Re: [PATCH] selftests/mm/uffd: remove static address usage in shmem_allocate_area()

On Tue, Nov 11, 2025 at 09:54:27PM +0100, Mehdi Ben Hadj Khelifa wrote:

The current shmem_allocate_area() implementation uses a hardcoded virtual base address(BASE_PMD_ADDR) as a hint for mmap() when creating shmem-backed test areas. This approach is fragile and may fail on systems with ASLR or different virtual memory layouts, where the chosen address is unavailable.

Replace the static base address with a dynamically reserved address range obtained via mmap(NULL, ..., PROT_NONE). The memfd-backed areas and their alias are then mapped into that reserved region using MAP_FIXED, preserving the original layout and aliasing semantics while avoiding collisions with unrelated mappings.

This change improves robustness and portability of the test suite without altering its behavior or coverage.

Signed-off-by: Mehdi Ben Hadj Khelifa mehdi.benhadjkhelifa@gmail.com

Testing: A diff between running the mm selftests on 6.18-rc5 from before and after the change show no regression on x86_64 architecture with 32GB DDR5 RAM. tools/testing/selftests/mm/uffd-common.c | 25 +++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c index 994fe8c03923..492b21c960bb 100644 --- a/tools/testing/selftests/mm/uffd-common.c +++ b/tools/testing/selftests/mm/uffd-common.c @@ -6,11 +6,11 @@ */ #include "uffd-common.h" +#include "asm-generic/mman-common.h"

Please drop this. There's already include <sys/mman.h> via uffd-common.h/vm_util.h.

uffd_test_ops_t *uffd_test_ops; uffd_test_case_ops_t *uffd_test_case_ops; -#define BASE_PMD_ADDR ((void *)(1UL << 30)) /* pthread_mutex_t starts at page offset 0 */ pthread_mutex_t *area_mutex(char *area, unsigned long nr, uffd_global_test_opts_t *gopts) @@ -142,30 +142,37 @@ static int shmem_allocate_area(uffd_global_test_opts_t *gopts, void **alloc_area unsigned long offset = is_src ? 0 : bytes; char *p = NULL, *p_alias = NULL; int mem_fd = uffd_mem_fd_create(bytes * 2, false);

• size_t region_size = bytes * 2 + hpage_size;

• /* TODO: clean this up. Use a static addr is ugly */
• p = BASE_PMD_ADDR;
• if (!is_src)

• /* src map + alias + interleaved hpages */
  

• p += 2 * (bytes + hpage_size);
  

• void *reserve = mmap(NULL, region_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS,

• 	-1, 0);
  

• if (reserve == MAP_FAILED) {

• close(mem_fd);
  

• return -errno;
  

• }
• p = (char *)reserve;

No need for casting here.

p_alias = p; p_alias += bytes; p_alias += hpage_size; /* Prevent src/dst VMA merge */

• *alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,

• *alloc_area = mmap(p, bytes, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, mem_fd, offset); if (*alloc_area == MAP_FAILED) {

• munmap(reserve, region_size);
  

I think it'll be more readable to put munmap() after setting *alloc_area to NULL.

*alloc_area = NULL;

• close(mem_fd);
  

  return -errno; } if (*alloc_area != p) err("mmap of memfd failed at %p", p);

• area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_SHARED,

• area_alias = mmap(p_alias, bytes, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, mem_fd, offset); if (area_alias == MAP_FAILED) {

• munmap(*alloc_area, bytes);
  

• munmap(reserve, region_size);
  

Here as well.

*alloc_area = NULL;

• close(mem_fd);
  

  return -errno; } if (area_alias != p_alias)

-- 2.51.2