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681900d296
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.
53 lines
1.6 KiB
C
53 lines
1.6 KiB
C
/* Test printf with x86-specific non-normal long double value.
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Copyright (C) 2020 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <stdio.h>
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#include <string.h>
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#include <support/check.h>
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/* Fill the stack with non-zero values. This makes a crash in
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snprintf more likely. */
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static void __attribute__ ((noinline, noclone))
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fill_stack (void)
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{
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char buffer[65536];
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memset (buffer, 0xc0, sizeof (buffer));
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asm ("" ::: "memory");
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}
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static int
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do_test (void)
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{
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fill_stack ();
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long double value;
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memcpy (&value, "\x00\x04\x00\x00\x00\x00\x00\x00\x00\x04", 10);
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char buf[30];
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int ret = snprintf (buf, sizeof (buf), "%Lg", value);
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TEST_COMPARE (ret, strlen (buf));
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if (strcmp (buf, "nan") != 0)
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/* If snprintf does not recognize the non-normal number as a NaN,
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it has added the missing explicit MSB. */
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TEST_COMPARE_STRING (buf, "3.02201e-4624");
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return 0;
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}
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#include <support/test-driver.c>
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