Fix strtod overflow detection (bug 23279).

System Internals / glibc - Joseph Myers [codesourcery.com] - 13 June 2018 16:06 EDT

As shown by bug 23279, strtod's round_and_return has an off-by-one error in its overflow detection, only counting an exponent greater than MAX_EXP as overflowing when an exponent of MAX_EXP also means overflow (recall the ISO C definition of DBL_MAX_EXP etc. is based on a floating-point model where 2^exp is multiplied by a value in the interval [0.5, 1), so 2^MAX_EXP is not representable).

For decimal arguments to strtod, a separate overflow check in the main implementation covers the case where the integer part of the argument (truncated to the nearest integer towards zero) has more than MAX_EXP bits, meaning that this issue in round_and_return only affects cases (arguments with absolute value strictly between the maximum representable value and 2^MAX_EXP) where overflow depends on the rounding mode; in such cases, the returned value would still have been correct on overflow but without the overflow exception being raised or errno being set to ERANGE. For hex float arguments, however, other cases can arise, as shown in bug 23279, where a value with exponent already set to MAX_EXP is passed into round_and_return and a result can wrongly end up being NaN, or infinity instead of the largest finite value.

This patch fixes the off-by-one error, adds testing of overflow exceptions to the tst-strtod-round framework, and adds tests of these issues.

Tested for x86_64. Also ran the tst-strtod-round tests for powerpc to make sure the new tests didn't introduce any new failures for IBM long double.

[BZ #23279]
- stdlib/strtod_l.c (round_and_return): Handle an exponent of MAX_EXP as overflowing.
- stdlib/gen-tst-strtod-round.c (string_to_fp): Clear MPFR overflow flag. (round_str): Output also whether result overflows in each rounding mode.
- stdlib/tst-strtod-round-data: Add more tests.
- stdlib/tst-strtod-round-data.h: Regenerated.
- stdlib/tst-strtod-round-skeleton.c (_XNTRY): Update comment. (TEST): Handle extra arguments for overflow flags. (struct test_overflow): New type. [!FE_OVERFLOW] (FE_OVERFLOW): Define to 0. (GEN_ONE_TEST): Clear all exceptions. Test overflow flag. (test_in_one_mode): Take argument with overflow information. (do_test): Update calls to test_in_one_mode.

fcd6b5ac36 Fix strtod overflow detection (bug 23279).
ChangeLog | 19 +
stdlib/gen-tst-strtod-round.c | 26 +-
stdlib/strtod_l.c | 4 +-
stdlib/tst-strtod-round-data | 7 +
stdlib/tst-strtod-round-data.h | 9091 +++++++++++++++++++-----------------
stdlib/tst-strtod-round-skeleton.c | 108 +-
6 files changed, 4864 insertions(+), 4391 deletions(-)

Upstream: sourceware.org


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