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We use check_add_overflow(a, b, d) here to validate that (off, len) pairs do not exceed the max value type. The kernel conveniently has several macros to sort out the problems with signed or unsigned types. However, we're not interested in purely seeing whether (a + b) overflows, because we're using this for (off, len) overflow checks, where the bytes we read are from 0 to len -1. We must therefore call this check with (b) being "len - 1". I've made sure that we don't accidentally fail when (len == 0) in all cases by making sure we've already checked this condition before, and moving code around as needed to ensure that (len > 0) in all cases where we check. The macro check_add_overflow requires a (d) argument in which temporarily the result of the addition is stored and then checked to see if an overflow occurred. We put a `tmp` variable on the stack of the correct type as needed to make the checks function. simple-release-extents test mistakenly relied on this buggy wrap code, so it needs fixing. The move-blocks test also got it wrong. Signed-off-by: Auke Kok <auke.kok@versity.com>