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[82.64.211.94]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45d9ec39806sm34279591f8f.9.2026.05.18.00.36.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 18 May 2026 00:36:31 -0700 (PDT) Message-ID: <3550f13e-124c-492f-a5b5-ae6ad95f09f0@redhat.com> Date: Mon, 18 May 2026 09:36:29 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v2] usbnet: limit max_mtu based on device's hard_mtu To: Matthias May Cc: linux-usb@vger.kernel.org, Oliver Neukum , Jakub Kicinski , netdev@vger.kernel.org, Stefano Brivio , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260119075518.2774373-1-lvivier@redhat.com> <539202f5-43ba-4938-a9b2-393c1bb3e072@westermo.com> Content-Language: en-US From: Laurent Vivier Autocrypt: addr=lvivier@redhat.com; keydata= xsFNBFYFJhkBEAC2me7w2+RizYOKZM+vZCx69GTewOwqzHrrHSG07MUAxJ6AY29/+HYf6EY2 WoeuLWDmXE7A3oJoIsRecD6BXHTb0OYS20lS608anr3B0xn5g0BX7es9Mw+hV/pL+63EOCVm SUVTEQwbGQN62guOKnJJJfphbbv82glIC/Ei4Ky8BwZkUuXd7d5NFJKC9/GDrbWdj75cDNQx UZ9XXbXEKY9MHX83Uy7JFoiFDMOVHn55HnncflUncO0zDzY7CxFeQFwYRbsCXOUL9yBtqLer Ky8/yjBskIlNrp0uQSt9LMoMsdSjYLYhvk1StsNPg74+s4u0Q6z45+l8RAsgLw5OLtTa+ePM JyS7OIGNYxAX6eZk1+91a6tnqfyPcMbduxyBaYXn94HUG162BeuyBkbNoIDkB7pCByed1A7q q9/FbuTDwgVGVLYthYSfTtN0Y60OgNkWCMtFwKxRaXt1WFA5ceqinN/XkgA+vf2Ch72zBkJL RBIhfOPFv5f2Hkkj0MvsUXpOWaOjatiu0fpPo6Hw14UEpywke1zN4NKubApQOlNKZZC4hu6/ 8pv2t4HRi7s0K88jQYBRPObjrN5+owtI51xMaYzvPitHQ2053LmgsOdN9EKOqZeHAYG2SmRW LOxYWKX14YkZI5j/TXfKlTpwSMvXho+efN4kgFvFmP6WT+tPnwARAQABzSNMYXVyZW50IFZp dmllciA8bHZpdmllckByZWRoYXQuY29tPsLBeAQTAQIAIgUCVgVQgAIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQ8ww4vT8vvjwpgg//fSGy0Rs/t8cPFuzoY1cex4limJQfReLr SJXCANg9NOWy/bFK5wunj+h/RCFxIFhZcyXveurkBwYikDPUrBoBRoOJY/BHK0iZo7/WQkur 6H5losVZtrotmKOGnP/lJYZ3H6OWvXzdz8LL5hb3TvGOP68K8Bn8UsIaZJoeiKhaNR0sOJyI YYbgFQPWMHfVwHD/U+/gqRhD7apVysxv5by/pKDln1I5v0cRRH6hd8M8oXgKhF2+rAOL7gvh jEHSSWKUlMjC7YwwjSZmUkL+TQyE18e2XBk85X8Da3FznrLiHZFHQ/NzETYxRjnOzD7/kOVy gKD/o7asyWQVU65mh/ECrtjfhtCBSYmIIVkopoLaVJ/kEbVJQegT2P6NgERC/31kmTF69vn8 uQyW11Hk8tyubicByL3/XVBrq4jZdJW3cePNJbTNaT0d/bjMg5zCWHbMErUib2Nellnbg6bc 2HLDe0NLVPuRZhHUHM9hO/JNnHfvgiRQDh6loNOUnm9Iw2YiVgZNnT4soUehMZ7au8PwSl4I KYE4ulJ8RRiydN7fES3IZWmOPlyskp1QMQBD/w16o+lEtY6HSFEzsK3o0vuBRBVp2WKnssVH qeeV01ZHw0bvWKjxVNOksP98eJfWLfV9l9e7s6TaAeySKRRubtJ+21PRuYAxKsaueBfUE7ZT 7zfOwU0EVgUmGQEQALxSQRbl/QOnmssVDxWhHM5TGxl7oLNJms2zmBpcmlrIsn8nNz0rRyxT 460k2niaTwowSRK8KWVDeAW6ZAaWiYjLlTunoKwvF8vP3JyWpBz0diTxL5o+xpvy/Q6YU3BN efdq8Vy3rFsxgW7mMSrI/CxJ667y8ot5DVugeS2NyHfmZlPGE0Nsy7hlebS4liisXOrN3jFz asKyUws3VXek4V65lHwB23BVzsnFMn/bw/rPliqXGcwl8CoJu8dSyrCcd1Ibs0/Inq9S9+t0 VmWiQWfQkz4rvEeTQkp/VfgZ6z98JRW7S6l6eophoWs0/ZyRfOm+QVSqRfFZdxdP2PlGeIFM C3fXJgygXJkFPyWkVElr76JTbtSHsGWbt6xUlYHKXWo+xf9WgtLeby3cfSkEchACrxDrQpj+ Jt/JFP+q997dybkyZ5IoHWuPkn7uZGBrKIHmBunTco1+cKSuRiSCYpBIXZMHCzPgVDjk4viP brV9NwRkmaOxVvye0vctJeWvJ6KA7NoAURplIGCqkCRwg0MmLrfoZnK/gRqVJ/f6adhU1oo6 z4p2/z3PemA0C0ANatgHgBb90cd16AUxpdEQmOCmdNnNJF/3Zt3inzF+NFzHoM5Vwq6rc1JP jfC3oqRLJzqAEHBDjQFlqNR3IFCIAo4SYQRBdAHBCzkM4rWyRhuVABEBAAHCwV8EGAECAAkF AlYFJhkCGwwACgkQ8ww4vT8vvjwg9w//VQrcnVg3TsjEybxDEUBm8dBmnKqcnTBFmxN5FFtI WlEuY8+YMiWRykd8Ln9RJ/98/ghABHz9TN8TRo2b6WimV64FmlVn17Ri6FgFU3xNt9TTEChq AcNg88eYryKsYpFwegGpwUlaUaaGh1m9OrTzcQy+klVfZWaVJ9Nw0keoGRGb8j4XjVpL8+2x OhXKrM1fzzb8JtAuSbuzZSQPDwQEI5CKKxp7zf76J21YeRrEW4WDznPyVcDTa+tz++q2S/Bp P4W98bXCBIuQgs2m+OflERv5c3Ojldp04/S4NEjXEYRWdiCxN7ca5iPml5gLtuvhJMSy36gl U6IW9kn30IWuSoBpTkgV7rLUEhh9Ms82VWW/h2TxL8enfx40PrfbDtWwqRID3WY8jLrjKfTd R3LW8BnUDNkG+c4FzvvGUs8AvuqxxyHbXAfDx9o/jXfPHVRmJVhSmd+hC3mcQ+4iX5bBPBPM oDqSoLt5w9GoQQ6gDVP2ZjTWqwSRMLzNr37rJjZ1pt0DCMMTbiYIUcrhX8eveCJtY7NGWNyx FCRkhxRuGcpwPmRVDwOl39MB3iTsRighiMnijkbLXiKoJ5CDVvX5yicNqYJPKh5MFXN1bvsB kmYiStMRbrD0HoY1kx5/VozBtc70OU0EB8Wrv9hZD+Ofp0T3KOr1RUHvCZoLURfFhSQ= In-Reply-To: <539202f5-43ba-4938-a9b2-393c1bb3e072@westermo.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 5/18/26 09:21, Matthias May wrote: > On 19/01/2026 8:55 am, Laurent Vivier wrote: >> The usbnet driver initializes net->max_mtu to ETH_MAX_MTU before calling >> the device's bind() callback. When the bind() callback sets >> dev->hard_mtu based the device's actual capability (from CDC Ethernet's >> wMaxSegmentSize descriptor), max_mtu is never updated to reflect this >> hardware limitation). >> >> This allows userspace (DHCP or IPv6 RA) to configure MTU larger than the >> device can handle, leading to silent packet drops when the backend sends >> packet exceeding the device's buffer size. >> >> Fix this by limiting net->max_mtu to the device's hard_mtu after the >> bind callback returns. >> >> See https://urldefense.com/v3/__https://gitlab.com/qemu-project/qemu/-/issues/3268__;!! >> I9LPvj3b!H-nIZIscCCh_2FnbJInagPxXTe0XcNu58-8k3NqGYKRdDy8LBOBjWiTIc1E- >> cC2wnv91MtZrak2pu7K-4cU$  and >>      https://urldefense.com/v3/__https://bugs.passt.top/attachment.cgi?bugid=189__;!! >> I9LPvj3b!H-nIZIscCCh_2FnbJInagPxXTe0XcNu58-8k3NqGYKRdDy8LBOBjWiTIc1E- >> cC2wnv91MtZrak2pq4lrvZI$ >> >> Fixes: f77f0aee4da4 ("net: use core MTU range checking in USB NIC drivers") >> Signed-off-by: Laurent Vivier >> Link: https://urldefense.com/v3/__https://bugs.passt.top/show_bug.cgi?id=189__;!! >> I9LPvj3b!H-nIZIscCCh_2FnbJInagPxXTe0XcNu58-8k3NqGYKRdDy8LBOBjWiTIc1E- >> cC2wnv91MtZrak2p8csXaww$ >> Reviewed-by: Stefano Brivio >> --- >>   drivers/net/usb/usbnet.c | 9 ++++++--- >>   1 file changed, 6 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c >> index 36742e64cff7..1093c2a412d9 100644 >> --- a/drivers/net/usb/usbnet.c >> +++ b/drivers/net/usb/usbnet.c >> @@ -1821,9 +1821,12 @@ usbnet_probe(struct usb_interface *udev, const struct >> usb_device_id *prod) >>           if ((dev->driver_info->flags & FLAG_NOARP) != 0) >>               net->flags |= IFF_NOARP; >> -        /* maybe the remote can't receive an Ethernet MTU */ >> -        if (net->mtu > (dev->hard_mtu - net->hard_header_len)) >> -            net->mtu = dev->hard_mtu - net->hard_header_len; >> +        if (net->max_mtu > (dev->hard_mtu - net->hard_header_len)) >> +            net->max_mtu = dev->hard_mtu - net->hard_header_len; >> + >> +        if (net->mtu > net->max_mtu) >> +            net->mtu = net->max_mtu; >> + >>       } else if (!info->in || !info->out) >>           status = usbnet_get_endpoints(dev, udev); >>       else { > > Hi Laurent Hi Matthias, > > This change was backported to 6.6.* and caused a regression with wwan devices via USB when > using a mux. > > Tested on a Quectel EM12 running the firmware EM12GPAR01A21M4G_01.300.01.300. > > Tue May  5 09:49:35.638 2026 kern.info kernel: [   10.819620] qmi_wwan 1-1.2:1.4: cdc- > wdm0: USB WDM device > Tue May  5 09:49:35.638 2026 kern.info kernel: [   10.829601] qmi_wwan 1-1.2:1.4 > cellular0: register 'qmi_wwan' at usb-fsl-ehci.0-1.2, WWAN/QMI device, 6a:c3:49:88:47:b1 > Tue May  5 09:49:35.638 2026 kern.info kernel: [   10.840579] usbcore: registered new > interface driver qmi_wwan > > The parent interface (we renamed it "cellular0") requires an MTU of 1504 (4 bytes overhead > from the muxer). > The actual wwan0, wwan1 interfaces have an MTU of 1500. > > With this change it's no longer possible to set an MTU of 1504 on cellular0. This should be fixed by 55f854dd5bdd ("qmi_wwan: allow max_mtu above hard_mtu to control rx_urb_size") which is in v7.0. Could you have a try? Thanks, Laurent