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

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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.
This commit is contained in:
Florian Weimer 2020-09-22 19:07:48 +02:00
parent 90ccfdf176
commit 681900d296
3 changed files with 64 additions and 0 deletions
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4
sysdeps/x86/Makefile
View File

@ -11,6 +11,10 @@ tests += tst-get-cpu-features tst-get-cpu-features-static \
tests-static += tst-get-cpu-features-static
endif
ifeq ($(subdir),math)
tests += tst-ldbl-nonnormal-printf
endif # $(subdir) == math
ifeq ($(subdir),setjmp)
gen-as-const-headers += jmp_buf-ssp.sym
sysdep_routines += __longjmp_cancel

8
sysdeps/x86/ldbl2mpn.c
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@ -115,6 +115,14 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
&& res_ptr[N - 1] == 0)
/* Pseudo zero. */
*expt = 0;
else
/* Unlike other floating point formats, the most significant bit
is explicit and expected to be set for normal numbers. Set it
in case it is cleared in the input. Otherwise, callers will
not be able to produce the expected multi-precision integer
layout by shifting. */
res_ptr[N - 1] |= (mp_limb_t) 1 << (LDBL_MANT_DIG - 1
- ((N - 1) * BITS_PER_MP_LIMB));
return N;
}

52
sysdeps/x86/tst-ldbl-nonnormal-printf.c Normal file
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@ -0,0 +1,52 @@
/* Test printf with x86-specific non-normal long double value.
Copyright (C) 2020 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <stdio.h>
#include <string.h>
#include <support/check.h>
/* Fill the stack with non-zero values. This makes a crash in
snprintf more likely. */
static void __attribute__ ((noinline, noclone))
fill_stack (void)
{
char buffer[65536];
memset (buffer, 0xc0, sizeof (buffer));
asm ("" ::: "memory");
}
static int
do_test (void)
{
fill_stack ();
long double value;
memcpy (&value, "\x00\x04\x00\x00\x00\x00\x00\x00\x00\x04", 10);
char buf[30];
int ret = snprintf (buf, sizeof (buf), "%Lg", value);
TEST_COMPARE (ret, strlen (buf));
if (strcmp (buf, "nan") != 0)
/* If snprintf does not recognize the non-normal number as a NaN,
it has added the missing explicit MSB. */
TEST_COMPARE_STRING (buf, "3.02201e-4624");
return 0;
}
#include <support/test-driver.c>