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Date: Wed, 10 Apr 2024 14:34:14 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, willemdebruijn.kernel@...il.com,
davem@...emloft.net, kuba@...nel.org, cong.wang@...edance.com,
xiaochun.lu@...edance.com
Subject: Re: [External] Re: [PATCH net-next 1/3] sock: add MSG_ZEROCOPY_UARG
On 4/10/24 11:57 AM, Eric Dumazet wrote:
> On Wed, Apr 10, 2024 at 7:01 PM Zijian Zhang <zijianzhang@...edance.com> wrote:
>>
>> On 4/9/24 2:18 PM, Eric Dumazet wrote:
>>> On Tue, Apr 9, 2024 at 10:53 PM <zijianzhang@...edance.com> wrote:
>>>>
>>>> From: Zijian Zhang <zijianzhang@...edance.com>
>>>>
>>>> The MSG_ZEROCOPY flag enables copy avoidance for socket send calls.
>>>> However, zerocopy is not a free lunch. Apart from the management of user
>>>> pages, the combination of poll + recvmsg to receive notifications incurs
>>>> unignorable overhead in the applications. The overhead of such sometimes
>>>> might be more than the CPU savings from zerocopy. We try to solve this
>>>> problem with a new option for TCP and UDP, MSG_ZEROCOPY_UARG.
>>>> This new mechanism aims to reduce the overhead associated with receiving
>>>> notifications by embedding them directly into user arguments passed with
>>>> each sendmsg control message. By doing so, we can significantly reduce
>>>> the complexity and overhead for managing notifications. In an ideal
>>>> pattern, the user will keep calling sendmsg with MSG_ZEROCOPY_UARG
>>>> flag, and the notification will be delivered as soon as possible.
>>>>
>>>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>>>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
>>>> ---
>>>> include/linux/skbuff.h | 7 +-
>>>> include/linux/socket.h | 1 +
>>>> include/linux/tcp.h | 3 +
>>>> include/linux/udp.h | 3 +
>>>> include/net/sock.h | 17 +++
>>>> include/net/udp.h | 1 +
>>>> include/uapi/asm-generic/socket.h | 2 +
>>>> include/uapi/linux/socket.h | 17 +++
>>>> net/core/skbuff.c | 137 ++++++++++++++++++++++--
>>>> net/core/sock.c | 50 +++++++++
>>>> net/ipv4/ip_output.c | 6 +-
>>>> net/ipv4/tcp.c | 7 +-
>>>> net/ipv4/udp.c | 9 ++
>>>> net/ipv6/ip6_output.c | 5 +-
>>>> net/ipv6/udp.c | 9 ++
>>>> net/vmw_vsock/virtio_transport_common.c | 2 +-
>>>> 16 files changed, 258 insertions(+), 18 deletions(-)
>>>>
>>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>>> index 03ea36a82cdd..19b94ba01007 100644
>>>> --- a/include/linux/skbuff.h
>>>> +++ b/include/linux/skbuff.h
>>>> @@ -1663,12 +1663,14 @@ static inline void skb_set_end_offset(struct sk_buff *skb, unsigned int offset)
>>>> #endif
>>>>
>>>> struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
>>>> - struct ubuf_info *uarg);
>>>> + struct ubuf_info *uarg, bool user_args_notification);
>>>>
>>>> void msg_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref);
>>>>
>>>> void msg_zerocopy_callback(struct sk_buff *skb, struct ubuf_info *uarg,
>>>> bool success);
>>>> +void msg_zerocopy_uarg_callback(struct sk_buff *skb, struct ubuf_info *uarg,
>>>> + bool success);
>>>>
>>>> int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
>>>> struct sk_buff *skb, struct iov_iter *from,
>>>> @@ -1763,7 +1765,8 @@ static inline void net_zcopy_put(struct ubuf_info *uarg)
>>>> static inline void net_zcopy_put_abort(struct ubuf_info *uarg, bool have_uref)
>>>> {
>>>> if (uarg) {
>>>> - if (uarg->callback == msg_zerocopy_callback)
>>>> + if (uarg->callback == msg_zerocopy_callback ||
>>>> + uarg->callback == msg_zerocopy_uarg_callback)
>>>> msg_zerocopy_put_abort(uarg, have_uref);
>>>> else if (have_uref)
>>>> net_zcopy_put(uarg);
>>>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>>>> index 139c330ccf2c..de01392344e1 100644
>>>> --- a/include/linux/socket.h
>>>> +++ b/include/linux/socket.h
>>>> @@ -326,6 +326,7 @@ struct ucred {
>>>> * plain text and require encryption
>>>> */
>>>>
>>>> +#define MSG_ZEROCOPY_UARG 0x2000000 /* MSG_ZEROCOPY with UARG notifications */
>>>> #define MSG_ZEROCOPY 0x4000000 /* Use user data in kernel path */
>>>> #define MSG_SPLICE_PAGES 0x8000000 /* Splice the pages from the iterator in sendmsg() */
>>>> #define MSG_FASTOPEN 0x20000000 /* Send data in TCP SYN */
>>>> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
>>>> index 55399ee2a57e..e973f4990646 100644
>>>> --- a/include/linux/tcp.h
>>>> +++ b/include/linux/tcp.h
>>>> @@ -501,6 +501,9 @@ struct tcp_sock {
>>>> */
>>>> struct request_sock __rcu *fastopen_rsk;
>>>> struct saved_syn *saved_syn;
>>>> +
>>>> +/* TCP MSG_ZEROCOPY_UARG related information */
>>>> + struct tx_msg_zcopy_queue tx_zcopy_queue;
>>>> };
>>>>
>>>> enum tsq_enum {
>>>> diff --git a/include/linux/udp.h b/include/linux/udp.h
>>>> index 3748e82b627b..502b393eac67 100644
>>>> --- a/include/linux/udp.h
>>>> +++ b/include/linux/udp.h
>>>> @@ -95,6 +95,9 @@ struct udp_sock {
>>>>
>>>> /* Cache friendly copy of sk->sk_peek_off >= 0 */
>>>> bool peeking_with_offset;
>>>> +
>>>> + /* This field is used by sendmsg zcopy user arg mode notification */
>>>> + struct tx_msg_zcopy_queue tx_zcopy_queue;
>>>> };
>>>>
>>>> #define udp_test_bit(nr, sk) \
>>>> diff --git a/include/net/sock.h b/include/net/sock.h
>>>> index 2253eefe2848..f7c045e98213 100644
>>>> --- a/include/net/sock.h
>>>> +++ b/include/net/sock.h
>>>> @@ -544,6 +544,23 @@ struct sock {
>>>> netns_tracker ns_tracker;
>>>> };
>>>>
>>>> +struct tx_msg_zcopy_node {
>>>> + struct list_head node;
>>>> + struct tx_msg_zcopy_info info;
>>>> + struct sk_buff *skb;
>>>> +};
>>>> +
>>>> +struct tx_msg_zcopy_queue {
>>>> + struct list_head head;
>>>> + spinlock_t lock; /* protects head queue */
>>>> +};
>>>> +
>>>> +static inline void tx_message_zcopy_queue_init(struct tx_msg_zcopy_queue *q)
>>>> +{
>>>> + spin_lock_init(&q->lock);
>>>> + INIT_LIST_HEAD(&q->head);
>>>> +}
>>>> +
>>>> enum sk_pacing {
>>>> SK_PACING_NONE = 0,
>>>> SK_PACING_NEEDED = 1,
>>>> diff --git a/include/net/udp.h b/include/net/udp.h
>>>> index 488a6d2babcc..9e4d7b128de4 100644
>>>> --- a/include/net/udp.h
>>>> +++ b/include/net/udp.h
>>>> @@ -182,6 +182,7 @@ static inline void udp_lib_init_sock(struct sock *sk)
>>>> skb_queue_head_init(&up->reader_queue);
>>>> up->forward_threshold = sk->sk_rcvbuf >> 2;
>>>> set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
>>>> + tx_message_zcopy_queue_init(&up->tx_zcopy_queue);
>>>> }
>>>>
>>>> /* hash routines shared between UDPv4/6 and UDP-Litev4/6 */
>>>> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
>>>> index 8ce8a39a1e5f..86aa4b5cb7f1 100644
>>>> --- a/include/uapi/asm-generic/socket.h
>>>> +++ b/include/uapi/asm-generic/socket.h
>>>> @@ -135,6 +135,8 @@
>>>> #define SO_PASSPIDFD 76
>>>> #define SO_PEERPIDFD 77
>>>>
>>>> +#define SO_ZEROCOPY_NOTIFICATION 78
>>>> +
>>>> #if !defined(__KERNEL__)
>>>>
>>>> #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
>>>> diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h
>>>> index d3fcd3b5ec53..469ed8f4e6c8 100644
>>>> --- a/include/uapi/linux/socket.h
>>>> +++ b/include/uapi/linux/socket.h
>>>> @@ -35,4 +35,21 @@ struct __kernel_sockaddr_storage {
>>>> #define SOCK_TXREHASH_DISABLED 0
>>>> #define SOCK_TXREHASH_ENABLED 1
>>>>
>>>> +/*
>>>> + * Given the fact that MSG_ZEROCOPY_UARG tries to copy notifications
>>>> + * back to user as soon as possible, 8 should be sufficient.
>>>> + */
>>>> +#define SOCK_USR_ZC_INFO_MAX 8
>>>> +
>>>> +struct tx_msg_zcopy_info {
>>>> + __u32 lo;
>>>> + __u32 hi;
>>>> + __u8 zerocopy;
>>>> +};
>>>> +
>>>> +struct tx_usr_zcopy_info {
>>>> + int length;
>>>> + struct tx_msg_zcopy_info info[SOCK_USR_ZC_INFO_MAX];
>>>> +};
>>>> +
>>>> #endif /* _UAPI_LINUX_SOCKET_H */
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index 2a5ce6667bbb..d939b2c14d55 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -1661,6 +1661,16 @@ void mm_unaccount_pinned_pages(struct mmpin *mmp)
>>>> }
>>>> EXPORT_SYMBOL_GPL(mm_unaccount_pinned_pages);
>>>>
>>>> +static void init_ubuf_info_msgzc(struct ubuf_info_msgzc *uarg, struct sock *sk, size_t size)
>>>> +{
>>>> + uarg->id = ((u32)atomic_inc_return(&sk->sk_zckey)) - 1;
>>>> + uarg->len = 1;
>>>> + uarg->bytelen = size;
>>>> + uarg->zerocopy = 1;
>>>> + uarg->ubuf.flags = SKBFL_ZEROCOPY_FRAG | SKBFL_DONT_ORPHAN;
>>>> + refcount_set(&uarg->ubuf.refcnt, 1);
>>>> +}
>>>> +
>>>> static struct ubuf_info *msg_zerocopy_alloc(struct sock *sk, size_t size)
>>>> {
>>>> struct ubuf_info_msgzc *uarg;
>>>> @@ -1682,12 +1692,38 @@ static struct ubuf_info *msg_zerocopy_alloc(struct sock *sk, size_t size)
>>>> }
>>>>
>>>> uarg->ubuf.callback = msg_zerocopy_callback;
>>>> - uarg->id = ((u32)atomic_inc_return(&sk->sk_zckey)) - 1;
>>>> - uarg->len = 1;
>>>> - uarg->bytelen = size;
>>>> - uarg->zerocopy = 1;
>>>> - uarg->ubuf.flags = SKBFL_ZEROCOPY_FRAG | SKBFL_DONT_ORPHAN;
>>>> - refcount_set(&uarg->ubuf.refcnt, 1);
>>>> + init_ubuf_info_msgzc(uarg, sk, size);
>>>> + sock_hold(sk);
>>>> +
>>>> + return &uarg->ubuf;
>>>> +}
>>>> +
>>>> +static struct ubuf_info *msg_zerocopy_uarg_alloc(struct sock *sk, size_t size)
>>>> +{
>>>> + struct sk_buff *skb;
>>>> + struct ubuf_info_msgzc *uarg;
>>>> + struct tx_msg_zcopy_node *zcopy_node_p;
>>>> +
>>>> + WARN_ON_ONCE(!in_task());
>>>> +
>>>> + skb = sock_omalloc(sk, sizeof(*zcopy_node_p), GFP_KERNEL);
>>>> + if (!skb)
>>>> + return NULL;
>>>> +
>>>> + BUILD_BUG_ON(sizeof(*uarg) > sizeof(skb->cb));
>>>> + uarg = (void *)skb->cb;
>>>> + uarg->mmp.user = NULL;
>>>> + zcopy_node_p = (struct tx_msg_zcopy_node *)skb_put(skb, sizeof(*zcopy_node_p));
>>>> +
>>>> + if (mm_account_pinned_pages(&uarg->mmp, size)) {
>>>> + kfree_skb(skb);
>>>> + return NULL;
>>>> + }
>>>> +
>>>> + INIT_LIST_HEAD(&zcopy_node_p->node);
>>>> + zcopy_node_p->skb = skb;
>>>> + uarg->ubuf.callback = msg_zerocopy_uarg_callback;
>>>> + init_ubuf_info_msgzc(uarg, sk, size);
>>>> sock_hold(sk);
>>>>
>>>> return &uarg->ubuf;
>>>> @@ -1699,7 +1735,7 @@ static inline struct sk_buff *skb_from_uarg(struct ubuf_info_msgzc *uarg)
>>>> }
>>>>
>>>> struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
>>>> - struct ubuf_info *uarg)
>>>> + struct ubuf_info *uarg, bool usr_arg_notification)
>>>> {
>>>> if (uarg) {
>>>> struct ubuf_info_msgzc *uarg_zc;
>>>> @@ -1707,7 +1743,8 @@ struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
>>>> u32 bytelen, next;
>>>>
>>>> /* there might be non MSG_ZEROCOPY users */
>>>> - if (uarg->callback != msg_zerocopy_callback)
>>>> + if (uarg->callback != msg_zerocopy_callback &&
>>>> + uarg->callback != msg_zerocopy_uarg_callback)
>>>> return NULL;
>>>>
>>>> /* realloc only when socket is locked (TCP, UDP cork),
>>>> @@ -1744,6 +1781,8 @@ struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
>>>> }
>>>>
>>>> new_alloc:
>>>> + if (usr_arg_notification)
>>>> + return msg_zerocopy_uarg_alloc(sk, size);
>>>> return msg_zerocopy_alloc(sk, size);
>>>> }
>>>> EXPORT_SYMBOL_GPL(msg_zerocopy_realloc);
>>>> @@ -1830,6 +1869,86 @@ void msg_zerocopy_callback(struct sk_buff *skb, struct ubuf_info *uarg,
>>>> }
>>>> EXPORT_SYMBOL_GPL(msg_zerocopy_callback);
>>>>
>>>> +static bool skb_zerocopy_uarg_notify_extend(struct tx_msg_zcopy_node *node, u32 lo, u16 len)
>>>> +{
>>>> + u32 old_lo, old_hi;
>>>> + u64 sum_len;
>>>> +
>>>> + old_lo = node->info.lo;
>>>> + old_hi = node->info.hi;
>>>> + sum_len = old_hi - old_lo + 1ULL + len;
>>>> +
>>>> + if (sum_len >= (1ULL << 32))
>>>> + return false;
>>>> +
>>>> + if (lo != old_hi + 1)
>>>> + return false;
>>>> +
>>>> + node->info.hi += len;
>>>> + return true;
>>>> +}
>>>> +
>>>> +static void __msg_zerocopy_uarg_callback(struct ubuf_info_msgzc *uarg)
>>>> +{
>>>> + struct sk_buff *skb = skb_from_uarg(uarg);
>>>> + struct sock *sk = skb->sk;
>>>> + struct tx_msg_zcopy_node *zcopy_node_p, *tail;
>>>> + struct tx_msg_zcopy_queue *zcopy_queue;
>>>> + unsigned long flags;
>>>> + u32 lo, hi;
>>>> + u16 len;
>>>> +
>>>> + mm_unaccount_pinned_pages(&uarg->mmp);
>>>> +
>>>> + /* if !len, there was only 1 call, and it was aborted
>>>> + * so do not queue a completion notification
>>>> + */
>>>> + if (!uarg->len || sock_flag(sk, SOCK_DEAD))
>>>> + goto release;
>>>> +
>>>> + /* only support TCP and UCP currently */
>>>> + if (sk_is_tcp(sk)) {
>>>> + zcopy_queue = &tcp_sk(sk)->tx_zcopy_queue;
>>>> + } else if (sk_is_udp(sk)) {
>>>> + zcopy_queue = &udp_sk(sk)->tx_zcopy_queue;
>>>> + } else {
>>>> + pr_warn("MSG_ZEROCOPY_UARG only support TCP && UDP sockets");
>>>> + goto release;
>>>> + }
>>>> +
>>>> + len = uarg->len;
>>>> + lo = uarg->id;
>>>> + hi = uarg->id + len - 1;
>>>> +
>>>> + zcopy_node_p = (struct tx_msg_zcopy_node *)skb->data;
>>>> + zcopy_node_p->info.lo = lo;
>>>> + zcopy_node_p->info.hi = hi;
>>>> + zcopy_node_p->info.zerocopy = uarg->zerocopy;
>>>> +
>>>> + spin_lock_irqsave(&zcopy_queue->lock, flags);
>>>> + tail = list_last_entry(&zcopy_queue->head, struct tx_msg_zcopy_node, node);
>>>> + if (!tail || !skb_zerocopy_uarg_notify_extend(tail, lo, len)) {
>>>> + list_add_tail(&zcopy_node_p->node, &zcopy_queue->head);
>>>> + skb = NULL;
>>>> + }
>>>> + spin_unlock_irqrestore(&zcopy_queue->lock, flags);
>>>> +release:
>>>> + consume_skb(skb);
>>>> + sock_put(sk);
>>>> +}
>>>> +
>>>> +void msg_zerocopy_uarg_callback(struct sk_buff *skb, struct ubuf_info *uarg,
>>>> + bool success)
>>>> +{
>>>> + struct ubuf_info_msgzc *uarg_zc = uarg_to_msgzc(uarg);
>>>> +
>>>> + uarg_zc->zerocopy = uarg_zc->zerocopy & success;
>>>> +
>>>> + if (refcount_dec_and_test(&uarg->refcnt))
>>>> + __msg_zerocopy_uarg_callback(uarg_zc);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(msg_zerocopy_uarg_callback);
>>>> +
>>>> void msg_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref)
>>>> {
>>>> struct sock *sk = skb_from_uarg(uarg_to_msgzc(uarg))->sk;
>>>> @@ -1838,7 +1957,7 @@ void msg_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref)
>>>> uarg_to_msgzc(uarg)->len--;
>>>>
>>>> if (have_uref)
>>>> - msg_zerocopy_callback(NULL, uarg, true);
>>>> + uarg->callback(NULL, uarg, true);
>>>> }
>>>> EXPORT_SYMBOL_GPL(msg_zerocopy_put_abort);
>>>>
>>>> diff --git a/net/core/sock.c b/net/core/sock.c
>>>> index 5ed411231fc7..a00ebd71f6ed 100644
>>>> --- a/net/core/sock.c
>>>> +++ b/net/core/sock.c
>>>> @@ -2843,6 +2843,56 @@ int __sock_cmsg_send(struct sock *sk, struct cmsghdr *cmsg,
>>>> case SCM_RIGHTS:
>>>> case SCM_CREDENTIALS:
>>>> break;
>>>> + case SO_ZEROCOPY_NOTIFICATION:
>>>> + if (sock_flag(sk, SOCK_ZEROCOPY)) {
>>>> + int i = 0;
>>>> + struct tx_usr_zcopy_info sys_zcopy_info;
>>>> + struct tx_msg_zcopy_node *zcopy_node_p, *tmp;
>>>> + struct tx_msg_zcopy_queue *zcopy_queue;
>>>> + struct tx_msg_zcopy_node *zcopy_node_ps[SOCK_USR_ZC_INFO_MAX];
>>>> + unsigned long flags;
>>>> +
>>>> + if (cmsg->cmsg_len != CMSG_LEN(sizeof(void *)))
>>>> + return -EINVAL;
>>>> +
>>>> + if (sk_is_tcp(sk))
>>>> + zcopy_queue = &tcp_sk(sk)->tx_zcopy_queue;
>>>> + else if (sk_is_udp(sk))
>>>> + zcopy_queue = &udp_sk(sk)->tx_zcopy_queue;
>>>> + else
>>>> + return -EINVAL;
>>>> +
>>>> + spin_lock_irqsave(&zcopy_queue->lock, flags);
>>>> + list_for_each_entry_safe(zcopy_node_p, tmp, &zcopy_queue->head, node) {
>>>> + sys_zcopy_info.info[i].lo = zcopy_node_p->info.lo;
>>>> + sys_zcopy_info.info[i].hi = zcopy_node_p->info.hi;
>>>> + sys_zcopy_info.info[i].zerocopy = zcopy_node_p->info.zerocopy;
>>>> + list_del(&zcopy_node_p->node);
>>>> + zcopy_node_ps[i++] = zcopy_node_p;
>>>> + if (i == SOCK_USR_ZC_INFO_MAX)
>>>> + break;
>>>> + }
>>>> + spin_unlock_irqrestore(&zcopy_queue->lock, flags);
>>>> +
>>>> + if (i > 0) {
>>>> + sys_zcopy_info.length = i;
>>>> + if (unlikely(copy_to_user(*(void **)CMSG_DATA(cmsg),
>>>
>>> This is going to break if user space is 32bit, and kernel 64bit ?
>>>
>>
>> Nice catch, thanks for pointing this out!
>>
>> I may update the code like this?
>>
>> ```
>> if (cmsg->cmsg_len != CMSG_LEN(sizeof(void *)) &&
>> cmsg->cmsg_len != CMSG_LEN(sizeof(u32)))
>> return -EINVAL;
>>
>> ...
>
> I would instead use u64 convention, instead of "void *" or u32.
>
>>
>> void __user *user;
>> if (cmsg->cmsg_len == 8)
>> user = *(void **)CMSG_DATA(cmsg);
>> else if (cmsg->cmsg_len == 4) {
>> u32 val = *(u32 *)CMSG_DATA(cmsg);
>> user = (void *)(uintptr_t)val;
>> }
>> copy_to_user(user, ...)
>> ```
>>
>> I assume it's user's duty to pass in the right address and cmsg_len.
>> Malicious address or invalid address because of wrong cmsg_len will be
>> protected by copy_to_user.
>
>
>
>>
>>> Also SOCK_USR_ZC_INFO_MAX is put in stone, it won't change in the future.
>>>
>>
>> In the scenario where users keep calling sendmsg, at most time, the user
>> will get one notification from the previous sendmsg. Sometimes, users
>> get two notifications. Thus, we give SOCK_USR_ZC_INFO_MAX a small but
>> sufficient number 8. We put it in stone here to avoid dynamic allocation
>> and memory management of the array.
>>
>> If the memory management worth it, we can change it to a sysctl
>> variable?
>
> Convention for user space -> kernel space , is to pass not only a base
> pointer, but also a length.
>
> Of course the kernel implementation can still use a local/automatic
> array in the stack with 8 elems,
> but this could change eventually
>
> Few system calls that did not follow this convention had to be
> extended to other system calls.
>
>>
>> Or, how about we keep SOCK_USR_ZC_INFO_MAX, the max value here, and let
>> the users pass in the length (could be any number less than MAX) of info
>> they want to get?
>>
>>>> + &sys_zcopy_info,
>>>> + sizeof(sys_zcopy_info))
>>>
>>> Your are leaking to user space part of kernel stack, if (i <
>>> SOCK_USR_ZC_INFO_MAX)
>>>
>>
>> Agree, I should do memset to clear the array.
>
> Or copy only the part that has been initialized, struct_size() might help ...
Thanks for the suggestions, I will update in the next version :)
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