Re: [PATCH 9/10] drivers/mmc: Move a dereference below a NULL test

[Adrian Hunter]

arun c wrote:
> On Mon, Jul 20, 2009 at 1:31 PM, Adrian Hunter<adrian.hunter@xxxxxxxxx> wrote:
> > Julia Lawall wrote:
> > > From: Julia Lawall <julia@xxxxxxx>
> > >
> > > If the NULL test is necessary, then the dereference should be moved below
> > > the NULL test.
> > >
> > > The semantic patch that makes this change is as follows:
> > > (http://www.emn.fr/x-info/coccinelle/)
> > >
> > > // <smpl>
> > > @@
> > > type T;
> > > expression E,E1;
> > > identifier i,fld;
> > > statement S;
> > > @@
> > >
> > > - T i = E->fld;
> > > + T i;
> > >  ... when != E=E1
> > >      when != i
> > >  BUG_ON (E == NULL||
> > > -     i
> > > +     E->fld
> > >       == NULL || ...);
> > > + i = E->fld;
> > > // </smpl>
> > >
> > > Signed-off-by: Julia Lawall <julia@xxxxxxx>
> > >
> > > ---
> > >  drivers/mmc/host/omap.c             |    5 +++--
> > >  1 files changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/mmc/host/omap.c b/drivers/mmc/host/omap.c
> > > index e7a331d..89281ab 100644
> > > --- a/drivers/mmc/host/omap.c
> > > +++ b/drivers/mmc/host/omap.c
> > > @@ -255,11 +255,12 @@ static void mmc_omap_slot_release_work(struct
> > > work_struct *work)
> > >   static void mmc_omap_release_slot(struct mmc_omap_slot *slot, int
> > > clk_enabled)
> > >  {
> > > -       struct mmc_omap_host *host = slot->host;
> > > +       struct mmc_omap_host *host;
> > >        unsigned long flags;
> > >        int i;
> > >  -       BUG_ON(slot == NULL || host->mmc == NULL);
> > > +       BUG_ON(slot == NULL || slot->host->mmc == NULL);
> > > +       host = slot->host;
> > >          if (clk_enabled)
> > >                /* Keeps clock running for at least 8 cycles on valid freq
> > > */
> > > --
> > If slot is NULL it will oops anyway, so the following is better IMHO:
> >
> > static void mmc_omap_release_slot(struct mmc_omap_slot *slot, int
> > clk_enabled)
> > {
> >        struct mmc_omap_host *host = slot->host;
> >        unsigned long flags;
> >        int i;
> >
> > > -       BUG_ON(slot == NULL || host->mmc == NULL);
> > > +       BUG_ON(host->mmc == NULL);
> >        if (clk_enabled)
> >                /* Keeps clock running for at least 8 cycles on valid freq */
> > --
>
> According to http://isc.sans.org/diary.html?storyid=6820&rss "NULL check"
> must be done before assigning the values. Does Julia Lawall's version
> is the more correct one??
>
> Arun

It is not a "NULL check" it is an assertion, which can be compiled out.
On some platforms BUG is implemented as a null dereference anyway
e.g. on ARM  it is *(int *)0 = 0;

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/