Re: [PATCH] kasan: fix unit tests with CONFIG_UBSAN_LOCAL_BOUNDS enabled

[Peter Collingbourne <pcc@google.com>]

On Thu, May 6, 2021 at 3:12 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Thu, May 6, 2021 at 11:20 PM Peter Collingbourne <pcc@google.com> wrote:
> >
> > These tests deliberately access these arrays out of bounds,
> > which will cause the dynamic local bounds checks inserted by
> > CONFIG_UBSAN_LOCAL_BOUNDS to fail and panic the kernel. To avoid this
> > problem, access the arrays via volatile pointers, which will prevent
> > the compiler from being able to determine the array bounds.
> >
> > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > Cc: stable@vger.kernel.org
> > Link: https://linux-review.googlesource.com/id/I90b1713fbfa1bf68ff895aef099ea77b98a7c3b9
> > ---
> >  lib/test_kasan.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> > index dc05cfc2d12f..2a078e8e7b8e 100644
> > --- a/lib/test_kasan.c
> > +++ b/lib/test_kasan.c
> > @@ -654,8 +654,8 @@ static char global_array[10];
> >
> >  static void kasan_global_oob(struct kunit *test)
> >  {
> > -       volatile int i = 3;
> > -       char *p = &global_array[ARRAY_SIZE(global_array) + i];
> > +       char *volatile array = global_array;
> > +       char *p = &array[ARRAY_SIZE(global_array) + 3];
>
> Nit: in the kernel, "volatile" usually comes before the pointer type.

That would refer to a different type. "volatile char *" is a pointer
to volatile char, while "char *volatile" is a volatile pointer to
char. The latter is what we want here, because we want to prevent the
compiler from inferring things about the pointer itself (i.e. its
array bounds), not the data that it refers to.

Peter