Re: [Patch bpf-next v3 2/4] skmsg: implement slab allocator cache for sk_msg

[Zijian Zhang <zijianzhang@xxxxxxxxxxxxx>]

On 5/28/25 5:04 PM, John Fastabend wrote:
> On 2025-05-19 13:36:26, Cong Wang wrote:
> > From: Zijian Zhang <zijianzhang@xxxxxxxxxxxxx>
> >
> > Optimizing redirect ingress performance requires frequent allocation and
> > deallocation of sk_msg structures. Introduce a dedicated kmem_cache for
> > sk_msg to reduce memory allocation overhead and improve performance.
> >
> > Reviewed-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> > Signed-off-by: Zijian Zhang <zijianzhang@xxxxxxxxxxxxx>
> > ---
> >   include/linux/skmsg.h | 21 ++++++++++++---------
> >   net/core/skmsg.c      | 28 +++++++++++++++++++++-------
> >   net/ipv4/tcp_bpf.c    |  5 ++---
> >   3 files changed, 35 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> > index d6f0a8cd73c4..bf28ce9b5fdb 100644
> > --- a/include/linux/skmsg.h
> > +++ b/include/linux/skmsg.h
> > @@ -121,6 +121,7 @@ struct sk_psock {
> >   	struct rcu_work			rwork;
> >   };
> >   
> > +struct sk_msg *sk_msg_alloc(gfp_t gfp);
> >   int sk_msg_expand(struct sock *sk, struct sk_msg *msg, int len,
> >   		  int elem_first_coalesce);
> >   int sk_msg_clone(struct sock *sk, struct sk_msg *dst, struct sk_msg *src,
> > @@ -143,6 +144,8 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> >   		   int len, int flags);
> >   bool sk_msg_is_readable(struct sock *sk);
> >   
> > +extern struct kmem_cache *sk_msg_cachep;
> > +
> >   static inline void sk_msg_check_to_free(struct sk_msg *msg, u32 i, u32 bytes)
> >   {
> >   	WARN_ON(i == msg->sg.end && bytes);
> > @@ -319,6 +322,13 @@ static inline void sock_drop(struct sock *sk, struct sk_buff *skb)
> >   	kfree_skb(skb);
> >   }
> >   
> > +static inline void kfree_sk_msg(struct sk_msg *msg)
> > +{
> > +	if (msg->skb)
> > +		consume_skb(msg->skb);
> > +	kmem_cache_free(sk_msg_cachep, msg);
> > +}
> > +
> >   static inline bool sk_psock_queue_msg(struct sk_psock *psock,
> >   				      struct sk_msg *msg)
> >   {
> > @@ -330,7 +340,7 @@ static inline bool sk_psock_queue_msg(struct sk_psock *psock,
> >   		ret = true;
> >   	} else {
> >   		sk_msg_free(psock->sk, msg);
> > -		kfree(msg);
> > +		kfree_sk_msg(msg);
>
> Isn't this a potential use after free on msg->skb? The sk_msg_free() a
> line above will consume_skb() if it exists and its not nil set so we would
> consume_skb() again?

Thanks to sk_msg_free, after consuming the skb, it invokes sk_msg_init
to make msg->skb NULL to prevent further double free.

To avoid the confusion, we can replace kfree_sk_msg here with
kmem_cache_free.


> >   		ret = false;
> >   	}
> >   	spin_unlock_bh(&psock->ingress_lock);
> > @@ -378,13 +388,6 @@ static inline bool sk_psock_queue_empty(const struct sk_psock *psock)
> >   	return psock ? list_empty(&psock->ingress_msg) : true;
> >   }
> >   
> > -static inline void kfree_sk_msg(struct sk_msg *msg)
> > -{
> > -	if (msg->skb)
> > -		consume_skb(msg->skb);
> > -	kfree(msg);
> > -}
> > -
> >   static inline void sk_psock_report_error(struct sk_psock *psock, int err)
> >   {
> >   	struct sock *sk = psock->sk;
> > @@ -441,7 +444,7 @@ static inline void sk_psock_cork_free(struct sk_psock *psock)
> >   {
> >   	if (psock->cork) {
> >   		sk_msg_free(psock->sk, psock->cork);
> > -		kfree(psock->cork);
> > +		kfree_sk_msg(psock->cork);
>
> Same here.
>
> >   		psock->cork = NULL;
> >   	}
> >   }