[Bug stdio/26649] printf should handle non-normal x86 long double numbers gracefully

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26649

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |2.33
         Resolution|---                         |FIXED

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
Fixed for glibc 2.33 via:

commit 681900d29683722b1cb0a8e565a0585846ec5a61
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Sep 22 19:07:48 2020 +0200

    x86: Harden printf against non-normal long double values (bug 26649)

    The behavior of isnan/__builtin_isnan on bit patterns that do not
    correspond to something that the CPU would produce from valid inputs
    is currently under-defined in the toolchain. (The GCC built-in and
    glibc disagree.)

    The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c
    assumes the GCC behavior that returns true for non-normal numbers
    which are not specified as NaN. (The glibc implementation returns
    false for such numbers.)

    At present, passing non-normal numbers to __mpn_extract_long_double
    causes this function to produce irregularly shaped multi-precision
    integers, triggering undefined behavior in __printf_fp_l.

    With GCC 10 and glibc 2.32, this behavior is not visible because
    __builtin_isnan is used, which avoids calling
    __mpn_extract_long_double in this case.  This commit updates the
    implementation of __mpn_extract_long_double so that regularly shaped
    multi-precision integers are produced in this case, avoiding
    undefined behavior in __printf_fp_l.

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