Re: [PATCH v2] mm/slub: Fix incorrect interpretation of s->offset

[Waiman Long]

On 4/27/20 12:10 PM, Christopher Lameter wrote:
> On Mon, 27 Apr 2020, Waiman Long wrote:
>
> > To fix it, use the check "s->offset == s->inuse" in the new helper
> > function freeptr_after_object() instead. Also add another helper function
> > get_info_end() to return the end of info block (inuse + free pointer
> > if not overlapping with object).
> >
> > Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   mm/slub.c | 37 ++++++++++++++++++++++---------------
> >   1 file changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 0e736d66bb42..68f1b4b1c309 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -551,15 +551,29 @@ static void print_section(char *level, char *text, u8 *addr,
> >   	metadata_access_disable();
> >   }
> >
> > +static inline bool freeptr_after_object(struct kmem_cache *s)
> bool freeptr_outside_of_object()?
I can change to that name. It doesn't really matter to me.
> > +{
> > +	return s->offset == s->inuse;
> s->offset >= s->inuse?
>
> There may be a redzone after the object.
Technically inuse is object + red zone. According to calculate_sizes():

        s->inuse = size;

        if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
                s->ctor)) {
                /*
                 * Relocate free pointer after the object if it is not
                 * permitted to overwrite the first word of the object on
                 * kmem_cache_free.
                 *
                 * This is the case if we do RCU, have a constructor or
                 * destructor or are poisoning the objects.
                 */
                s->offset = size;
                size += sizeof(void *);

So (s->offset == s->inuse) when the free pointer is outside of the object.

> > +static inline unsigned int get_info_end(struct kmem_cache *s)
> static inline track_offset()?
The main reason why I don't use that is because there is a track data 
structure in slub. There are functions name get_track() and set_track(). 
I don't want to confuse with them.

Cheers,
Longman