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

[Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@gmail.com>]

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"
 
 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;
 	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);
 		*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);
 		*alloc_area = NULL;
+		close(mem_fd);
 		return -errno;
 	}
 	if (area_alias != p_alias)
-- 
2.51.2