On 2006.08.08 08:26:57 -0700, Dave Hansen wrote:
> On Tue, 2006-08-08 at 14:22 +0400, Kirill Korotaev wrote:
> > sys_getppid() optimization can access a freed memory.
> > On kernels with DEBUG_SLAB turned ON, this results in
> > Oops.
> ...
> > +#else
> > + /*
> > + * ->real_parent could be released before dereference and
> > + * we accessed freed kernel memory, which faults with debugging on.
> > + * Keep it simple and stupid.
> > + */
> > + read_lock(&tasklist_lock);
> > + pid = current->group_leader->real_parent->tgid;
> > + read_unlock(&tasklist_lock);
> > +#endif
> > return pid;
> > }
>
> Accessing freed memory is a bug, always, not just *only* when slab
> debugging is on, right? Doesn't this mean we could get junk, or that
> the reader could potentially run off a bad pointer?
>
> It seems that this patch only papers over the problem in the case when
> it is observed, but doesn't really even fix the normal case.
>
> Could we use a seqlock to determine when real_parent is in flux, and
> re-read real_parent until we get a consistent one? We could use in in
> lieu of the existing for() loop.
See this thread: http://lkml.org/lkml/2006/8/8/215
Björn
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