[llvm-dev] UBSan, StringRef and Allocator.h

[Pete Cooper via llvm-dev]

Hi all

(No idea if I have the correct audience.  Please CC more people as needed).

I have an UBSan failure in BumpPtrAllocatorImpl.Allocate.

The problem is that lld requests that we StringRef::copy an empty string.  This passes a length of 0 to a BumpPtrAllocator.  The BumpPtrAllocator happened to not have anything allocated yet so the CurPtr is nullptr, but given that we need 0 space we think we have enough space and return an allocation of size 0 at address nullptr.  This therefore returns nullptr from Allocate, but that method is marked with LLVM_ATTRIBUTE_RETURNS_NONNULL and LLVM_ATTRIBUTE_RETURNS_NOALIAS, both of which aren’t true in this case.

To put this in code, if I have

BumpPtrAllocator allocator;
StringRef s;
s.copy(allocator);

then i’m going to allocate 0 bytes in the allocator and get a StringRef(nullptr, 0).  Its a valid StringRef, but an UBSan failures in the allocator.

Lang and I looked up malloc behaviour online as this is fairly analogous.  The answer there is that you are allowed to return nullptr, or not, its implementation defined.  So no help there.

So the question is, how do we want this to behave in our code?

Some options:
- Assert that Allocate never gets a size 0 allocation.  So fix StringRef::copy to see this case
- Remove the attributes from Allocate but continue to return nullptr (or base of the allocator?) in this case
- Keep the attributes on Allocate and treat size 0 allocations as size 1

Thanks,
Pete

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