On Tue, Dec 23, 2008 at 07:01:59AM +0100, Julia Lawall wrote:
> On Mon, 22 Dec 2008, Greg KH wrote:
>
> > On Fri, Dec 19, 2008 at 06:56:46PM +0100, Julia Lawall wrote:
> > > Considering code such as the following (drivers/acpi/osl.c):
> > >
> > > struct acpi_os_dpc *dpc = container_of(work, struct acpi_os_dpc, work);
> > > if (!dpc) {
> > > printk(KERN_ERR PREFIX "Invalid (NULL) context\n");
> > > return;
> > > }
> > >
> > > Is it very likely that container_of can return NULL? Container_of
> > > computes an offset from a pointer, so I have the impression that if given
> > > a NULL value it would normally return a negative number (or a very large
> > > positive one, depending on how it is interpreted).
> >
> > You are correct, that check will never happen, and should be removed.
>
> Actually, someone else pointed out that one could call the function with
> &x->fld, and if x is NULL, then container_of(&x->fld,...) would go back
> down to NULL. I haven't looked at the call sites though to see if that
> can happen.
No, if you were to evaluate x->fld, you would oops :)
So again, the check is pointless. And I say this as someone who has
written such checks many times in the past...
thanks,
greg k-h
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