Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

Andrew Morton
2007-06-01T22:33:53Z

On Fri, 1 Jun 2007 15:20:05 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Fri, 1 Jun 2007, Andrew Morton wrote:
>
> > If slab was smart enough, it would have poisoned those 8 bytes to some
> > known pattern, and then checked that they still had that pattern when the
> > memory got freed again.
>
> So this is new feature request?
>
> > But it isn't smart enough, so the bug went undetected.
>
> I should make SLUB put poisoning values in unused areas of a kmalloced
> object?

hm, I hadn't thought of it that way actually. I was thinking it was
specific to kmalloc(0) but as you point out, the situation is
generalisable.

Yes, if someone does kmalloc(42) and we satisfy the allocation from the
size-64 slab, we should poison and then check the allegedly-unused 22
bytes.

Please ;)

(vaguely stunned that we didn't think of doing this years ago).

It'll be a large patch, I expect?

Actually, I have this vague memory that slab would take that kmalloc(42)
and would then return kmalloc(64)+22, so the returned memory is
"right-aligned". This way the existing overrun-detection is applicable to
all kmallocs. Maybe I dreamed it.