Re: [PATCH v2 net-next 1/2] net: dsa: lan9303: Add port_fast_age and port_fdb_dump methods

[Egil Hjelmeland]

Den 19. okt. 2017 17:42, skrev Egil Hjelmeland:
> On 19. okt. 2017 17:15, David Laight wrote:
> > From: Andrew Lunn
> > > Sent: 19 October 2017 15:15
> > > > > +/* Clear learned (non-static) entry on given port */
> > > > > +static void alr_loop_cb_del_port_learned(struct lan9303 *chip, u32 
> > > > > dat0,
> > > > > +                     u32 dat1, int portmap, void *ctx)
> > > > > +{
> > > > > +    int *port = ctx;
> > > >
> > > > You can get the value directly to make the line below more readable:
> > > >
> > > >      int port = *(int *)ctx;
> > >
> > > You have to be a bit careful with this. You often see people
> > > submitting patches taking away casts for void * pointers.
> > > If they do that here, it should at least not compile...
> > >
> > > So maybe do it in two steps?
> > >
> > >     int * pport = ctx;
> > >     int port = *pport;
> >
> > IMHO it is best to define a struct for the 'ctx and then do:
> >     ..., void *v_ctx)
> > {
> >     foo_ctx *ctx = v_ctx;
> >     int port = ctx->port;
> >
> > That stops anyone having to double-check that the *(int *)
> > is operating on a pointer to an integer of the correct size.
>
> Does casting to a struct pointer require less manual double-check than
> to a int-pointer? In neither cases the compiler can protect us, IFAIK.
> But on the other hand, a the text "foo_ctx" can searched in the editor.
> So in that respect it can somewhat aid to the double-checking.
>
> So I can do that.

I understand now that the caller side (lan9303_port_fast_age) is
vulnerable. Say somebody has the idea to change the "port" param
of .port_fast_age from int to u8, then my code is a trap.

Thanks for the education.

Egil