[Bug 191719] bin/expr doesn't detect some overflow errors with multiplication

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191719

Jilles Tjoelker <jilles at FreeBSD.org> changed:

           What    |Removed                     |Added
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             Status|Needs Triage                |Open
                 CC|                            |jilles at FreeBSD.org

--- Comment #2 from Jilles Tjoelker <jilles at FreeBSD.org> ---
All the overflow checks except for division and modulo are fundamentally broken
because they first let overflow happen and then attempt to check for it. Since
signed integer overflow is undefined behaviour, this allows the compiler to
optimize the overflow checks away, or worse.

Also, INTMAX_MIN modulo -1 causes SIGFPE on amd64, like

expr -e -- $((-0x7fffffffffffffff - 1)) % -1

Either op_rem() should not sabotage assert_div()'s check or op_rem() should
return a zero-valued integer for anything modulo -1.

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