selftests/bpf: Avoid a page fault in rcu_reentry while the handler is armed

The test arms a handler at rcu_segcblist_enqueue() for its thread. Then it
deletes an element of a task storage map. The delete gets to
rcu_segcblist_enqueue(), and the handler does a nested delete there.

The handler fires at the first rcu_segcblist_enqueue() in the thread. A
page fault can be the first:

  skel->bss->target_pid = tid      the handler is armed
  the next insn                    is in a page that is not in memory
    asm_exc_page_fault
      filemap_fault
        v9fs_issue_read
          p9_req_put
            irq_work_run
              rcu_defer_drain
                rcu_segcblist_enqueue
                  the handler deletes the element
  bpf_map_delete_elem()            -ENOENT

  test_rcu_reentry:FAIL:boot_delete unexpected error: -2 (errno 2)

The result depends on the layout of test_progs. In the binary that failed,
test_rcu_reentry() crosses the end of a page immediately after the store.
This binary fails each time when it is on 9p. A copy of the same file on
ext4 passes.

Move the store and the delete to a function. Call it one time without a pid
and with a bad fd. This call brings the text and .bss into memory, and it
does not arm the handler.

Fixes: 0464f587c61a ("selftests/bpf: Add a call_srcu() re-entry reproducer")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261027190314.1440199-11-alexei.starovoitov@gmail.com

1 files changed, 15 insertions, 4 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/rcu_reentry.c b/tools/testing/selftests/bpf/prog_tests/rcu_reentry.c
index de23a14b3d40..10b4ea913921 100644
--- a/tools/testing/selftests/bpf/prog_tests/rcu_reentry.c
+++ b/tools/testing/selftests/bpf/prog_tests/rcu_reentry.c
@@ -23,10 +23,17 @@ static bool have_tree_srcu(void)
 	return kallsyms_find("srcu_expedite_current", &addr) == 0;
 }
 
+/* Arm the handler for this thread, then trigger call_rcu_tasks_trace(). */
+static noinline int arm_and_delete(struct rcu_reentry *skel, int pid, int map_fd, int *pidfd)
+{
+	skel->bss->target_pid = pid;
+	return bpf_map_delete_elem(map_fd, pidfd);
+}
+
 void test_rcu_reentry(void)
 {
 	struct rcu_reentry *skel;
-	int err, pidfd = -1, map_fd;
+	int err, pidfd = -1, bad_fd = -1, map_fd;
 	cpu_set_t set, old_set;
 	bool affinity_saved;
 	__u64 val = 1;
@@ -63,9 +70,13 @@ void test_rcu_reentry(void)
 	if (!ASSERT_OK(err, "boot_create"))
 		goto restore;
 
-	/* Arm the handler for this thread, then trigger call_rcu_tasks_trace(). */
-	skel->bss->target_pid = syscall(__NR_gettid);
-	err = bpf_map_delete_elem(map_fd, &pidfd);
+	/*
+	 * The handler fires at the first rcu_segcblist_enqueue() in this thread, and it deletes
+	 * the element. A page fault can get there too, when the file system reads the page.
+	 * Go through the same code first, without a pid, to bring its text and .bss in.
+	 */
+	arm_and_delete(skel, 0, map_fd, &bad_fd);
+	err = arm_and_delete(skel, syscall(__NR_gettid), map_fd, &pidfd);
 	if (!ASSERT_OK(err, "boot_delete"))
 		goto restore;