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

[Pete Cooper via llvm-dev]

> On Mar 22, 2016, at 5:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Tue, Mar 22, 2016 at 5:30 PM, Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
> 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
> 
> I believe the last is closer to 'new's behavior - which I think returns a unique non-null address (well, unique amongst current allocations - can be recycled once deleted) if I recall correctly.
That’s what I would have expected too.  Its like sizeof(struct {}) which can be a 1 depending on factors we don’t need to get in to here.
> Can check for wording if that's helpful/desired.
No need for my benefit :)  I’m in agreement that this is a good behavior to go for, but will leave it to others to say if they’d like the extra detail.

One thing I did forget to say is that I’d like to fix StringRef::copy in all of the above cases.  I think that this method should always avoid the allocator and return StringRef(nullptr, 0) when length is 0.  I’ll get a patch up on llvm-commits if there’s no objections there.

Thanks,
Pete
>  
> 
> Thanks,
> Pete

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