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SubjectRe: [PATCH RFC 04/10] mm, kfence: insert KFENCE hooks for SLAB
On Fri, 11 Sep 2020 at 09:17, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Sep 7, 2020 at 3:41 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?

Thanks,
-- Marco

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