Hi Leon,

On 02/09/2018 11:36 AM, Gustavo A. R. Silva wrote:

Quoting Leon Romanovsky <l...@kernel.org>:

On Fri, Feb 09, 2018 at 09:56:00AM -0600, Gustavo A. R. Silva wrote:

Quoting Leon Romanovsky <l...@kernel.org>:

> On Fri, Feb 09, 2018 at 07:36:49AM -0600, Gustavo A. R. Silva wrote:
> > Hi Leon,
> >
> > Quoting Leon Romanovsky <l...@kernel.org>:
> >
> > > On Fri, Feb 09, 2018 at 12:37:02AM -0600, Gustavo A. R. Silva wrote: > > > > In case the message header and payload cannot be stored, function
> > > > nlmsg_put returns null.
> > > >
> > > > Fix this by adding multiple sanity checks and avoid a potential
> > > > null dereference on _nlh_ when calling nlmsg_end.
> > > >
> > > > Addresses-Coverity-ID: 1454215 ("Dereference null return value")
> > > > Addresses-Coverity-ID: 1454223 ("Dereference null return value")
> > > > Addresses-Coverity-ID: 1454224 ("Dereference null return value")
> > > > Addresses-Coverity-ID: 1464669 ("Dereference null return value")
> > > > Addresses-Coverity-ID: 1464670 ("Dereference null return value")
> > > > Addresses-Coverity-ID: 1464672 ("Dereference null return value")
> > > > Fixes: e5c9469efcb1 ("RDMA/netlink: Add nldev device doit
> > implementation")
> > > > Fixes: c3f66f7b0052 ("RDMA/netlink: Implement nldev port doit callback")
> > > > Fixes: 7d02f605f0dc ("RDMA/netlink: Add nldev port dumpit
> > implementation")
> > > > Fixes: b5fa635aab8f ("RDMA/nldev: Provide detailed QP information") > > > > Fixes: bf3c5a93c523 ("RDMA/nldev: Provide global resource utilization")
> > > > Signed-off-by: Gustavo A. R. Silva <gust...@embeddedor.com>
> > > > ---
> > > >  drivers/infiniband/core/nldev.c | 26 +++++++++++++++++++++++++-
> > > >  1 file changed, 25 insertions(+), 1 deletion(-)
> > > >
> > >
> > > It will be much better to fix the tool instead of fixing ghost case.
> > > This scenario is impossible for all those flows.
> > > We can receive the skv/msg in two ways:
> > >  * First by allocating new message with NLMSG_DEFAULT_SIZE, which has
> > > more room
> > >    than nlmsg_total_size(payload), payload is 0.
> > >  * Second by getting from netlink.c and it will be at least "struct
> > > nlmsghdr" too.
> > >
> > > Can you please add this info to the commit message?
> > >
> >
> > Actually, I was planing to send a new version of this patch. This time using
> > the unlikely macro for all the null checks on nlh.
> >
> > What do you think?
>
> It is not datapath, so "unlikely" is not needed. Let's assume that smart
> enough
> compiler will optimize such flow anyway, because nlmsg_put returns NULL > in unlikely scenario, so this check will be unlikely automatically too.
>

I'm curious about why the return value of nlmsg_put is null checked 118 out
of 129 times (based on Coverity reports) in the last linux-next tree.

So based on what you mention, do you think all those checks are actually
unnecessary and, maybe they should be removed?

I honestly don't know about all cases, but if message is allocated with
NLMSG_DEFAULT_SIZE and payload is 0, this check won't be needed.


I got it.

So go ahead, add check if (!...) in all places, but be cautious with
"potential null dereference" claims, it is not always true.



I've finally decided to document all these cases as False Positives in the Coverity platform.

I think it is better to do that than adding unnecessary code. I will also add a link to this conversation to the Coverity database.

Thanks a lot for your feedback.
--
Gustavo

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