Re: [PATCH net 1/1] tcp: reject non zerocopy devmem tx

[Pavel Begunkov <asml.silence@gmail.com>]

On 8/26/26 18:29, Mina Almasry wrote:
> On Wed, Aug 26, 2026 at 7:36 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>
>> On 8/23/26 16:58, Mina Almasry wrote:
>>> On Sat, Aug 22, 2026 at 3:57 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>> ...> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>>> index b4237d0e994d6..1e34f50770446 100644
>>> --- a/net/ipv4/tcp.c
>>> +++ b/net/ipv4/tcp.c
>>> @@ -1170,7 +1170,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct
>>> msghdr *msg, size_t size)
>>>           }
>>>
>>>           if (!sockc_err && sockc.dmabuf_id &&
>>> -           (!(flags & MSG_ZEROCOPY) || !sock_flag(sk, SOCK_ZEROCOPY))) {
>>> +           (!(flags & MSG_ZEROCOPY) || !sock_flag(sk, SOCK_ZEROCOPY) ||
>>
>> This check before your diff fells off. From a quick look, via io_uring
>> you can have a sendmsg call with dmabuf_id that would happily pass this
>> check but never goes through net_devmem_get_binding() and
>> msg_zerocopy_realloc(). It's not much too problematic for now but feels
>> dangerous to rely on it in the current form. I think we want sth like
>> this in the end.
>>
>> if (sockc.dmabuf_id && !sockc_err) {
>>          if (!binding || zc != MSG_ZEROCOPY)
>>                  // fail;
>> }
>>
> 
> OK it sorta makes sense to me. I think we're making a mistake checking
> SOCK_ZEROCOPY/MSG_ZEROCOPY when instead all these are conveniently
> condensed into 'zc == MSG_ZEROCOPY' when everything lines up. The
> point of this line was roughly 'if the user is trying to send devmem
> but not MSG_ZEROCOPY'd, fail'.

Alright, good to know as I was assuming that. Let's do it like that
then, I'll send a patch.

-- 
Pavel Begunkov