<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi all<div class=""><br class=""></div><div class="">(No idea if I have the correct audience. Please CC more people as needed).</div><div class=""><br class=""></div><div class="">I have an UBSan failure in BumpPtrAllocatorImpl.Allocate.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">To put this in code, if I have</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">BumpPtrAllocator allocator;</div><div class="">StringRef s;</div><div class="">s.copy(allocator);</div></blockquote><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">So the question is, how do we want this to behave in our code?</div><div class=""><br class=""></div><div class="">Some options:</div><div class="">- Assert that Allocate never gets a size 0 allocation. So fix StringRef::copy to see this case</div><div class="">- Remove the attributes from Allocate but continue to return nullptr (or base of the allocator?) in this case</div><div class="">- Keep the attributes on Allocate and treat size 0 allocations as size 1</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Pete</div><div class=""><br class=""></div></body></html>