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[82.132.213.132]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49b91c9cf95sm46448655e9.1.2026.08.28.05.37.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Aug 2026 05:37:37 -0700 (PDT) Message-ID: <0db1151c-35a0-4597-b69c-091b60cb864c@gmail.com> Date: Fri, 28 Aug 2026 13:37:36 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net 1/1] tcp: reject non zerocopy devmem tx To: Mina Almasry Cc: "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org References: <75cf5a10-f7ae-4b4d-ae47-1556a30d76c9@gmail.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 8/26/26 18:29, Mina Almasry wrote: > On Wed, Aug 26, 2026 at 7:36 AM Pavel Begunkov wrote: >> >> On 8/23/26 16:58, Mina Almasry wrote: >>> On Sat, Aug 22, 2026 at 3:57 AM Pavel Begunkov 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