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[libc][fuzzing] Improve printf long double fuzzing #172113

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Merged
michaelrj-google merged 2 commits into from
Dec 16, 2025
Merged

[libc][fuzzing] Improve printf long double fuzzing #172113

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[libc][fuzzing] Improve printf long double fuzzing
michaelrj-google Dec 12, 2025
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michaelrj-google Dec 16, 2025
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17 changes: 12 additions & 5 deletions libc/fuzzing/stdio/printf_float_conv_fuzz.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -87,10 +87,12 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
// data = raw_data;
// size = sizeof(raw_data);
double num = 0.0;
long double ld_num = 0.0L;
int prec = 0;
int width = 0;

LIBC_NAMESPACE::fputil::FPBits<double>::StorageType raw_num = 0;
LIBC_NAMESPACE::fputil::FPBits<long double>::StorageType ld_raw_num = 0;

// Copy as many bytes of data as will fit into num, prec, and with. Any extras
// are ignored.
Expand All @@ -101,16 +103,20 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
prec = (prec << 8) + data[cur];
} else if (cur < sizeof(raw_num) + sizeof(prec) + sizeof(width)) {
width = (width << 8) + data[cur];
} else if (cur < sizeof(raw_num) + sizeof(prec) + sizeof(width) +
sizeof(ld_raw_num)) {
ld_raw_num = (ld_raw_num << 8) + data[cur];
}
}

num = LIBC_NAMESPACE::fputil::FPBits<double>(raw_num).get_val();
ld_num = LIBC_NAMESPACE::fputil::FPBits<long double>(ld_raw_num).get_val();

// While we could create a "ld_raw_num" from additional bytes, it's much
// easier to stick with simply casting num to long double. This avoids the
// issues around 80 bit long doubles, especially unnormal and pseudo-denormal
// numbers, which MPFR doesn't handle well.
long double ld_num = static_cast<long double>(num);
// checking the same value in double and long double could help find
// mismatches. It also ensures long doubles are being tested even before the
// input data is long enough. Mostly this is here to match previous behavior
// where this was the only long double value checked.
long double num_as_ld = static_cast<long double>(num);

if (width > MAX_SIZE) {
width = MAX_SIZE;
Expand All @@ -130,6 +136,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
TestResult result;
if (fmt_arr[cur_fmt][fmt_len - 2] == 'L') {
result = test_vals<long double>(fmt_arr[cur_fmt], ld_num, prec, width);
result = test_vals<long double>(fmt_arr[cur_fmt], num_as_ld, prec, width);
} else {
result = test_vals<double>(fmt_arr[cur_fmt], num, prec, width);
}
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