On 2015-11-09 18:53, Willem de Bruijn wrote: > On Sat, Nov 7, 2015 at 8:11 AM, Felix Fietkau <n...@openwrt.org> wrote: >> On 2015-07-31 00:15, Martin Blumenstingl wrote: >>> On Wed, Jul 29, 2015 at 8:05 AM, Willem de Bruijn <will...@google.com> >>> wrote: >>>> Martin, to return to your initial statement that PPPoE PADI packets can >>>> have a zero payload: the PPPoE RFC states that PADI packets "MUST >>>> contain exactly one TAG of TAG_TYPE Service-Name, indicating the >>>> service the Host is requesting, and any number of other TAG types." >>>> (RFC 2516, 5.1). Is the observed behavior (no payload) perhaps >>>> incorrect? >>> As far as I can see you are right, but the real world seems to be different. >>> My ISP for example lists the PPPoE connection settings, but they are >>> nowhere mentioning the "service name". >>> >>> I have also re-read pppd's source code again and that seems to confirm >>> what you are reading in the RFC: Leaving the service name away makes >>> seems to violate the RFC, but pppd still accepts those configurations. >>> >>>> Even if it is, if this is breaking established userspace expectations, >>>> we should look into it. Ethernet specifies a minimum payload size of >>>> 46 on the wire, but perhaps that is handled with padding, so that >>>> 0 length should be valid within the stack. Also, there may be other >>>> valid uses of 0 length payload on top of link layers that are not Ethernet. >>> Good catch. I would also like to note that the documentation for >>> "hard_header_len" describes it as "Hardware header length". When the >>> purpose of this field we should check whether the documentation should >>> be updated to "Minimum hardware header length" -> that would mean the >>> condition has to be a "len < hard_header_len" instead of a "len <= >>> hard_header_len" (as it is now). >>> >>> PS: I have also added the pppd maintainer (Paul Mackerras) to this >>> thread because I think he should know about this issue (and he can >>> probably provide more details if required). >>> As a quick summary for him: linux >= 3.19 rejects PADI packets when >>> no service name is configured. >> Any news on this? Users are complaining about this regression: >> https://dev.openwrt.org/ticket/20707 > > I took another look. This hinges on the question what the contract with > device drivers is on skb network data and length. Is passing an skb with > skb->len == 0 to ndo_start_xmit allowed? > > From what I gather from the ethernet spec [1], sending frames with an > empty head is allowed on that medium, at least. > > A quick scan of a few drivers and the loopback path also does not show > anything that would break. In some cases, skb_network_header points > beyond the end of the buffer (ETH_HLEN), but the length is correctly > reported as 0. > > The tap device can also generate packets consisting of only a link layer > header: compares len < ETH_HLEN in tun_get_user. > > So, I think that this change should be correct: > > static bool ll_header_truncated(const struct net_device *dev, int len) > { > - /* net device doesn't like empty head */ > - if (unlikely(len <= dev->hard_header_len)) { > + if (unlikely(len < dev->hard_header_len)) { > > but a definitive answer would require an audit of all device drivers > (including bonding, ..) or at least the certainty that it has always > been correct to send a packet of only link layer header to > ndo_start_xmit. > > [1] IEEE 802.3™-2012 – Section One, {3.2.8, 4.2.3.3} Yeah, I agree that such an audit is required. However, I think it's *much* more important to add this change as soon as possible to fix the regression. The old code may have had theoretical driver issues, but the current code breaks real-world user setups.
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