[LTP] [PATCH] syscalls/mprotect04: align exec_func to 64 bytes

[Daniel Mentz]

Consider consolidating page_sz (currently in get_func()) and copy_sz
(global) since they are now identical. Maybe make page_sz a global
variable.

I had some concerns around "-falign-functions=64": For example,
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html states
"Enabled at levels -O2, -O3.". The way I read this is that
-falign-functions is ignored for optimization levels other than O2 and
O3. An ad-hoc test with gcc 7.3.0, however, showed that functions are
indeed aligned on 64 byte boundaries even with O0. I also tried clang
(llvm), and it also behaves as desired (for x86-64 and aarch64).

I guess one thing you could do is to calculate the difference between
the address of exec_func and the end of the page and programmatically
verify that there is enough cushion between the start of exec_func and
the end of the page. This would catch the case where some compiler
ignores "-falign-functions=64".

On Mon, Feb 11, 2019 at 7:04 AM Jan Stancek <jstancek@redhat.com> wrote:
>
> exec_func() is dummy/empty function. If we make sure it's aligned,
> we can be pretty confident that it will located in single page and
> can drop code that deals with 2nd page.
>
> Signed-off-by: Jan Stancek <jstancek@redhat.com>
> ---
>  testcases/kernel/syscalls/mprotect/Makefile     |  2 ++
>  testcases/kernel/syscalls/mprotect/mprotect04.c | 12 ++----------
>  2 files changed, 4 insertions(+), 10 deletions(-)
>
> diff --git a/testcases/kernel/syscalls/mprotect/Makefile b/testcases/kernel/syscalls/mprotect/Makefile
> index bd617d806675..bc5c8bc10395 100644
> --- a/testcases/kernel/syscalls/mprotect/Makefile
> +++ b/testcases/kernel/syscalls/mprotect/Makefile
> @@ -20,4 +20,6 @@ top_srcdir            ?= ../../../..
>
>  include $(top_srcdir)/include/mk/testcases.mk
>
> +mprotect04: CFLAGS += -falign-functions=64
> +
>  include $(top_srcdir)/include/mk/generic_leaf_target.mk
> diff --git a/testcases/kernel/syscalls/mprotect/mprotect04.c b/testcases/kernel/syscalls/mprotect/mprotect04.c
> index 60941a4220d5..3125f344795d 100644
> --- a/testcases/kernel/syscalls/mprotect/mprotect04.c
> +++ b/testcases/kernel/syscalls/mprotect/mprotect04.c
> @@ -88,7 +88,7 @@ static void setup(void)
>  {
>         tst_tmpdir();
>         tst_sig(NOFORK, sighandler, cleanup);
> -       copy_sz = getpagesize() * 2;
> +       copy_sz = getpagesize();
>
>         TEST_PAUSE;
>  }
> @@ -133,7 +133,7 @@ static void testfunc_protnone(void)
>
>  #ifdef __ia64__
>
> -static char exec_func[] = {
> +static char exec_func[] __attribute__ ((aligned (64))) = {
>         0x11, 0x00, 0x00, 0x00, 0x01, 0x00, /* nop.m 0x0             */
>         0x00, 0x00, 0x00, 0x02, 0x00, 0x80, /* nop.i 0x0             */
>         0x08, 0x00, 0x84, 0x00,             /* br.ret.sptk.many b0;; */
> @@ -237,14 +237,6 @@ static void *get_func(void *mem)
>         }
>         memcpy(mem, page_to_copy, page_sz);
>
> -       /* copy 2nd page if possible */
> -       mem += page_sz;
> -       page_to_copy += page_sz;
> -       if (page_present(page_to_copy))
> -               memcpy(mem, page_to_copy, page_sz);
> -       else
> -               memset(mem, 0, page_sz);
> -
>         clear_cache(mem_start, copy_sz);
>
>         /* return pointer to area where copy of exec_func resides */
> --
> 1.8.3.1
>