[Linux-kernel-mentees] [PATCH v2] lib: kunit: Provides a userspace memory context when tests are compiled as module

[Vitor Massaru Iha]

KUnit test cases run on kthreads, and kthreads don't have an
adddress space (current->mm is NULL), but processes have mm.

The purpose of this patch is to allow to borrow mm to KUnit kthread
after userspace is brought up, because we know that there are processes
running, at least the process that loaded the module to borrow mm.

This allows, for example, tests such as user_copy_kunit, which uses
vm_mmap, which needs current->mm.

Signed-off-by: Vitor Massaru Iha <vitor at massaru.org>
---
v2:
    * splitted patch in 3:
        - Allows to install and load modules in root filesystem;
        - Provides an userspace memory context when tests are compiled
          as module;
        - Convert test_user_copy to KUnit test;
    * added documentation;
    * added more explanation;
    * tested a pointer;
    * released mput();
---
 Documentation/dev-tools/kunit/usage.rst | 14 ++++++++++++++
 include/kunit/test.h                    | 12 ++++++++++++
 lib/kunit/try-catch.c                   | 15 ++++++++++++++-
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index 3c3fe8b5fecc..9f909157be34 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -448,6 +448,20 @@ We can now use it to test ``struct eeprom_buffer``:
 
 .. _kunit-on-non-uml:
 
+User-space context
+------------------
+
+I case you need a user-space context, for now this is only possible through
+tests compiled as a module. And it will be necessary to use a root filesystem
+and uml_utilities.
+
+Example:
+
+.. code-block:: bash
+
+   ./tools/testing/kunit/kunit.py run --timeout=60 --uml_rootfs_dir=.uml_rootfs
+
+
 KUnit on non-UML architectures
 ==============================
 
diff --git a/include/kunit/test.h b/include/kunit/test.h
index 59f3144f009a..ae3337139c65 100644
--- a/include/kunit/test.h
+++ b/include/kunit/test.h
@@ -222,6 +222,18 @@ struct kunit {
 	 * protect it with some type of lock.
 	 */
 	struct list_head resources; /* Protected by lock. */
+	/*
+	 * KUnit test cases run on kthreads, and kthreads don't have an
+	 * adddress space (current->mm is NULL), but processes have mm.
+	 *
+	 * The purpose of this mm_struct is to allow to borrow mm to KUnit kthread
+	 * after userspace is brought up, because we know that there are processes
+	 * running, at least the process that loaded the module to borrow mm.
+	 * 
+	 * This allows, for example, tests such as user_copy_kunit, which uses
+	 * vm_mmap, which needs current->mm.
+	 */
+	struct mm_struct *mm;
 };
 
 void kunit_init_test(struct kunit *test, const char *name, char *log);
diff --git a/lib/kunit/try-catch.c b/lib/kunit/try-catch.c
index 0dd434e40487..d03e2093985b 100644
--- a/lib/kunit/try-catch.c
+++ b/lib/kunit/try-catch.c
@@ -11,7 +11,8 @@
 #include <linux/completion.h>
 #include <linux/kernel.h>
 #include <linux/kthread.h>
-
+#include <linux/sched/mm.h>
+#include <linux/sched/task.h>
 #include "try-catch-impl.h"
 
 void __noreturn kunit_try_catch_throw(struct kunit_try_catch *try_catch)
@@ -24,8 +25,17 @@ EXPORT_SYMBOL_GPL(kunit_try_catch_throw);
 static int kunit_generic_run_threadfn_adapter(void *data)
 {
 	struct kunit_try_catch *try_catch = data;
+	struct kunit *test = try_catch->test;
+	
+	if (test != NULL && test->mm != NULL)
+		kthread_use_mm(test->mm);
 
 	try_catch->try(try_catch->context);
+	if (test != NULL && test->mm != NULL) {
+		kthread_unuse_mm(test->mm);
+		mmput(test->mm);
+		test->mm = NULL;
+	}
 
 	complete_and_exit(try_catch->try_completion, 0);
 }
@@ -65,6 +75,9 @@ void kunit_try_catch_run(struct kunit_try_catch *try_catch, void *context)
 	try_catch->context = context;
 	try_catch->try_completion = &try_completion;
 	try_catch->try_result = 0;
+
+	test->mm = get_task_mm(current);
+
 	task_struct = kthread_run(kunit_generic_run_threadfn_adapter,
 				  try_catch,
 				  "kunit_try_catch_thread");

base-commit: 725aca9585956676687c4cb803e88f770b0df2b2
prerequisite-patch-id: 5e5f9a8a05c5680fda1b04c9ab1b95ce91dc88b2
prerequisite-patch-id: 4d997940f4a9f303424af9bac412de1af861f9d9
prerequisite-patch-id: 582b6d9d28ce4b71628890ec832df6522ca68de0
-- 
2.26.2