[CRIU] [PATCH 22/23] zdtm/grow_map: fix test failure for clang

[Andrei Vagin]

On Tue, Oct 11, 2016 at 06:47:00PM -0700, Kir Kolyshkin wrote:
> When this code is compiled by clang, it optimizes two byte writes into
> one word write, like this:
> 
> >        fake_grow_down[0] = 'c';
> >        *(fake_grow_down - 1) = 'b';
> > 401b60:       66 41 c7 46 ff 62 63    movw   $0x6362,-0x1(%r14)
> 
> This is incorrect, as the stack is supposed to grow page by page,
> so we need to touch one page then another, i.e. the order is important.
> 
> To fix, let's use volatile pointer. After this change, the following
> (correct) code is generated:
> 
> >        *p-- = 'c';
> >   401b60:       41 c6 06 63             movb   $0x63,(%r14)
> >        *p = 'b';
> >   401b64:       41 c6 46 ff 62          movb   $0x62,-0x1(%r14)
> 
> [v2: same fix for another similar place]
> 
> Cc: Andrei Vagin <avagin at virtuozzo.com>

Acked-by: Andrei Vagin <avagin at virtuozzo.com>

> Signed-off-by: Kir Kolyshkin <kir at openvz.org>
> ---
>  test/zdtm/static/grow_map.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/test/zdtm/static/grow_map.c b/test/zdtm/static/grow_map.c
> index c26cfc8..dd1dffd 100644
> --- a/test/zdtm/static/grow_map.c
> +++ b/test/zdtm/static/grow_map.c
> @@ -11,6 +11,7 @@ const char *test_author	= "Andrew Vagin <avagin at openvz.org>";
>  int main(int argc, char **argv)
>  {
>  	char *start_addr, *fake_grow_down, *test_addr, *grow_down;
> +	volatile char *p;
>  	test_init(argc, argv);
>  
>  	start_addr = mmap(NULL, PAGE_SIZE * 10, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> @@ -28,8 +29,9 @@ int main(int argc, char **argv)
>  		return 1;
>  	}
>  
> -	fake_grow_down[0] = 'c';
> -	*(fake_grow_down - 1) = 'b';
> +	p = fake_grow_down;
> +	*p-- = 'c';
> +	*p = 'b';
>  
>  	/* overlap the guard page of fake_grow_down */
>  	test_addr = mmap(start_addr + PAGE_SIZE * 3, PAGE_SIZE,
> @@ -57,8 +59,9 @@ int main(int argc, char **argv)
>  		return 1;
>  	}
>  
> -	grow_down[0] = 'z';
> -	*(grow_down - 1) = 'x';
> +	p = grow_down;
> +	*p-- = 'z';
> +	*p = 'x';
>  
>  	pass();
>  
> -- 
> 2.7.4
>