Re: [PATCH] staging: rtl8192u: ieee80211_rx: Fix incorrect type in assignments

[Gaston Gonzalez <gascoar@gmail.com>]

On Thu, Jun 25, 2015 at 02:06:44PM +0200, Arnd Bergmann wrote:
> On Wednesday 24 June 2015 13:34:58 Gaston Gonzalez wrote:
> > On Tue, Jun 23, 2015 at 12:13:47PM +0200, Arnd Bergmann wrote:
> > > On Sunday 21 June 2015 19:12:09 Gaston Gonzalez wrote:
> > > >                 /* WMM spec P.11: The minimum value for AIFSN shall be 2 */
> > > >                 qos_param->aifs[aci] = (qos_param->aifs[aci] < 2) ? 2:qos_param->aifs[aci];
> > > >  
> > > > -               qos_param->cw_min[aci] = ac_params->ecw_min_max & 0x0F;
> > > > +               qos_param->cw_min[aci] =
> > > > +                       cpu_to_le16(ac_params->ecw_min_max & 0x0F);
> > > >  
> > > > -               qos_param->cw_max[aci] = (ac_params->ecw_min_max & 0xF0) >> 4;
> > > > +               qos_param->cw_max[aci] =
> > > > +                       cpu_to_le16((ac_params->ecw_min_max & 0xF0) >> 4);
> > > >  
> > > >                 qos_param->flag[aci] =
> > > >                     (ac_params->aci_aifsn & 0x10) ? 0x01 : 0x00;
> > > > -               qos_param->tx_op_limit[aci] = le16_to_cpu(ac_params->tx_op_limit);
> > > > +               qos_param->tx_op_limit[aci] = ac_params->tx_op_limit;
> > > >         }
> > > >         return 0;
> > > 
> > > This certainly needs a more thorough description of how you determined that
> > > the byte swaps that you add are in fact required. Did you test it on
> > > a big-endian machine?
> > > 
> > > 	Arnd
> > 
> > Hello Arnd,
> > 
> > Thank you for reviewing this.
> > After your email and reviwing this again I'm getting a bit suspicious
> > myself, but this is what I saw:
> > 
> > -- First warning:
> > 
> > qos_param->cw_min[aci] is defined as __le16() in ieee80211.h
> > (ieee80211_qos_parameters structure)
> > 
> > ac_params-> ecw_min_max is defined as u8 in ieee80211.h
> > (ieee80211_qos_ac_parameter structure)
> > 
> > So the assignment is: __le16 = u8 & 0x0F;
> > 
> > -- Second warning:
> > 
> > qos_param->cw_max[aci] is __le16()
> > ac_params-> ecw_min_max is u8
> > 
> > The assignment is: __le16 = (u8 & 0xF0) >> 4;
> > 
> > Thus, for the warning 1 and 2, I understand that the result won't be the
> > same if the machine is big-endian or little-endian, and that's why we
> > need a cpu_to_le16. Am I missing something?
> 
> I think your analysis is right, as long as the __le16 annotation is
> actually correct. It usually helps to look at the git history to
> see what the intent of the patch was that introduced the assignment
> and the patch that introduced the __le16 type. Presumably one of them
> was incorrect, and it would be good to figure out where it went wrong,
> and to add a 'Fixes:' tag in your patch description that pinpoints
> the exact mistake.
> 

Ok, will do.

> > -- Third warning:
> > 
> > In this case both sides of the assignment are already defined as __le16:
> > 
> > qos_param->tx_op_limit[aci] (ieee80211_qos_parameters structure defined
> > in ieee80211.h))
> > 
> > ac_params->tx_op_limit (ieee80211_qos_ac_parameter structure defined in
> > ieee80211.h)
> > 
> > So the assignment is: __le16() = le16_to_cpu(__le16)
> > 
> > Im getting suspicious now, but it sounded wrong to me.
> > In the case the right part is correct, I guess the left part should be
> > u16 type?
> 
> Again, your logic sounds good: there is clearly something wrong here, but
> it's not obvious to conclude whether it is an incorrect annotation or
> an extraneous byte swap. Besides looking at the git history, it also
> helps to look at all other uses of the two sides of the assignment:
> 
> See if qos_param->tx_op_limit is in fact used as a little-endian
> value (e.g. by copying to memory or a register), and if the value that
> got written to ac_params->tx_op_limit was byte-swapped already at
> the time of assignment.
> 
Ok, I'll do it too.

> > Regarding the test: I tested it on my machine, but is of course little-
> > endian :( I could built a qemu virtual machine to test it on a
> > big-endian emulated platform. Should that work?
> 
> Yes, that would work: you can assign the USB device to the qemu machine
> and run a kernel in there. The easiest emulation to try is probably
> a PowerPC PAPR machine with a file system from
> https://people.debian.org/~aurel32/qemu/powerpc/.
> MIPS should work as well.
> 

Ok, thanks a lot for all the pointers.

Gaston
> 	Arnd