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

[Jan Stancek]

----- Original Message -----
> 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".

That sounds like reasonable precaution, I'll send v2 shortly.

> 
> 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
> >
>