Re: [PATCH RFC 04/10] mm, kfence: insert KFENCE hooks for SLAB

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, Sep 11, 2020 at 2:24 PM Marco Elver <elver@google.com> wrote:
> > > From: Alexander Potapenko <glider@google.com>
> > >
> > > Inserts KFENCE hooks into the SLAB allocator.
> > >
> > > We note the addition of the 'orig_size' argument to slab_alloc*()
> > > functions, to be able to pass the originally requested size to KFENCE.
> > > When KFENCE is disabled, there is no additional overhead, since these
> > > functions are __always_inline.
> > >
> > > Co-developed-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Alexander Potapenko <glider@google.com>
> > > ---
> > >  mm/slab.c        | 46 ++++++++++++++++++++++++++++++++++------------
> > >  mm/slab_common.c |  6 +++++-
> > >  2 files changed, 39 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/mm/slab.c b/mm/slab.c
> > > index 3160dff6fd76..30aba06ae02b 100644
> > > --- a/mm/slab.c
> > > +++ b/mm/slab.c
> > > @@ -100,6 +100,7 @@
> > >  #include       <linux/seq_file.h>
> > >  #include       <linux/notifier.h>
> > >  #include       <linux/kallsyms.h>
> > > +#include       <linux/kfence.h>
> > >  #include       <linux/cpu.h>
> > >  #include       <linux/sysctl.h>
> > >  #include       <linux/module.h>
> > > @@ -3206,7 +3207,7 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
> > >  }
> > >
> > >  static __always_inline void *
> > > -slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
> > > +slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid, size_t orig_size,
> > >                    unsigned long caller)
> > >  {
> > >         unsigned long save_flags;
> > > @@ -3219,6 +3220,10 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
> > >         if (unlikely(!cachep))
> > >                 return NULL;
> > >
> > > +       ptr = kfence_alloc(cachep, orig_size, flags);
> > > +       if (unlikely(ptr))
> > > +               goto out_hooks;
> > > +
> > >         cache_alloc_debugcheck_before(cachep, flags);
> > >         local_irq_save(save_flags);
> > >
> > > @@ -3251,6 +3256,7 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
> > >         if (unlikely(slab_want_init_on_alloc(flags, cachep)) && ptr)
> > >                 memset(ptr, 0, cachep->object_size);
> > >
> > > +out_hooks:
> > >         slab_post_alloc_hook(cachep, objcg, flags, 1, &ptr);
> > >         return ptr;
> > >  }
> > > @@ -3288,7 +3294,7 @@ __do_cache_alloc(struct kmem_cache *cachep, gfp_t flags)
> > >  #endif /* CONFIG_NUMA */
> > >
> > >  static __always_inline void *
> > > -slab_alloc(struct kmem_cache *cachep, gfp_t flags, unsigned long caller)
> > > +slab_alloc(struct kmem_cache *cachep, gfp_t flags, size_t orig_size, unsigned long caller)
> > >  {
> > >         unsigned long save_flags;
> > >         void *objp;
> > > @@ -3299,6 +3305,10 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, unsigned long caller)
> > >         if (unlikely(!cachep))
> > >                 return NULL;
> > >
> > > +       objp = kfence_alloc(cachep, orig_size, flags);
> > > +       if (unlikely(objp))
> > > +               goto leave;
> > > +
> > >         cache_alloc_debugcheck_before(cachep, flags);
> > >         local_irq_save(save_flags);
> > >         objp = __do_cache_alloc(cachep, flags);
> > > @@ -3309,6 +3319,7 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, unsigned long caller)
> > >         if (unlikely(slab_want_init_on_alloc(flags, cachep)) && objp)
> > >                 memset(objp, 0, cachep->object_size);
> > >
> > > +leave:
> > >         slab_post_alloc_hook(cachep, objcg, flags, 1, &objp);
> > >         return objp;
> > >  }
> > > @@ -3414,6 +3425,11 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> > >  static __always_inline void __cache_free(struct kmem_cache *cachep, void *objp,
> > >                                          unsigned long caller)
> > >  {
> > > +       if (kfence_free(objp)) {
> > > +               kmemleak_free_recursive(objp, cachep->flags);
> > > +               return;
> > > +       }
> > > +
> > >         /* Put the object into the quarantine, don't touch it for now. */
> > >         if (kasan_slab_free(cachep, objp, _RET_IP_))
> > >                 return;
> > > @@ -3479,7 +3495,7 @@ void ___cache_free(struct kmem_cache *cachep, void *objp,
> > >   */
> > >  void *kmem_cache_alloc(struct kmem_cache *cachep, gfp_t flags)
> > >  {
> > > -       void *ret = slab_alloc(cachep, flags, _RET_IP_);
> > > +       void *ret = slab_alloc(cachep, flags, cachep->object_size, _RET_IP_);
> >
> >
> > It's kinda minor, but since we are talking about malloc fast path:
> > will passing 0 instead of cachep->object_size (here and everywhere
> > else) and then using cachep->object_size on the slow path if 0 is
> > passed as size improve codegen?
>
> It doesn't save us much, maybe 1 instruction based on what I'm looking
> at right now. The main worry I have is that the 'orig_size' argument
> is now part of slab_alloc, and changing its semantics may cause
> problems in future if it's no longer just passed to kfence_alloc().
> Today, we can do the 'size = size ?: cache->object_size' trick inside
> kfence_alloc(), but at the cost breaking the intuitive semantics of
> slab_alloc's orig_size argument for future users. Is it worth it?

I don't have an answer to this question. I will leave this to others.
If nobody has strong support for changing semantics, let's leave it as
is. Maybe keep in mind as potential ballast.
FWIW most likely misuse of 0 size for other future purposes should
manifest itself in a quite straightforward way.