Re: [PATCH] selftests/futex: Fix futex_numa_mpol's memory out of range subtest

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-08-27 13:58:50 [-0400], Waiman Long wrote:
> On 8/27/25 11:44 AM, André Almeida wrote:
> > --- a/tools/testing/selftests/futex/functional/futex_numa_mpol.c
> > +++ b/tools/testing/selftests/futex/functional/futex_numa_mpol.c
> > @@ -168,6 +168,17 @@ int main(int argc, char *argv[])
> >   	ksft_set_plan(1);
> >   	mem_size = sysconf(_SC_PAGE_SIZE);
> > +
> > +	/*
> > +	 * The "Memory out of range" test depends on having a pointer to an
> > +	 * invalid address. To make this test deterministic, and to not depend
> > +	 * on the memory layout of the process, create a "buffer zone" with
> > +	 * PROT_NONE just before the valid memory (*futex_ptr).
> > +	 */
> > +	buffer_zone = mmap(NULL, mem_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
> > +	if (buffer_zone == MAP_FAILED)
> > +		ksft_exit_fail_msg("mmap() for %d bytes failed\n", mem_size);
> > +
> >   	futex_ptr = mmap(NULL, mem_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
> >   	if (futex_ptr == MAP_FAILED)
> >   		ksft_exit_fail_msg("mmap() for %d bytes failed\n", mem_size);
> 
> This patch makes the assumption that consecutive mmap() calls will allocate
> pages consecutively downward from a certain address. I don't know if this
> assumption will be valid in all cases. I think it will be safer to just
> allocate the 2-page memory block and then change the 2nd page protection to
> PROT_NONE to make it a guard page.

You shouldn't make any assumption about mmap()'s returned pointer unless
you pass it as the addr argument and expect it to be followed.
Using two pages and making the second a guard page sounds sane.

> Cheers,
> Longman

Sebastian