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

[Waiman Long]

On 4/27/20 9:18 AM, Waiman Long wrote:
> On 4/27/20 8:38 AM, Matthew Wilcox wrote:
> > On Sun, Apr 26, 2020 at 10:02:12PM -0400, Waiman Long wrote:
> > > In a couple of places in the slub memory allocator, the code uses
> > > "s->offset" as a check to see if the free pointer is put right after 
> > > the
> > > object. That check is no longer true with commit 3202fa62fb43 ("slub:
> > > relocate freelist pointer to middle of object").
> > >
> > > As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
> > > may cause a bunch of "Freepointer corrupt" error reports to appear with
> > > the system in panic afterwards.
> > >
> > > To fix it, use the check "s->offset == s->inuse" instead.
> > I think a little refactoring would make this more clear.
> >
> > unsigned int track_offset(const struct kmem_cache *s)
> > {
> > ÂÂÂÂreturn s->inuse + (s->offset == s->inuse) ? sizeof(void *) : 0;
> > }
>
> Yes, that was what I am thinking of doing in v2. 

BTW, "+" has a higher priority than "?:". So we need a parenthesis 
around "?:".

Cheers,
Longman