[PATCH] x86: Add a testcase for BZ #29863

[Noah Goldstein]

On Wed, Dec 14, 2022 at 9:29 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> When a thread is updating the memory area of memcmp and another thread
> is doing memcmp, the memcmp return value is undefined.  Add a testcase
> to verify that memcmp won't segfault in this case.
> ---
>  sysdeps/x86/Makefile               | 11 +++++
>  sysdeps/x86/tst-memcmp-race-sse2.c |  1 +
>  sysdeps/x86/tst-memcmp-race.c      | 75 ++++++++++++++++++++++++++++++
>  3 files changed, 87 insertions(+)
>  create mode 100644 sysdeps/x86/tst-memcmp-race-sse2.c
>  create mode 100644 sysdeps/x86/tst-memcmp-race.c
>
> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
> index 56fd5fc805..3e07e18c7d 100644
> --- a/sysdeps/x86/Makefile
> +++ b/sysdeps/x86/Makefile
> @@ -257,3 +257,14 @@ tests += \
>  tests-static += \
>    tst-sysconf-cache-linesize-static
>  endif
> +
> +ifeq ($(subdir),nptl)
> +tests += \
> +  tst-memcmp-race-sse2 \
> +  tst-memcmp-race \
> +# tests
> +
> +CFLAGS-tst-memcmp-race-sse2.c += -O0
> +CFLAGS-tst-memcmp-race.c += -O0
> +tst-memcmp-race-sse2-ENV = GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX2
> +endif
> diff --git a/sysdeps/x86/tst-memcmp-race-sse2.c b/sysdeps/x86/tst-memcmp-race-sse2.c
> new file mode 100644
> index 0000000000..2986e418f3
> --- /dev/null
> +++ b/sysdeps/x86/tst-memcmp-race-sse2.c
> @@ -0,0 +1 @@
> +#include "tst-memcmp-race.c"
> diff --git a/sysdeps/x86/tst-memcmp-race.c b/sysdeps/x86/tst-memcmp-race.c
> new file mode 100644
> index 0000000000..f8f8e47f6e
> --- /dev/null
> +++ b/sysdeps/x86/tst-memcmp-race.c
> @@ -0,0 +1,75 @@
> +/* Test case for memcmp with race condition.
> +   Copyright (C) 2022 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 <stdint.h>
> +#include <string.h>
> +#include <support/xthread.h>
> +
> +#define        NUM_THREADS     2
> +#define        LEN             17

Imo if we test this `LEN` should be a variable and we should test
all 2^N + 1 for N in [0, 12]. I wouldn't be suprised if there are
other cases.

> +#define        STR             "abcdefghijklmnopq"
> +#define LOOP1          100
> +#define LOOP2          1000000
> +
> +char buf1[LEN], buf2[LEN];

I had to make this volatile to reproduce the failure
with GCC 12, does this properly segfault w.o the
fix patch?
> +
> +static void *
> +childThread (void *tArgs)
> +{
> +  int i;
> +
> +  if (0 == (size_t)tArgs % 2)
> +    {
> +      for (i = 0; i < LOOP1; i++)
> +       {
> +         int result;
> +
> +         result = memcmp (buf1, buf2, LEN);

If we are testing this I think it should test each impl.

> +         printf ("i = %d : result = %d\n", i, result);

IMO remove the print and make `result` volatile.
> +       }
> +    }
> +  else
> +    {
> +      for (i = 0; i < LOOP2; i++)
> +       buf1[1] = (0 == (i % 2)) ? 'b' : 'c';
> +    }
> +
> +  return NULL;
> +}
> +
> +static int
> +do_test (void)
> +{
> +  int i;
> +  pthread_t threads[NUM_THREADS];
> +
> +  memcpy (buf1, STR, LEN);
> +  memcpy (buf2, STR, LEN);
> +
> +  for (i = 0; i < NUM_THREADS; ++i)
> +    threads[i] = xpthread_create (NULL, childThread,
> +                                 (void *)(uintptr_t) i);
> +
> +  for (i = 0; i < NUM_THREADS; ++i)
> +    xpthread_join (threads[i]);
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> --
> 2.38.1
>

Not sure we want this, given its not behavior we have decided
to actively support its failures could be misleading.