Re: [PATCH v2] mbuf: fix packet copy

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 16 Jan 2026 11:16:21 +0000
Morten Brørup <mb@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@smartsharesystems.com>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@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.