Signed integer overflow in __secs_to_tm

[Brian Mastenbrook <brian@mastenbrook.net>]

__secs_to_tm (used by gmtime_r et al) may invoke undefined behavior due to signed integer overflow in two places. At __secs_to_tm.c:58, 400*qc_cycles may overflow. At __secs_to_tm.c:63, there is a nonsensical comparison between an already overflowed value and INT_MAX or INT_MIN; the compiler will delete this test due to overflow. Here are some example values that provoke the overflow:

t = -67771633420944000

__secs_to_tm.c:58:[kernel] warning: signed overflow. assert -2147483648 ≤ 400*qc_cycles;

t = 67768037838810496

__secs_to_tm.c:63:[kernel] warning: signed overflow. assert years+100 ≤ 2147483647;

These errors were found using KLEE and clang's undefined behavior sanitizer together. (Unfortunately KLEE also produced a false report of an out-of-bounds access to the days_in_month array due to a solver bug.)

--
Brian Mastenbrook
brian@mastenbrook.net
http://brian.mastenbrook.net/