[ubsan] Teach the pointer overflow check that "p - <unsigned> <= p" (PR33430)
The pointer overflow check gives false negatives when dealing with
expressions in which an unsigned value is subtracted from a pointer.
This is summarized in PR33430 [1]: ubsan permits the result of the
subtraction to be greater than "p", but it should not.
To fix the issue, we should track whether or not the pointer expression
is a subtraction. If it is, and the indices are unsigned, we know to
expect "p - <unsigned> <= p".
I've tested this by running check-{llvm,clang} with a stage2 ubsan-enabled build. I've also added some tests to compiler-rt, which I'll upload in a separate patch.
[1] https://bugs.llvm.org/show_bug.cgi?id=33430
4 files changed
