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Prevent int underflow when parsing exponents

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When parsing negative exponents, the current implementation takes
precautions for |exp| to not underflow int.

But that is not sufficient: later on [1], |exp + expFrac| is also
stored to an int - so we must ensure that the sum stays within int
representable values.

Update the exp clamping logic to take expFrac into account.

[1] https://github.com/Tencent/rapidjson/blob/master/include/rapidjson/reader.h#L1690
This commit is contained in:
Florin Malita 2018-05-15 22:48:07 -04:00
parent a091035846
commit 8269bc2bc2
2 changed files with 11 additions and 1 deletions
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11
include/rapidjson/reader.h
View File

@ -1632,9 +1632,18 @@ private:
if (RAPIDJSON_LIKELY(s.Peek() >= '0' && s.Peek() <= '9')) {
exp = static_cast<int>(s.Take() - '0');
if (expMinus) {
// (exp + expFrac) must not underflow int => we're detecting when -exp gets
// dangerously close to INT_MIN (a pessimistic next digit 9 would push it into
// underflow territory):
//
// -(exp * 10 + 9) + expFrac >= INT_MIN
// <=> exp <= (expFrac - INT_MIN - 9) / 10
RAPIDJSON_ASSERT(expFrac <= 0);
int maxExp = (expFrac + 2147483639) / 10;
while (RAPIDJSON_LIKELY(s.Peek() >= '0' && s.Peek() <= '9')) {
exp = exp * 10 + static_cast<int>(s.Take() - '0');
if (exp >= 214748364) { // Issue #313: prevent overflow exponent
if (RAPIDJSON_UNLIKELY(exp > maxExp)) {
while (RAPIDJSON_UNLIKELY(s.Peek() >= '0' && s.Peek() <= '9')) // Consume the rest of exponent
s.Take();
}

1
test/unittest/readertest.cpp
View File

@ -242,6 +242,7 @@ static void TestParseDouble() {
TEST_DOUBLE(fullPrecision, "1e-214748363", 0.0); // Maximum supported negative exponent
TEST_DOUBLE(fullPrecision, "1e-214748364", 0.0);
TEST_DOUBLE(fullPrecision, "1e-21474836311", 0.0);
TEST_DOUBLE(fullPrecision, "1.00000000001e-2147483638", 0.0);
TEST_DOUBLE(fullPrecision, "0.017976931348623157e+310", 1.7976931348623157e+308); // Max double in another form
// Since