[LTP] [PATCH] mmap04.c: Avoid vma merging

[Martin Doucha]

Hi,
some comments below.

On 23. 01. 24 17:55, Avinesh Kumar wrote:
> We hit a scenario where new mapping was merged with existing mapping of
> same permission and the return address from the mmap was hidden in the
> merged mapping in /proc/self/maps, causing the test to fail.
> To avoid this, we first  create a 2-page mapping with the different
> permissions, and then remap the 2nd page with the perms being tested.
> 
> Signed-off-by: Avinesh Kumar <akumar@suse.de>
> Reported-by: Martin Doucha <mdoucha@suse.cz>
> ---
>   testcases/kernel/syscalls/mmap/mmap04.c | 49 +++++++++++++++----------
>   1 file changed, 30 insertions(+), 19 deletions(-)
> 
> diff --git a/testcases/kernel/syscalls/mmap/mmap04.c b/testcases/kernel/syscalls/mmap/mmap04.c
> index f6f4f7c98..f0f87b7f5 100644
> --- a/testcases/kernel/syscalls/mmap/mmap04.c
> +++ b/testcases/kernel/syscalls/mmap/mmap04.c
> @@ -17,28 +17,28 @@
>   #include "tst_test.h"
>   #include <stdio.h>
>   
> -#define MMAPSIZE 1024
> -static char *addr;
> +static char *addr1;
> +static char *addr2;
>   
>   static struct tcase {
>   	int prot;
>   	int flags;
>   	char *exp_perms;
>   } tcases[] = {
> -	{PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, "---p"},
> -	{PROT_NONE, MAP_ANONYMOUS | MAP_SHARED, "---s"},
> -	{PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, "r--p"},
> -	{PROT_READ, MAP_ANONYMOUS | MAP_SHARED, "r--s"},
> -	{PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, "-w-p"},
> -	{PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, "-w-s"},
> -	{PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, "rw-p"},
> -	{PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, "rw-s"},
> -	{PROT_READ | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE, "r-xp"},
> -	{PROT_READ | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED, "r-xs"},
> -	{PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE, "-wxp"},
> -	{PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED, "-wxs"},
> -	{PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE, "rwxp"},
> -	{PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED, "rwxs"}
> +	{PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "---p"},
> +	{PROT_NONE, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "---s"},
> +	{PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "r--p"},
> +	{PROT_READ, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "r--s"},
> +	{PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "-w-p"},
> +	{PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "-w-s"},
> +	{PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "rw-p"},
> +	{PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "rw-s"},
> +	{PROT_READ | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "r-xp"},
> +	{PROT_READ | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "r-xs"},
> +	{PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "-wxp"},
> +	{PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "-wxs"},
> +	{PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, "rwxp"},
> +	{PROT_READ | PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, "rwxs"}

The MAP_FIXED flag doesn't belong in the testcases, it should be added 
in the mmap() call instead: SAFE_MMAP(..., tc->flags | MAP_FIXED, ...);
It's an implementation detail not related to the testcases themselves. 
You don't want to rewrite all the test cases again if we decide to not 
use MAP_FIXED for whatever reason in the future.

>   };
>   
>   static void run(unsigned int i)
> @@ -47,10 +47,21 @@ static void run(unsigned int i)
>   	char addr_str[20];
>   	char perms[8];
>   	char fmt[1024];
> +	unsigned int pagesize;
>   
> -	addr = SAFE_MMAP(NULL, MMAPSIZE, tc->prot, tc->flags, -1, 0);
> +	pagesize = SAFE_SYSCONF(_SC_PAGESIZE);
>   
> -	sprintf(addr_str, "%" PRIxPTR, (uintptr_t)addr);
> +	/* To avoid new mapping getting merged with existing mappings, we first
> +	   create a 2-page mapping with the different permissions, and then remap
> +	   the 2nd page with the perms being tested. */
> +	if ((tc->prot == PROT_NONE) && (tc->flags == (MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED)))
> +		addr1 = SAFE_MMAP(NULL, pagesize * 2, PROT_READ, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> +	else
> +		addr1 = SAFE_MMAP(NULL, pagesize * 2, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

This would be cleaner (just invert the shared/private flag):
int flags = (tc->flags & MAP_PRIVATE) ? MAP_SHARED : MAP_PRIVATE;
addr1 = SAFE_MMAP(NULL, pagesize * 2, PROT_NONE, MAP_ANONYMOUS | flags, 
-1, 0);

> +
> +	addr2 = SAFE_MMAP(addr1 + pagesize, pagesize, tc->prot, tc->flags, -1, 0);
> +
> +	sprintf(addr_str, "%" PRIxPTR, (uintptr_t)addr2);

Why not merge the two sprintf()s into one?
sprintf(fmt, "%" PRIxPTR "-%%*x %%s", (uintptr_t)addr2);

>   	sprintf(fmt, "%s-%%*x %%s", addr_str);
>   	SAFE_FILE_LINES_SCANF("/proc/self/maps", fmt, perms);
>   
> @@ -61,7 +72,7 @@ static void run(unsigned int i)
>   						tc->exp_perms, perms);
>   	}
>   
> -	SAFE_MUNMAP(addr, MMAPSIZE);
> +	SAFE_MUNMAP(addr1, pagesize * 2);
>   }
>   
>   static struct tst_test test = {

-- 
Martin Doucha   mdoucha@suse.cz
SW Quality Engineer
SUSE LINUX, s.r.o.
CORSO IIa
Krizikova 148/34
186 00 Prague 8
Czech Republic