From ad055ac3cdbe49afef08cd035572b428a2ae1d93 Mon Sep 17 00:00:00 2001
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: Mon, 20 Jul 2026 15:38:40 +0100
Subject: [PATCH] tools/testing/vma: add test to assert MAP_PRIVATE-/dev/zero
 is anon

Now we've made MAP_PRIVATE-mapped /dev/zero mappings truly anonymous, add
a VMA userland test to assert that this is the case and everything is as
we would expect for an anonymous mapping.

Link: https://lore.kernel.org/20260720-b4-scalable-cow-virt-pgoff-v2-14-2d549757a76f@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: Harry Yoo <harry@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 tools/testing/vma/include/dup.h |  1 +
 tools/testing/vma/tests/mmap.c  | 50 +++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index bb902a4b33e1..800e5fa02d78 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -20,6 +20,7 @@ struct task_struct *get_current(void);
 
 #define MAJOR(dev)	((unsigned int) ((dev) >> MINORBITS))
 #define MINOR(dev)	((unsigned int) ((dev) & MINORMASK))
+#define MKDEV(ma, mi)	(((ma) << MINORBITS) | (mi))
 
 #define S_IFMT  00170000
 #define S_IFCHR  0020000
diff --git a/tools/testing/vma/tests/mmap.c b/tools/testing/vma/tests/mmap.c
index c85bc000d1cb..b720deb4227d 100644
--- a/tools/testing/vma/tests/mmap.c
+++ b/tools/testing/vma/tests/mmap.c
@@ -45,7 +45,57 @@ static bool test_mmap_region_basic(void)
 	return true;
 }
 
+static int dummy_mmap_prepare(struct vm_area_desc *desc)
+{
+	return 0;
+}
+
+static bool test_pure_anon_dev_zero(void)
+{
+	const vma_flags_t vma_flags = mk_vma_flags(VMA_READ_BIT, VMA_WRITE_BIT,
+			VMA_MAYREAD_BIT, VMA_MAYWRITE_BIT);
+	const struct file_operations f_op = {
+		.mmap_prepare = dummy_mmap_prepare,
+	};
+	struct inode inode = {
+		.i_mode = S_IFCHR,
+		.i_rdev = MKDEV(MEM_MAJOR, DEVZERO_MINOR),
+	};
+	struct file file = {
+		.f_inode = &inode,
+		.f_op = &f_op,
+	};
+	struct mm_struct mm = {};
+	struct vm_area_struct *vma;
+	unsigned long addr;
+	VMA_ITERATOR(vmi, &mm, 0);
+
+	current->mm = &mm;
+
+	/*
+	 * Map a MAP_PRIVATE-/dev/zero mapping at address 0x300000 with a page
+	 * offset of 0x10, which we expect to be reset to the virtual page
+	 * offset.
+	 */
+	addr = __mmap_region(&file, 0x300000, 0x3000, vma_flags, 0x10, NULL);
+	ASSERT_EQ(addr, 0x300000);
+
+	/* Assert that it truly is an anonymous mapping. */
+	vma = vma_lookup(&mm, addr);
+	ASSERT_NE(vma, NULL);
+	ASSERT_TRUE(vma_is_anonymous(vma));
+	ASSERT_EQ(vma->vm_file, NULL);
+	ASSERT_EQ(vma->vm_private_data, NULL);
+	/* Expect virtual page offsets. */
+	ASSERT_EQ(vma->vm_pgoff, 0x300);
+	ASSERT_EQ(vma_start_virt_pgoff(vma), 0x300);
+
+	cleanup_mm(&mm, &vmi);
+	return true;
+}
+
 static void run_mmap_tests(int *num_tests, int *num_fail)
 {
 	TEST(mmap_region_basic);
+	TEST(pure_anon_dev_zero);
 }