Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

[Marco Elver]

On Thu, 13 May 2021 at 08:28, Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> On Wed, May 12, 2021 at 08:40:24PM -0700, Andrew Morton wrote:
> > On Thu, 13 May 2021 12:12:20 +0900 Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> > > On Wed, May 12, 2021 at 07:52:27PM -0700, Andrew Morton wrote:
> > > > This explodes in mysterious ways.  The patch as I have it is appended,
> > > > for reference.
> > > >
> > > > gcc-10.3.0 allmodconfig.
> > > >
> > > > This patch suppresses the error:
> >
> > Ah, yes, of course, your patch changes kmalloc_index() to require that
> > it always is called with a constant `size'.  kfence_test doesn't do
> > that.
> >
> > kfence is being a bit naughty here - the other kmalloc_index() callers
> > only comple up the call after verifying that `size' is a compile-time
> > constant.
> >
> > Would something like this work?
> >  include/linux/slab.h    |   12 ++++++++----
> >  mm/kfence/kfence_test.c |    4 ++--
> >  2 files changed, 10 insertions(+), 6 deletions(-)
> >
> > --- a/include/linux/slab.h~b
> > +++ a/include/linux/slab.h
> > @@ -374,7 +374,8 @@ static __always_inline enum kmalloc_cach
> >   * Note: there's no need to optimize kmalloc_index because it's evaluated
> >   * in compile-time.
> >   */
> > -static __always_inline unsigned int kmalloc_index(size_t size)
> > +static __always_inline unsigned int kmalloc_index(size_t size,
> > +                                               bool size_is_constant)
> >  {
> >       if (!size)
> >               return 0;
> > @@ -410,7 +411,10 @@ static __always_inline unsigned int kmal
> >       if (size <=  16 * 1024 * 1024) return 24;
> >       if (size <=  32 * 1024 * 1024) return 25;
> >
> > -     BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> > +     if (size_is_constant)
> > +             BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> > +     else
> > +             BUG();
>
>
> kfence is randomly generating size. because kfence is using non-constant
> size, we should do run-time assertion or compile-time assertion depending
> on situation.
>
> I think we can use __builtin_constant_p here. we don't need to modify
> kmalloc_index's prototype.
>
> so what about this?
> if you think it makes sense, I'll send patch v4.
>
> I used KMALLOC_MAX_CACHE_SIZE to assure it's safe size.
> it's safer than putting BUILD_BUG_ON_MSG(1, ...) to below if statements
> because KMALLOC_MAX_CACHE_SIZE can be less than 32MB.

I'm actually inclined to say that Andrew's patch with
'size_is_constant' is the better option, because we want to be
explicit about where it's using constant size and where it isn't. I
think in tests like kfence_test, it should be permitted to use
non-constant size, it's a test after all and performance is no
concern. For non-test code, however, we want to ensure size is
constant, and therefore having the distinguishing argument makes
sense. That way non-test code will not compile if our intent does not
match reality.

Thanks,
-- Marco