* [PATCH] x86: Harden printf against non-normal long double values (bug 26649)
@ 2020-09-22 14:31 Florian Weimer
2020-09-22 14:58 ` H.J. Lu
0 siblings, 1 reply; 2+ messages in thread
From: Florian Weimer @ 2020-09-22 14:31 UTC (permalink / raw)
To: libc-alpha
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.
---
sysdeps/i386/ldbl2mpn.c | 7 +++++
sysdeps/x86/Makefile | 4 +++
sysdeps/x86/tst-ldbl-nonnormal-printf.c | 52 +++++++++++++++++++++++++++++++++
3 files changed, 63 insertions(+)
diff --git a/sysdeps/i386/ldbl2mpn.c b/sysdeps/i386/ldbl2mpn.c
index ec8464eef7..867c66ef8d 100644
--- a/sysdeps/i386/ldbl2mpn.c
+++ b/sysdeps/i386/ldbl2mpn.c
@@ -115,6 +115,13 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
&& res_ptr[N - 1] == 0)
/* Pseudo zero. */
*expt = 0;
+ else
+ /* The sign bit is explicit, but add it in case it is missing in
+ the input. Otherwise, callers will not be able to produce the
+ expected multi-precision integer layout by shifting the sign
+ bit into the MSB. */
+ res_ptr[N - 1] |= (mp_limb_t) 1 << (LDBL_MANT_DIG - 1
+ - ((N - 1) * BITS_PER_MP_LIMB));
return N;
}
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index c369faf00d..081cc72e93 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -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
diff --git a/sysdeps/x86/tst-ldbl-nonnormal-printf.c b/sysdeps/x86/tst-ldbl-nonnormal-printf.c
new file mode 100644
index 0000000000..72797fc33b
--- /dev/null
+++ b/sysdeps/x86/tst-ldbl-nonnormal-printf.c
@@ -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 sign bit. */
+ TEST_COMPARE_STRING (buf, "3.02201e-4624");
+ return 0;
+}
+
+#include <support/test-driver.c>
--
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] x86: Harden printf against non-normal long double values (bug 26649)
2020-09-22 14:31 [PATCH] x86: Harden printf against non-normal long double values (bug 26649) Florian Weimer
@ 2020-09-22 14:58 ` H.J. Lu
0 siblings, 0 replies; 2+ messages in thread
From: H.J. Lu @ 2020-09-22 14:58 UTC (permalink / raw)
To: Florian Weimer; +Cc: GNU C Library
On Tue, Sep 22, 2020 at 7:33 AM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> 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.
>
> ---
> sysdeps/i386/ldbl2mpn.c | 7 +++++
> sysdeps/x86/Makefile | 4 +++
> sysdeps/x86/tst-ldbl-nonnormal-printf.c | 52 +++++++++++++++++++++++++++++++++
> 3 files changed, 63 insertions(+)
>
> diff --git a/sysdeps/i386/ldbl2mpn.c b/sysdeps/i386/ldbl2mpn.c
> index ec8464eef7..867c66ef8d 100644
> --- a/sysdeps/i386/ldbl2mpn.c
> +++ b/sysdeps/i386/ldbl2mpn.c
> @@ -115,6 +115,13 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
> && res_ptr[N - 1] == 0)
> /* Pseudo zero. */
> *expt = 0;
> + else
> + /* The sign bit is explicit, but add it in case it is missing in
> + the input. Otherwise, callers will not be able to produce the
> + expected multi-precision integer layout by shifting the sign
> + bit into the MSB. */
> + res_ptr[N - 1] |= (mp_limb_t) 1 << (LDBL_MANT_DIG - 1
> + - ((N - 1) * BITS_PER_MP_LIMB));
>
> return N;
> }
> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
> index c369faf00d..081cc72e93 100644
> --- a/sysdeps/x86/Makefile
> +++ b/sysdeps/x86/Makefile
> @@ -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
> diff --git a/sysdeps/x86/tst-ldbl-nonnormal-printf.c b/sysdeps/x86/tst-ldbl-nonnormal-printf.c
> new file mode 100644
> index 0000000000..72797fc33b
> --- /dev/null
> +++ b/sysdeps/x86/tst-ldbl-nonnormal-printf.c
> @@ -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 sign bit. */
> + TEST_COMPARE_STRING (buf, "3.02201e-4624");
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
>
> --
> Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
> Commercial register: Amtsgericht Muenchen, HRB 153243,
> Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill
>
Please create a patch to move i386/ldbl2mpn.c to x86 and remove
x86_64/ldbl2mpn.c.
And rebase your patch.
Thanks.
--
H.J.
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2020-09-22 14:59 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-09-22 14:31 [PATCH] x86: Harden printf against non-normal long double values (bug 26649) Florian Weimer
2020-09-22 14:58 ` H.J. Lu
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).