Re: [PATCH v2 19/20] kasan, mm: allow cache merging with no metadata

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Nov 11, 2020 at 4:13 PM Marco Elver <elver@google.com> wrote:
>
> On Tue, Nov 10, 2020 at 11:20PM +0100, Andrey Konovalov wrote:
> > The reason cache merging is disabled with KASAN is because KASAN puts its
> > metadata right after the allocated object. When the merged caches have
> > slightly different sizes, the metadata ends up in different places, which
> > KASAN doesn't support.
> >
> > It might be possible to adjust the metadata allocation algorithm and make
> > it friendly to the cache merging code. Instead this change takes a simpler
> > approach and allows merging caches when no metadata is present. Which is
> > the case for hardware tag-based KASAN with kasan.mode=prod.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > Link: https://linux-review.googlesource.com/id/Ia114847dfb2244f297d2cb82d592bf6a07455dba
> > ---
> >  include/linux/kasan.h | 26 ++++++++++++++++++++++++--
> >  mm/kasan/common.c     | 11 +++++++++++
> >  mm/slab_common.c      | 11 ++++++++---
> >  3 files changed, 43 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > index 534ab3e2935a..c754eca356f7 100644
> > --- a/include/linux/kasan.h
> > +++ b/include/linux/kasan.h
> > @@ -81,17 +81,35 @@ struct kasan_cache {
> >  };
> >
> >  #ifdef CONFIG_KASAN_HW_TAGS
> > +
> >  DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
> > +
> >  static inline kasan_enabled(void)
> >  {
> >       return static_branch_likely(&kasan_flag_enabled);
> >  }
> > -#else
> > +
> > +slab_flags_t __kasan_never_merge(slab_flags_t flags);
> > +static inline slab_flags_t kasan_never_merge(slab_flags_t flags)
> > +{
> > +     if (kasan_enabled())
> > +             return __kasan_never_merge(flags);
> > +     return flags;
> > +}
> > +
> > +#else /* CONFIG_KASAN_HW_TAGS */
> > +
> >  static inline kasan_enabled(void)
> >  {
> >       return true;
> >  }
> > -#endif
> > +
> > +static inline slab_flags_t kasan_never_merge(slab_flags_t flags)
> > +{
> > +     return flags;
> > +}
> > +
> > +#endif /* CONFIG_KASAN_HW_TAGS */
> >
> >  void __kasan_alloc_pages(struct page *page, unsigned int order);
> >  static inline void kasan_alloc_pages(struct page *page, unsigned int order)
> > @@ -240,6 +258,10 @@ static inline kasan_enabled(void)
> >  {
> >       return false;
> >  }
> > +static inline slab_flags_t kasan_never_merge(slab_flags_t flags)
> > +{
> > +     return flags;
> > +}
> >  static inline void kasan_alloc_pages(struct page *page, unsigned int order) {}
> >  static inline void kasan_free_pages(struct page *page, unsigned int order) {}
> >  static inline void kasan_cache_create(struct kmem_cache *cache,
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 940b42231069..25b18c145b06 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -81,6 +81,17 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
> >  }
> >  #endif /* CONFIG_KASAN_STACK */
> >
> > +/*
> > + * Only allow cache merging when stack collection is disabled and no metadata
> > + * is present.
> > + */
> > +slab_flags_t __kasan_never_merge(slab_flags_t flags)
> > +{
> > +     if (kasan_stack_collection_enabled())
> > +             return flags;
> > +     return flags & ~SLAB_KASAN;
> > +}
> > +
> >  void __kasan_alloc_pages(struct page *page, unsigned int order)
> >  {
> >       u8 tag;
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index f1b0c4a22f08..3042ee8ea9ce 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/seq_file.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/debugfs.h>
> > +#include <linux/kasan.h>
> >  #include <asm/cacheflush.h>
> >  #include <asm/tlbflush.h>
> >  #include <asm/page.h>
> > @@ -49,12 +50,16 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
> >                   slab_caches_to_rcu_destroy_workfn);
> >
> >  /*
> > - * Set of flags that will prevent slab merging
> > + * Set of flags that will prevent slab merging.
> > + * Use slab_never_merge() instead.
> >   */
> >  #define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
> >               SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \
> >               SLAB_FAILSLAB | SLAB_KASAN)
>
> Rather than changing this to require using slab_never_merge() which
> removes SLAB_KASAN, could we not just have a function
> kasan_never_merge() that returns KASAN-specific flags that should never
> result in merging -- because as-is now, making kasan_never_merge()
> remove the SLAB_KASAN flag seems the wrong way around.
>
> Could we not just do this:
>
>   #define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
>                 SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \
>                 SLAB_FAILSLAB | kasan_never_merge())
>
> ??

The issue here was that SLAB_KASAN is defined in slab.h, which
includes kasan.h, so we can't have a static inline definition of this
function for generic and software tag-based modes. So we can do this,
as long as we're fine with having kasan_never_merge() to be an actual
function call for all KASAN modes. I guess it's not a problem, so
let's do it this way.

>
> Of course that might be problematic if this always needs to be a
> compile-time constant, but currently that's not a requirement.
>
> > +/* KASAN allows merging in some configurations and will remove SLAB_KASAN. */
> > +#define slab_never_merge() (kasan_never_merge(SLAB_NEVER_MERGE))
>
> Braces unnecessary.