Re: [PATCH 2.6.17-rc6 7/9] Remove some of the kmemleak false positives

["Catalin Marinas" <catalin.marinas@gmail.com>]

On 13/06/06, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On 13/06/06, Pekka J Enberg <penberg@cs.helsinki.fi> wrote:
> > > As far as I understood, Ingo is worried about:
> > >
> > >         struct s { /* some fields */; char *buf; };
> > >
> > >         struct s *p = kmalloc(sizeof(struct s) + BUF_SIZE);
> > >         p->buf = p + sizeof(struct s);
> > >
> > > Which could lead to false negative due to p->buf pointing to p.  However,
> > > for us to even _find_ p->buf, we would need an incoming pointer _to_ p
> > > which makes me think this is not a problem in practice.  Hmm?
>
> On 6/13/06, Catalin Marinas <catalin.marinas@gmail.com> wrote:
> > Not exactly. In the above case, Ingo (and me) is worried about having
> > a incoming pointer (from other block) equal to p->buf and therefore
> > inside the block allocated with kmalloc.
>
> Ah, right, I overlooked that case. But, is it really a leak? That is,
> even though we currently don't have a pointer to the beginning fo the
> block, we don't know for sure it was a leak. You're now allowed to do:
>
>     p = kmalloc(...);
>     p = p + HDR_SIZE;
>
>     /* ... */
>
>     kfree(p - HDR_SIZE);
>
> Do you think we should ban the above?

I don't think you can ban this because of places where the structure
needs to be aligned to a certain value. Look in the false positives
patch for the memleak_padding() calls with the 2nd argument not zero.

--
Catalin
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