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py/parsenum: Avoid undefined behavior parsing floats with large exponents. 

Fuzz testing combined with the undefined behavior sanitizer found that
parsing unreasonable float literals like 1e+9999999999999 resulted in
undefined behavior due to overflow in signed integer arithmetic, and a
wrong result being returned.
This commit is contained in:
Jeff Epler 2018-05-19 10:56:34 -05:00 committed by Damien George
parent 5efc575067
commit 4f71a2a75a
1 changed files with 6 additions and 1 deletions
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7
py/parsenum.c
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@ -234,7 +234,12 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
if ('0' <= dig && dig <= '9') { if ('0' <= dig && dig <= '9') {
dig -= '0'; dig -= '0';
if (in == PARSE_DEC_IN_EXP) { if (in == PARSE_DEC_IN_EXP) {
exp_val = 10 * exp_val + dig; // don't overflow exp_val when adding next digit, instead just truncate
// it and the resulting float will still be correct, either inf or 0.0
// (use INT_MAX/2 to allow adding exp_extra at the end without overflow)
if (exp_val < (INT_MAX / 2 - 9) / 10) {
exp_val = 10 * exp_val + dig;
}
} else { } else {
if (dec_val < DEC_VAL_MAX) { if (dec_val < DEC_VAL_MAX) {
// dec_val won't overflow so keep accumulating // dec_val won't overflow so keep accumulating