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

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 25 Sep 2024 17:21:12 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> From: Morten Brørup <mb@smartsharesystems.com>
> To: "Robin Jarry" <rjarry@redhat.com>,  <dev@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@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@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?