[PATCH v2] mbuf: fix packet copy

[Morten Brørup]

> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Friday, 16 January 2026 18.06
> 
> On Fri, 16 Jan 2026 11:16:21 +0000
> Morten Brørup <mb at smartsharesystems.com> wrote:
> 
> > buf: fix packet copy
> >
> > Requests for copying the at the end of a packet incorrectly returned
> NULL,
> > as if copying past the end of a packet.
> >
> > When allocating the mbuf for the copy from a mempool using pinned
> external
> > buffers, the external flag in this mbuf was not preserved.
> >
> > Fixes: c3a90c381daa ("mbuf: add a copy routine")
> >
> > Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> > Acked-by: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> > ---
> > v2:
> > * Improved comment about preserving flags for newly allocated mbuf
> >   potentially using pinned external buffer.
> > * Added missing spaces in expression. (Stephen)
> > ---
> >  lib/mbuf/rte_mbuf.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/lib/mbuf/rte_mbuf.c b/lib/mbuf/rte_mbuf.c
> > index 0d931c7a15..a5d16e4c97 100644
> > --- a/lib/mbuf/rte_mbuf.c
> > +++ b/lib/mbuf/rte_mbuf.c
> > @@ -675,7 +675,7 @@ rte_pktmbuf_copy(const struct rte_mbuf *m, struct
> rte_mempool *mp,
> >  	__rte_mbuf_sanity_check(m, 1);
> >
> >  	/* check for request to copy at offset past end of mbuf */
> > -	if (unlikely(off >= m->pkt_len))
> > +	if (unlikely(off > m->pkt_len))
> >  		return NULL;
> >
> 
> I still think asking for a copy of data that isn't there should return
> NULL
> not a zero length mbuf.  Kind of academic since I dont think any code
> uses
> non-zero offset now.

Yes, I totally agree it's kind of academic.
But I insist that it is an off-by-one bug, so I fixed it.

Consider the function documentation:

* @param offset
 *   The number of bytes to skip before copying.
 *   If the mbuf does not have that many bytes, it is an error
 *   and NULL is returned.

An offset resulting in copying zero bytes is not an error according to this.

Also consider the comment at the comparison in the source code:
/* check for request to copy at offset past end of mbuf */

It says "past the end", not "at the end"... although I'm not confident enough in my English skills to determine if this means ">=" or ">".