Re: [PATCH v2] kselftest: mm: fix potential failure for merged VMA in guard-regions

[Lorenzo Stoakes (ARM)]

On Fri, Sep 11, 2026 at 03:29:03PM +0100, Yeoreum Yun wrote:
> check_vmflag_guard() uses /proc/self/smaps to retrieve the VMA flags,
> but this can fail if the mapping is merged with an adjacent VMA.
>
> To avoid this potential failure, first allocate a temporary region with
> extra pages at both ends, unmap it, and then map the test region within
> the temporary address range, leaving an unmapped page on each side to
> prevent VMA merging.
>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>

LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Thanks!

> ---
> Change in v2:
>   - change the prot as PROT_NONE for placeholder region.
>   - add comment for clarification.
>   - Link to v1: https://lore.kernel.org/all/20260911123534.1181501-1-yeoreum.yun@xxxxxxx/
> ---
>  tools/testing/selftests/mm/guard-regions.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> index 5c8ec3ca75d7..b724d62d2b75 100644
> --- a/tools/testing/selftests/mm/guard-regions.c
> +++ b/tools/testing/selftests/mm/guard-regions.c
> @@ -2257,8 +2257,18 @@ TEST_F(guard_regions, smaps)
>  	char *ptr, *ptr2;
>  	int i;
>
> -	/* Map a region. */
> -	ptr = mmap_(self, variant, NULL, 10 * page_size, PROT_READ | PROT_WRITE, 0, 0);
> +	/* Map then unmap placeholder to avoid adjacent merges */
> +	ptr = mmap_(self, variant, NULL, 12 * page_size, PROT_NONE, 0, 0);
> +	ASSERT_NE(ptr, MAP_FAILED);
> +	ASSERT_EQ(munmap(ptr, 12 * page_size), 0);
> +
> +	/*
> +	 * Map a region for the test. Since the preceding temporary mapping
> +	 * succeeded, this mapping should also succeed without merging with
> +	 * adjacent VMAs.
> +	 */
> +	ptr = mmap_(self, variant, ptr + page_size, 10 * page_size,
> +		    PROT_READ | PROT_WRITE, MAP_FIXED, 0);
>  	ASSERT_NE(ptr, MAP_FAILED);
>
>  	/* We shouldn't yet see a guard flag. */
> --
> LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
>

--
Cheers, Lorenzo