, I'll take them up later. |
sliceDff() was declared to return a BundleView (a non-owning view) but returned the owning Bundle produced by Graph::add<Dff>(). The implicit BundleView(const Bundle&) conversion stores a pointer to that temporary, which is destroyed when sliceDff() returns, so foldSequentials() then indexes a dangling view (BundleView::operator[] -> Bundle::operator[]). This is undefined behavior that optimized builds tolerate (sliceDff() is inlined, so the temporary's storage survives the read), but a DEBUG (unoptimized) build faults on. It reproduces as a SIGSEGV during synthesis of larger designs using the integrated syn flow (e.g. asap7/aes, asap7/jpeg with SYNTH_USE_SYN=1); AddressSanitizer reports a stack-use-after-return at ir/Bundle.cc. Return an owning Bundle from sliceDff() and hold it as a Bundle at the call site (a BundleView local would re-create the dangle). Signed-off-by: Matthew Guthaus <mrg@ucsc.edu>
Since the return value of `add` represents a contiguous block of net indices we can use `BundleView` for it which is both more efficient and prevents use-after-free should the caller cast to BundleView. Signed-off-by: Martin Povišer <povik@cutebit.org>
Signed-off-by: Martin Povišer <povik@cutebit.org>
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Summary
This is a revised fix from #10768.
Since the return value of
addrepresents a contiguous block of net indices we can useBundleViewfor it which is both more efficient and prevents use-after-free should the caller cast to BundleView.Type of Change