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

[Mehdi Amini via llvm-dev]

> On Mar 22, 2016, at 5:39 PM, Pete Cooper via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>> 
>> On Mar 22, 2016, at 5:35 PM, David Blaikie <dblaikie at gmail.com <mailto: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.

Well except that if sizeof(struct{}) is 1, the allocator is never called with a 0.

I would consider forbidding zero sized allocation in the allocator (assert()) by design (hey we're controlling every possible uses!), unless there is a real use-case for that.

This would also be in line with the C++ standard requirement for allocator which specifies that the result of "a.allocate(0)" is unspecified (ref: C++14 Table 28 — Allocator requirements).

-- 
Mehdi


>> 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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