Tests can allocate from virtual memory using kunit_vm_mmap(), which
transparently creates and attaches an mm_struct to the test runner if
one is not already attached. This is suitable for most cases, except for
when the code under test must access a task's mm before performing an
mmap. Expose kunit_attach_mm() as part of the interface for those
cases. This does not change the existing behavior.

Cc: David Gow <davidgow@google.com>
Signed-off-by: Tiffany Yang <ynaffit@google.com>
---
 include/kunit/test.h   | 12 ++++++++++++
 lib/kunit/user_alloc.c |  4 ++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/kunit/test.h b/include/kunit/test.h
index 39c768f87dc9..d958ee53050e 100644
--- a/include/kunit/test.h
+++ b/include/kunit/test.h
@@ -531,6 +531,18 @@ static inline char *kunit_kstrdup(struct kunit *test, const char *str, gfp_t gfp
  */
 const char *kunit_kstrdup_const(struct kunit *test, const char *str, gfp_t gfp);
 
+/**
+ * kunit_attach_mm() - Create and attach a new mm if it doesn't already exist.
+ *
+ * Allocates a &struct mm_struct and attaches it to @current. In most cases, call
+ * kunit_vm_mmap() without calling kunit_attach_mm() directly. Only necessary when
+ * code under test accesses the mm before executing the mmap (e.g., to perform
+ * additional initialization beforehand).
+ *
+ * Return: 0 on success, -errno on failure.
+ */
+int kunit_attach_mm(void);
+
 /**
  * kunit_vm_mmap() - Allocate KUnit-tracked vm_mmap() area
  * @test: The test context object.
diff --git a/lib/kunit/user_alloc.c b/lib/kunit/user_alloc.c
index 46951be018be..b8cac765e620 100644
--- a/lib/kunit/user_alloc.c
+++ b/lib/kunit/user_alloc.c
@@ -22,8 +22,7 @@ struct kunit_vm_mmap_params {
 	unsigned long offset;
 };
 
-/* Create and attach a new mm if it doesn't already exist. */
-static int kunit_attach_mm(void)
+int kunit_attach_mm(void)
 {
 	struct mm_struct *mm;
 
@@ -49,6 +48,7 @@ static int kunit_attach_mm(void)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(kunit_attach_mm);
 
 static int kunit_vm_mmap_init(struct kunit_resource *res, void *context)
 {
-- 
2.50.0.727.gbf7dc18ff4-goog

On Mon, Jul 14, 2025 at 11:53:16AM -0700, Tiffany Yang wrote:
> Tests can allocate from virtual memory using kunit_vm_mmap(), which
> transparently creates and attaches an mm_struct to the test runner if
> one is not already attached. This is suitable for most cases, except for
> when the code under test must access a task's mm before performing an
> mmap. Expose kunit_attach_mm() as part of the interface for those
> cases. This does not change the existing behavior.
> 
> Cc: David Gow <davidgow@google.com>
> Signed-off-by: Tiffany Yang <ynaffit@google.com>
> ---
>  include/kunit/test.h   | 12 ++++++++++++
>  lib/kunit/user_alloc.c |  4 ++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/include/kunit/test.h b/include/kunit/test.h
> index 39c768f87dc9..d958ee53050e 100644
> --- a/include/kunit/test.h
> +++ b/include/kunit/test.h
> @@ -531,6 +531,18 @@ static inline char *kunit_kstrdup(struct kunit *test, const char *str, gfp_t gfp
>   */
>  const char *kunit_kstrdup_const(struct kunit *test, const char *str, gfp_t gfp);
>  
> +/**
> + * kunit_attach_mm() - Create and attach a new mm if it doesn't already exist.
> + *
> + * Allocates a &struct mm_struct and attaches it to @current. In most cases, call
> + * kunit_vm_mmap() without calling kunit_attach_mm() directly. Only necessary when
> + * code under test accesses the mm before executing the mmap (e.g., to perform
> + * additional initialization beforehand).
> + *
> + * Return: 0 on success, -errno on failure.
> + */

Yay kern-doc! :)

> +int kunit_attach_mm(void);
> +
>  /**
>   * kunit_vm_mmap() - Allocate KUnit-tracked vm_mmap() area
>   * @test: The test context object.
> diff --git a/lib/kunit/user_alloc.c b/lib/kunit/user_alloc.c
> index 46951be018be..b8cac765e620 100644
> --- a/lib/kunit/user_alloc.c
> +++ b/lib/kunit/user_alloc.c
> @@ -22,8 +22,7 @@ struct kunit_vm_mmap_params {
>  	unsigned long offset;
>  };
>  
> -/* Create and attach a new mm if it doesn't already exist. */
> -static int kunit_attach_mm(void)
> +int kunit_attach_mm(void)
>  {
>  	struct mm_struct *mm;
>  
> @@ -49,6 +48,7 @@ static int kunit_attach_mm(void)
>  
>  	return 0;
>  }
> +EXPORT_SYMBOL_GPL(kunit_attach_mm);
>  
>  static int kunit_vm_mmap_init(struct kunit_resource *res, void *context)
>  {
> -- 
> 2.50.0.727.gbf7dc18ff4-goog

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

On Mon, Jul 14, 2025 at 11:53:16AM -0700, Tiffany Yang wrote:
> Tests can allocate from virtual memory using kunit_vm_mmap(), which
> transparently creates and attaches an mm_struct to the test runner if
> one is not already attached. This is suitable for most cases, except for
> when the code under test must access a task's mm before performing an
> mmap. Expose kunit_attach_mm() as part of the interface for those
> cases. This does not change the existing behavior.
>
> Cc: David Gow <davidgow@google.com>
> Signed-off-by: Tiffany Yang <ynaffit@google.com>
> ---
>  include/kunit/test.h   | 12 ++++++++++++
>  lib/kunit/user_alloc.c |  4 ++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/kunit/test.h b/include/kunit/test.h
> index 39c768f87dc9..d958ee53050e 100644
> --- a/include/kunit/test.h
> +++ b/include/kunit/test.h
> @@ -531,6 +531,18 @@ static inline char *kunit_kstrdup(struct kunit *test, const char *str, gfp_t gfp
>   */
>  const char *kunit_kstrdup_const(struct kunit *test, const char *str, gfp_t gfp);
>
> +/**
> + * kunit_attach_mm() - Create and attach a new mm if it doesn't already exist.
> + *
> + * Allocates a &struct mm_struct and attaches it to @current. In most cases, call
> + * kunit_vm_mmap() without calling kunit_attach_mm() directly. Only necessary when
> + * code under test accesses the mm before executing the mmap (e.g., to perform
> + * additional initialization beforehand).
> + *
> + * Return: 0 on success, -errno on failure.
> + */
> +int kunit_attach_mm(void);
> +
>  /**
>   * kunit_vm_mmap() - Allocate KUnit-tracked vm_mmap() area
>   * @test: The test context object.
> diff --git a/lib/kunit/user_alloc.c b/lib/kunit/user_alloc.c
> index 46951be018be..b8cac765e620 100644
> --- a/lib/kunit/user_alloc.c
> +++ b/lib/kunit/user_alloc.c
> @@ -22,8 +22,7 @@ struct kunit_vm_mmap_params {
>       unsigned long offset;
>  };
>
> -/* Create and attach a new mm if it doesn't already exist. */
> -static int kunit_attach_mm(void)
> +int kunit_attach_mm(void)
>  {
>       struct mm_struct *mm;
>
> @@ -49,6 +48,7 @@ static int kunit_attach_mm(void)
>
>       return 0;
>  }
> +EXPORT_SYMBOL_GPL(kunit_attach_mm);
>
>  static int kunit_vm_mmap_init(struct kunit_resource *res, void *context)
>  {
> --
> 2.50.0.727.gbf7dc18ff4-goog
>

LGTM!

Reviewed-by: Carlos Llamas <cmllamas@google.com>