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[82.192.128.231]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b56f5e59d3sm69153765e9.10.2026.11.05.04.32.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Nov 2026 04:32:27 -0800 (PST) Message-ID: <8e63cfcc-6120-4db5-ac2f-8214db2c1fe0@redhat.com> Date: Thu, 5 Nov 2026 13:32:26 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v2] ppp: remove SC_MUST_COMP To: Qingfang Deng , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Simon Horman , Tristan Madani , Norbert Szetei , Andrew Morton , Paul Mackerras , Matt Domsch , Brice Goglin , linux-ppp@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261103015615.2704-1-qingfang.deng@linux.dev> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20261103015615.2704-1-qingfang.deng@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/3/26 02:56, Qingfang Deng wrote: > SC_MUST_COMP accidentally shares the same value with SC_DC_ERROR. > PPPIOCGFLAGS combines configuration flags with transmit and receive > state, so userspace cannot distinguish them, and a read-modify-write > sequence can promote a decompression error into SC_MUST_COMP. > > With SC_MUST_COMP set, intentional or not, a fatal decompression error > (SC_DC_FERROR) causes the receive path to silently drop the failed > packet before delivering it to userspace. pppd therefore cannot detect > the error and reset the compressor's state. Subsequent incoming IP and > CCP packets hit the same check and are also dropped, preventing normal > recovery. > > The flag entered Linux in 2005 with commit b3f9b92a6ec1 ("[PPP]: add PPP > MPPE encryption module"). The original patch submission [1] said pppd > would set it when CCP came up with MPPE enabled. That intended > integration [2] appears never to have landed upstream. > > Remove the broken and unused SC_MUST_COMP from SC_MASK and SC_FLAG_BITS, > and remove its transmit and receive drop checks. Keep the uAPI macro and > mark it as obsolete. PPPIOCSFLAGS will discard this bit while > PPPIOCGFLAGS can still report it as SC_DC_ERROR. > > [1] https://lore.kernel.org/netdev/20050814141046.GB20944@lists.us.dell.com/ > [2] https://lore.kernel.org/netdev/20050814190529.GA26802@lists.us.dell.com/ > Fixes: b3f9b92a6ec1 ("[PPP]: add PPP MPPE encryption module") > Signed-off-by: Qingfang Deng Given the 'broken from beginning' status and the hopefully low impact, I'm applying this to net-next. /P