We call UnmapShadow before the actual munmap, at that point we don't yet
know if the provided address/size are sane. We can't call UnmapShadow
after the actual munmap becuase at that point the memory range can
already be reused for something else, so we can't rely on the munmap
return value to understand is the values are sane.
While calling munmap with insane values (non-canonical address, negative
size, etc) is an error, the kernel won't crash. We must also try to not
crash as the failure mode is very confusing (paging fault inside of the
runtime on some derived shadow address).
Such invalid arguments are observed on Chromium tests:
https://bugs.chromium.org/p/chromium/issues/detail?id=1275581
Reviewed By: melver
Differential Revision: https://reviews.llvm.org/D114944
Dmitry Vyukov