RE: [PATCH 1/3] tty: n_gsm: add keep alive support

Starke, Daniel
2023-02-01T09:49:07Z

> > > > index cb8693b39cb7..b64360aca1f9 100644
> > > > --- a/include/uapi/linux/gsmmux.h
> > > > +++ b/include/uapi/linux/gsmmux.h
> > > > @@ -19,7 +19,8 @@ struct gsm_config
> > > > unsigned int mtu;
> > > > unsigned int k;
> > > > unsigned int i;
> > > > - unsigned int unused[8]; /* Padding for expansion without
> > > > + unsigned int keep_alive;
> > > > + unsigned int unused[7]; /* Padding for expansion without
> > >
> > > "unsigned int" is not really a valid uapi variable type.
> > >
> > > Shouldn't this be __u32 instead?
> >
> > I know but changing it to a fixed size data type may break compatibility
> > as this may change the overall size of the structure.
>
> Will it? It shouldn't that's why using the correct data types is
> essencial.

Well, unsigned int is defined to be at least 16 bit. Using __u32 will break
systems where this is true. I am not sure if the Linux kernel targets any
system which defines unsigned int with 16 bit. But sure, I can change it to
__u32.

> > This is why I
> > took a field out of the "unused" array for the "keep_alive" parameter.
> > A value of zero disables keep-alive polling.
> >
> > > Should you document this field as to what the value is and the units as
> > > you are creating a new user/kernel api here.
> >
> > I will add a comment here. Comments for the other fields remain subject to
> > another patch.
> >
> > > And finally, "unused" here is being properly checked to be all 0, right?
> > > If not, then this change can't happen for obvious reasons :(
> >
> > This was not the case until now. I assumed there was some coding guideline
> > that unused fields need to be initialized to zero. Obviously, checking it
> > prevents misuse here. I will add relevant checks for this.
>
> If the value was not checked previously, then you can not use the field
> now, otherwise things will break, sorry. Those are useless fields and
> should be marked as such :(

What is the way forward here? Should I introduce a complete new ioctl?
Or should I use a different size for this structure to break existing code
intentionally? Does this mean that we cannot extend this structure at all
in the future? I had planned another extension here to properly support
parameter negotiation.
In case we need to keep the structure as it is: Would a comment be
sufficient to mark this field accordingly?

Best regards,
Daniel Starke