hctim added a comment.
In D127812#4012854 <https://reviews.llvm.org/D127812#4012854>, @ilinpv wrote:
> I don't think `std::equal` is underlying cause here. We have
> featuresStrs_size() comparison before calling it:
>
> if (CurClones && NewClones &&
> (CurClones->featuresStrs_size() != NewClones->featuresStrs_size()
> ||
> !std::equal(CurClones->featuresStrs_begin(),
> CurClones->featuresStrs_end(),
> NewClones->featuresStrs_begin()))) {
>
> Also even if we completely remove std::equal the use-of-uninitialized-value
> error still persist.
Sorry for the red herring. MSan is detecting use-after-destructor here.
`Strings` holds `StringRef`-references to the memory allocated in
`StringsBuffer`.
When StringsBuffer is template-constructed with two inline elements (i.e.
`SmallVector<SmallString<64>, 2> StringsBuffer;`), the third element that gets
added (in `clang::Sema::checkTargetClonesAttrString`, `SemaDeclAttr.cpp:3568`)
causes the `SmallVector` to move the three strings to the heap (the two already
existing inline and the new addition).
`SmallVector` cleans up the inline memory by calling the destructors on it.
MSan dutifully marks the destroyed memory as uninitialized, to detect
use-after-destruction. The reason why ASan doesn't generate a report here is
that the memory is still technically "reachable", it's possible to have
destroyed memory that's still part of a live stack frame or hasn't had its heap
allocation cleaned up yet.
Because `Strings` captured the reference of the two string objects when they
were inline-allocated, as soon as this move-to-heap happens, these two
references are dangling. Then, when any caller attempts to iterate over
`Strings`, it finally explodes as MSan correctly detects the use of destroyed
memory.
Allocating 3 objects inline solves the issue for now, as there's no uses that
end up needing more than 3 elements, but this leaves a footgun for anyone who
would add another element later.
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D127812/new/
https://reviews.llvm.org/D127812
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