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@arm.com>
---
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@arm.com/
---
 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}

On 9/11/26 16:29, 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@arm.com>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

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@arm.com>

LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

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@arm.com/
> ---
>  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