[PATCH dpdk] mbuf: fix strict aliasing error in allocator

[Robin Jarry]

Stephen Hemminger, Sep 25, 2024 at 11:23:
> On Wed, 25 Sep 2024 17:21:12 +0200
> Morten Brørup <mb at smartsharesystems.com> wrote:
>
> > From: Morten Brørup <mb at smartsharesystems.com>
> > To: "Robin Jarry" <rjarry at redhat.com>,  <dev at dpdk.org>
> > Subject: RE: [PATCH dpdk] mbuf: fix strict aliasing error in allocator
> > Date: Wed, 25 Sep 2024 17:21:12 +0200
> > 
> > > From: Robin Jarry [mailto:rjarry at redhat.com]
> > > Sent: Wednesday, 25 September 2024 10.00
> > > 
> > > When building an application with -fstrict-aliasing -Wstrict-
> > > aliasing=2,
> > > we get errors triggered by rte_mbuf_raw_alloc() which is called inline
> > > from rte_pktmbuf_alloc().
> > > 
> > >  ../dpdk/lib/mbuf/rte_mbuf.h: In function ‘rte_mbuf_raw_alloc’:
> > >  ../dpdk/lib/mbuf/rte_mbuf.h:600:42: error: dereferencing type-punned
> > >  pointer might break strict-aliasing rules [-Werror=strict-aliasing]
> > >    600 |         if (rte_mempool_get(mp, (void **)&m) < 0)
> > >        |                                          ^~
> > > 
> > > Avoid incorrect casting by changing the type of the returned variable.
> > > 
> > > Signed-off-by: Robin Jarry <rjarry at redhat.com>
> > > ---
> > >  lib/mbuf/rte_mbuf.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> > > index babe16c72ccb..bab1fb94d41d 100644
> > > --- a/lib/mbuf/rte_mbuf.h
> > > +++ b/lib/mbuf/rte_mbuf.h
> > > @@ -595,9 +595,9 @@ __rte_mbuf_raw_sanity_check(__rte_unused const
> > > struct rte_mbuf *m)
> > >   */
> > >  static inline struct rte_mbuf *rte_mbuf_raw_alloc(struct rte_mempool
> > > *mp)
> > >  {
> > > -	struct rte_mbuf *m;
> > > +	void *m;
> > > 
> > > -	if (rte_mempool_get(mp, (void **)&m) < 0)
> > > +	if (rte_mempool_get(mp, &m) < 0)
> > >  		return NULL;
> > >  	__rte_mbuf_raw_sanity_check(m);
> > >  	return m;  
> > 
> > Suggest:
> > 	__rte_mbuf_raw_sanity_check((struct rte_mbuf *)m);
> > 	return (struct rte_mbuf *)m;
>
> Another way to avoid the warning would be to use a union?

I can use an inline union, it will be less code.