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

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Message-ID: <c66ac1f6-1626-47d6-9132-1aeedf771032@bytedance.com>
Date: Wed, 28 May 2025 17:49:22 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: John Fastabend <john.fastabend@...il.com>,
 Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, zhoufeng.zf@...edance.com,
 jakub@...udflare.com, Cong Wang <cong.wang@...edance.com>
Subject: Re: [Patch bpf-next v3 2/4] skmsg: implement slab allocator cache for
 sk_msg

On 5/28/25 5:04 PM, John Fastabend wrote:
> On 2025-05-19 13:36:26, Cong Wang wrote:
>> From: Zijian Zhang <zijianzhang@...edance.com>
>>
>> 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@...edance.com>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> ---
>>   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;
>>   	}
>>   }