[PATCH v2] tools/testing/selftests: add merge test for partial msealed range

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

Commit 2697dd8ae721 ("mm/mseal: update VMA end correctly on merge") fixed
an issue in the loop which iterates through VMAs applying mseal, which was
triggered by mseal()'ing a range of VMAs where the second was mseal()'d and
the first mergeable with it, once mseal()'d.

Add a regression test to assert that this behaviour is correct. We place it
in the merge selftests as this is strictly an issue with merging (via a
vma_modify() invocation).

It also assert that mseal()'d ranges are correctly merged as you'd expect.

The test is implemented such that it is skipped if mseal() is not
available on the system.

Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
v2:
* Added tools/ based header so __NR_mseal should always be available.
* However, for completeness, also check to see if defined, and assume ENOSYS if
  not.
* Thanks to Mike for reporting issues in his build on this test!

v1:
https://lore.kernel.org/all/20260330135011.107036-1-ljs@kernel.org/

 tools/testing/selftests/mm/merge.c | 89 ++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)

diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
index 10b686102b79..f73803b3679a 100644
--- a/tools/testing/selftests/mm/merge.c
+++ b/tools/testing/selftests/mm/merge.c
@@ -2,6 +2,7 @@

 #define _GNU_SOURCE
 #include "kselftest_harness.h"
+#include <asm-generic/unistd.h>
 #include <linux/prctl.h>
 #include <fcntl.h>
 #include <stdio.h>
@@ -48,6 +49,19 @@ static pid_t do_fork(struct procmap_fd *procmap)
 	return 0;
 }

+#ifdef __NR_mseal
+static int sys_mseal(void *ptr, size_t len, unsigned long flags)
+{
+	return syscall(__NR_mseal, (unsigned long)ptr, len, flags);
+}
+#else
+static int sys_mseal(void *ptr, size_t len, unsigned long flags)
+{
+	errno = ENOSYS;
+	return -1;
+}
+#endif
+
 FIXTURE_SETUP(merge)
 {
 	self->page_size = psize();
@@ -1217,6 +1231,81 @@ TEST_F(merge, mremap_correct_placed_faulted)
 	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 15 * page_size);
 }

+TEST_F(merge, merge_vmas_with_mseal)
+{
+	unsigned int page_size = self->page_size;
+	struct procmap_fd *procmap = &self->procmap;
+	char *ptr, *ptr2, *ptr3;
+	/* We need our own as cannot munmap() once sealed. */
+	char *carveout;
+
+	/* Invalid mseal() call to see if implemented. */
+	ASSERT_EQ(sys_mseal(NULL, 0, ~0UL), -1);
+	if (errno == ENOSYS)
+		SKIP(return, "mseal not supported, skipping.");
+
+	/* Map carveout. */
+	carveout = mmap(NULL, 17 * page_size, PROT_NONE,
+			MAP_PRIVATE | MAP_ANON, -1, 0);
+	ASSERT_NE(carveout, MAP_FAILED);
+
+	/*
+	 * Map 3 separate VMAs:
+	 *
+	 * |-----------|-----------|-----------|
+	 * |    RW     |    RWE    |    RO     |
+	 * |-----------|-----------|-----------|
+	 *      ptr         ptr2        ptr3
+	 */
+	ptr = mmap(&carveout[page_size], 5 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+	ASSERT_NE(ptr, MAP_FAILED);
+	ptr2 = mmap(&carveout[page_size * 6], 5 * page_size,
+		    PROT_READ | PROT_WRITE | PROT_EXEC,
+		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+	ASSERT_NE(ptr2, MAP_FAILED);
+	ptr3 = mmap(&carveout[page_size * 11], 5 * page_size, PROT_READ,
+		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+	ASSERT_NE(ptr3, MAP_FAILED);
+
+	/*
+	 * mseal the second VMA:
+	 *
+	 * |-----------|-----------|-----------|
+	 * |    RW     |    RWES   |    RO     |
+	 * |-----------|-----------|-----------|
+	 *      ptr         ptr2        ptr3
+	 */
+	ASSERT_EQ(sys_mseal(ptr2, 5 * page_size, 0), 0);
+
+	/* Make first VMA mergeable upon mseal. */
+	ASSERT_EQ(mprotect(ptr, 5 * page_size,
+			   PROT_READ | PROT_WRITE | PROT_EXEC), 0);
+	/*
+	 * At this point we have:
+	 *
+	 * |-----------|-----------|-----------|
+	 * |    RWE    |    RWES   |    RO     |
+	 * |-----------|-----------|-----------|
+	 *      ptr         ptr2        ptr3
+	 *
+	 * Now mseal all of the VMAs.
+	 */
+	ASSERT_EQ(sys_mseal(ptr, 15 * page_size, 0), 0);
+
+	/*
+	 * We should end up with:
+	 *
+	 * |-----------------------|-----------|
+	 * |          RWES         |    ROS    |
+	 * |-----------------------|-----------|
+	 *            ptr               ptr3
+	 */
+	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
+	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
+	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 10 * page_size);
+}
+
 TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev)
 {
 	struct procmap_fd *procmap = &self->procmap;
--
2.53.0