RE: [PATCH v1 1/1] baseband/acc: fix check after deref and dead code

["Chautru, Nicolas" <nicolas.chautru@intel.com>]

Hi David, 

> -----Original Message-----
> From: David Marchand <david.marchand@redhat.com>
> Sent: Thursday, November 10, 2022 1:49 AM
> To: Vargas, Hernan <hernan.vargas@intel.com>
> Cc: dev@dpdk.org; gakhil@marvell.com; trix@redhat.com;
> maxime.coquelin@redhat.com; Chautru, Nicolas <nicolas.chautru@intel.com>;
> Zhang, Qi Z <qi.z.zhang@intel.com>
> Subject: Re: [PATCH v1 1/1] baseband/acc: fix check after deref and dead code
> 
> On Thu, Nov 3, 2022 at 8:57 PM Hernan Vargas <hernan.vargas@intel.com>
> wrote:
> >
> > Fix potential issue of dereferencing a pointer before null check.
> > Remove null check for value that could never be null.
> >
> > Coverity issue: 381646, 381631
> > Fixes: 989dec301a9 ("baseband/acc100: add ring companion address")
> >
> > Signed-off-by: Hernan Vargas <hernan.vargas@intel.com>
> > ---
> >  drivers/baseband/acc/rte_acc100_pmd.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/drivers/baseband/acc/rte_acc100_pmd.c
> > b/drivers/baseband/acc/rte_acc100_pmd.c
> > index 96daef87bc..30a718916d 100644
> > --- a/drivers/baseband/acc/rte_acc100_pmd.c
> > +++ b/drivers/baseband/acc/rte_acc100_pmd.c
> > @@ -4122,15 +4122,11 @@ acc100_dequeue_ldpc_enc(struct
> rte_bbdev_queue_data *q_data,
> >         struct rte_bbdev_enc_op *op;
> >         union acc_dma_desc *desc;
> >
> > -       if (q == NULL)
> > -               return 0;
> 
> I guess this protects badly written applications that would do stuff like pass an
> incorrect queue id, or call this callback while the queue has not been configured
> yet.
> This is something that should be caught at the bbdev layer (arguably under the
> RTE_LIBRTE_BBDEV_DEBUG if the performance is that much affected, though
> I'd like to see numbers).
> (edit: I see Maxime replied a similar comment).

That is not directly to that ticket but would be good to follow up. 
From previous discussion with Maxime, the new consensus was to avoid special check in debug mode (try to build the same code). It would be good to come up to a new consensus on this. 

> 
> Back to this particular patch, rather than remove the check, the right fix is to
> move acc_ring_avail_deq(q).
> This is what Coverity reports.
> 
> And this same pattern is used in other parts of the driver.
> It just happens that Coverity did not report them because some avec under
> RTE_LIBRTE_BBDEV_DEBUG...

I believe that we don't want to create discrepancies : each dequeue function should behave the same way. Ie. acc100_dequeue_ldpc_enc should not do things differently from others dequeue functions. 
Currently there is a discrepancy which is being resolved in that patch.

Either we remove the check as in that commit which could be approved as is, 
or we move the check under the debug as for the other functions which hides the Coverity issue without in reality fully addressing it, 
or we remove these check from all functions (including under debug) which is what we do for other PMD. 

That 4th option you seem to suggest would consist in effect to do thing differently just for the dequeue function which would lacks consistency really. 

Is there any concern just to approve as is, again that q == NULL is not done in production code anywhere else as you pointed out.

I agree that in next release we can remove much of the code under DEBUG flag which is not adding value nor being built/used in practice. 

Thanks
Nic

> 
> 
> 
> >  #ifdef RTE_LIBRTE_BBDEV_DEBUG
> >         if (unlikely(ops == 0))
> 
> And I also noticed this hunk.
> 
> DPDK coding style, ops should be compared against NULL, but see below...
> 
> 
> >                 return 0;
> >  #endif
> >         desc = q->ring_addr + (q->sw_ring_tail & q->sw_ring_wrap_mask);
> > -       if (unlikely(desc == NULL))
> > -               return 0;
> >         op = desc->req.op_addr;
> >         if (unlikely(ops == NULL || op == NULL))
> >                 return 0;
> 
> ... like here, so above check is redundant.
> 
> There is probably more cleanups to do in this driver.
> This can be done later.
> 
> 
> --
> David Marchand