https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99418
--- Comment #6 from Ivan Sorokin <vanyacpp at gmail dot com> --- (In reply to Jakub Jelinek from comment #4) > Asan can't by design detect neither #c0 nor #c1, only ubsan can. > The reason why ubsan has that off by one stuff is that in C/C++, > &mas[n - 1][m] is not undefined behavior, only mas[n - 1][m] is. That is very unfortunate. For standard containers subscripting with wrond index is undefined behavior no matter if it is followed by taking of address. I assumed the same rules apply for builtin arrays. If one need just a point one can easily write a + n instead of &a[n]. Now I see that this is not the case and built-in arrays behave differently. > For #c1, the big question is what exactly is UB in C++, whether already > binding a reference to the object after the end of the array or only > actually accessing that reference. If the former, ubsan could treat > REFERENCE_TYPE differently, if the latter, then I'm afraid it can't do that, > and ubsan by design has to be done early before all the optimizations change > the IL so much that it is completely lost what were the user errors in it. > For the method calls, there really isn't a reference in the IL either, this > argument is a pointer, but .UBSAN_BOUNDS calls are added in the FE and so > perhaps it could know it is a method call and treat it as a reference. > So, something can be done but we need answers on where the UB in C++ exactly > happens. For -fsanitize=null the rules are quite subtle: dereferencing by itself (*p) doesn't check for nullptr, but binding a reference (int& q = *p;) does. Perhaps similar rules can be employed for past-the-end element: taking pointer to it is fine, but passing the pointer as this parameter to function is UB? At least this would be consistent with null pointers.
