selftests/vm: anon_cow: add mprotect() optimization tests

Let's extend the test to cover the possible mprotect() optimization when
removing write-protection. mprotect() must not allow write-access to a
COW-shared page by accident.

Link: https://lkml.kernel.org/r/20221108174652.198904-8-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/tools/testing/selftests/vm/anon_cow.c b/tools/testing/selftests/vm/anon_cow.c
index 705bd0b..bbb251e 100644 (file)
--- a/tools/testing/selftests/vm/anon_cow.c
+++ b/tools/testing/selftests/vm/anon_cow.c
@@ -190,7 +190,8 @@ static int child_vmsplice_memcmp_fn(char *mem, size_t size,
 
 typedef int (*child_fn)(char *mem, size_t size, struct comm_pipes *comm_pipes);
 
-static void do_test_cow_in_parent(char *mem, size_t size, child_fn fn)
+static void do_test_cow_in_parent(char *mem, size_t size, bool do_mprotect,
+                                 child_fn fn)
 {
        struct comm_pipes comm_pipes;
        char buf;
@@ -212,6 +213,22 @@ static void do_test_cow_in_parent(char *mem, size_t size, child_fn fn)
 
        while (read(comm_pipes.child_ready[0], &buf, 1) != 1)
                ;
+
+       if (do_mprotect) {
+               /*
+                * mprotect() optimizations might try avoiding
+                * write-faults by directly mapping pages writable.
+                */
+               ret = mprotect(mem, size, PROT_READ);
+               ret |= mprotect(mem, size, PROT_READ|PROT_WRITE);
+               if (ret) {
+                       ksft_test_result_fail("mprotect() failed\n");
+                       write(comm_pipes.parent_ready[1], "0", 1);
+                       wait(&ret);
+                       goto close_comm_pipes;
+               }
+       }
+
        /* Modify the page. */
        memset(mem, 0xff, size);
        write(comm_pipes.parent_ready[1], "0", 1);
@@ -229,12 +246,22 @@ close_comm_pipes:
 
 static void test_cow_in_parent(char *mem, size_t size)
 {
-       do_test_cow_in_parent(mem, size, child_memcmp_fn);
+       do_test_cow_in_parent(mem, size, false, child_memcmp_fn);
+}
+
+static void test_cow_in_parent_mprotect(char *mem, size_t size)
+{
+       do_test_cow_in_parent(mem, size, true, child_memcmp_fn);
 }
 
 static void test_vmsplice_in_child(char *mem, size_t size)
 {
-       do_test_cow_in_parent(mem, size, child_vmsplice_memcmp_fn);
+       do_test_cow_in_parent(mem, size, false, child_vmsplice_memcmp_fn);
+}
+
+static void test_vmsplice_in_child_mprotect(char *mem, size_t size)
+{
+       do_test_cow_in_parent(mem, size, true, child_vmsplice_memcmp_fn);
 }
 
 static void do_test_vmsplice_in_parent(char *mem, size_t size,
@@ -969,6 +996,14 @@ static const struct test_case test_cases[] = {
                "Basic COW after fork()",
                test_cow_in_parent,
        },
+       /*
+        * Basic test, but do an additional mprotect(PROT_READ)+
+        * mprotect(PROT_READ|PROT_WRITE) in the parent before write access.
+        */
+       {
+               "Basic COW after fork() with mprotect() optimization",
+               test_cow_in_parent_mprotect,
+       },
        /*
         * vmsplice() [R/O GUP] + unmap in the child; modify in the parent. If
         * we miss to break COW, the child observes modifications by the parent.
@@ -978,6 +1013,14 @@ static const struct test_case test_cases[] = {
                "vmsplice() + unmap in child",
                test_vmsplice_in_child
        },
+       /*
+        * vmsplice() test, but do an additional mprotect(PROT_READ)+
+        * mprotect(PROT_READ|PROT_WRITE) in the parent before write access.
+        */
+       {
+               "vmsplice() + unmap in child with mprotect() optimization",
+               test_vmsplice_in_child_mprotect
+       },
        /*
         * vmsplice() [R/O GUP] in parent before fork(), unmap in parent after
         * fork(); modify in the child. If we miss to break COW, the parent