Re: 2.6.8-rc2-mm1

Johannes Stezenbach
2004-07-28T23:30:40Z

On Wed, Jul 28, 2004 at 11:44:23PM +0100, viro@parcelfarce.linux.theplanet.co.uk wrote:
> On Thu, Jul 29, 2004 at 12:24:55AM +0200, Johannes Stezenbach wrote:
> > Signed-off-by: Johannes Stezenbach <js@convergence.de>
> >
> > --- linux-2.6.8-rc2/drivers/media/dvb/dvb-core/dvb_functions.c.orig 2004-07-29 00:19:50.000000000 +0200
> > +++ linux-2.6.8-rc2/drivers/media/dvb/dvb-core/dvb_functions.c 2004-07-29 00:20:05.000000000 +0200
> > @@ -36,7 +36,7 @@ int dvb_usercopy(struct inode *inode, st
> > /* Copy arguments into temp kernel buffer */
> > switch (_IOC_DIR(cmd)) {
> > case _IOC_NONE:
> > - parg = NULL;
> > + parg = (void *) arg;
>
> Mind explaining why it is the right thing to do? You are creating a kernel
> pointer out of value passed to you by userland and feed it to a function
> that expects a kernel pointer. Which is an invitation for trouble - if
> it ends up dereferenced, we are screwed and won't notice that.

This is a hack introduced by someone years ago. The "pointer" is
actually an integer argument, e.g. in include/linux/dvb/audio.h:

#define AUDIO_SET_MUTE _IO('o', 6)

actually takes an integer argument (!0 mute, 0 unmute), so one can write

if (ioctl(fd, AUDIO_SET_MUTE, 1) == -1)
perror("mute");

It is unusual (maybe even wrong?), but we cannot change it without
losing binary API compatibility. However, I see that sparse might
flag this as a possible bug :-(

Johannes