Re: [PATCH v4 bluetooth] 6lowpan: fix incorrect return values in lowpan_rcv

[Alexander Aring <alex.aring@gmail.com>]

Hi Jukka,

On Tue, Sep 16, 2014 at 04:20:19PM +0300, Jukka Rissanen wrote:
> Hi Alex,
> 
> On ti, 2014-09-16 at 14:48 +0200, Alexander Aring wrote:
> > Hi Martin,
> > 
> > On Tue, Sep 16, 2014 at 01:40:24PM +0100, Martin Townsend wrote:
> > ...
> > > 
> > > Yes I see the problem now, maybe it's better to revert back to skb_inout, less chance of introducing bugs and then we have a well defined return value.
> > > 
> > 
> > No problem, for me it's okay, if this is okay for Jukka, we can change
> > it later to a better behaviour. Jukka please answer what you think about this.
> > 
> 
> What about doing things like this in your example?
> 

ehm yes, the example is only there to describe the current situation.

> > I also did a small c example because this now:
> > 
> > char *foo(char *buf)
> > {
> >         char *new;
> > 
> >         if (some_error)
> >                 return NULL;
> 
> In this case you should probably not return NULL but something like
> -EINVAL
> 
> if (some_error) {
> 	free(buf);
> 	return -EINVAL;
> }

yes, that's the second choice, let do consume_skb/kfree_skb inside
lowpan_process_data function.

> 
> > 
> >         if (some_error)
> >                 return NULL;
> 
> Ditto
> 
> > 
> >         new = expand(buf, 23);
> >         if (!new)
> >                 return NULL;
> 
> if (!new) {
> 	free(buf);
> 	return -ENOMEM;
> }
> 
> > 
> >         free(buf);
> >         buf = new;
> > 
> > 	/* buf is now different than the parameter buf */
> >         if (some_error)
> >                 return NULL;
> 
> if (some_error) {
> 	free(buf);
> 	return -EFOOBAR;
> }
> 
> >              
> >         return buf;
> > }             
> >               
> > int main(int argc, const char *argv[])
> > {             
> >         char *local_buf = malloc(42);
> >         char *buf;
> >              
> >         buf = foo(local_buf);
> >         if (!buf) {
> >                 /* BUG */
> >                 /* we don't know if local_buf is still valid. */
> >                 free(local_buf);
> >         }        
> 
> if (IS_ERR_OR_NULL(buf)) {
> 	fail();
> } else
> 	free(buf);
> 
> >                  
> >         return 0;
> > }
> > 
> > I think if you do buf = foo(buf) you can rescue it but this doesn't
> > look like a clean solution for me.
> > 
> > - Alex
> 
> 
> In this simplified example, the subroutine frees the buf which does not
> look nice I have to admit.
> 

I am also fine with this solution. Make something I will review it and
look if we run into trouble.

In my last mails stands, that we have two choices:

- make the skb_inout thingy
- handle error freeing into lowpan_process_data function.

You described the last one now. :-)

- Alex