selftest: use mmap instead of posix_memalign to allocate memory

This test passes pointers obtained from anon_allocate_area to the
userfaultfd and mremap APIs.  This causes a problem if the system
allocator returns tagged pointers because with the tagged address ABI
the kernel rejects tagged addresses passed to these APIs, which would
end up causing the test to fail.  To make this test compatible with such
system allocators, stop using the system allocator to allocate memory in
anon_allocate_area, and instead just use mmap.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://linux-review.googlesource.com/id/Icac91064fcd923f77a83e8e133f8631c5b8fc241
Fixes: c47174f ("userfaultfd: selftest")
Co-developed-by: Lokesh Gidra <[email protected]>
Signed-off-by: Lokesh Gidra <[email protected]>
Signed-off-by: Peter Collingbourne <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Dave Martin <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Alistair Delva <[email protected]>
Cc: William McVicker <[email protected]>
Cc: Evgenii Stepanov <[email protected]>
Cc: Mitch Phillips <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: <[email protected]>	[5.4]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

tools/testing/selftests/vm/userfaultfd.c

@@ -210,8 +210,10 @@ static void anon_release_pages(char *rel_area)
 
 static void anon_allocate_area(void **alloc_area)
 {
-	if (posix_memalign(alloc_area, page_size, nr_pages * page_size))
-		err("posix_memalign() failed");
+	*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
+			   MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	if (*alloc_area == MAP_FAILED)
+		err("mmap of anonymous memory failed");
 }
 
 static void noop_alias_mapping(__u64 *start, size_t len, unsigned long offset)