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Miller'" , "'Jakub Kicinski'" , "'Network Development'" , "'linux-kernel'" In-Reply-To: Subject: RE: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4 Date: Fri, 7 May 2021 17:25:49 +0900 Message-ID: <007001d7431a$96281960$c2784c20$@samsung.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Mailer: Microsoft Outlook 16.0 Thread-Index: AQKypHYW3xad5/j2XvChPebQmKG2owG+YoocAqFpyMsBZavlXQIxNaIYAiru3ucBq8RF2QKY6vTDAm6kVlUBadK04AMaqQ3GqHYXshA= Content-Language: ko X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFjrOJsWRmVeSWpSXmKPExsWy7bCmhW7fzykJBvNXSlh8/z2b2eLLz9vs Fp+PHGezWLzwG7PFnPMtLBZNO1YwWbz48ITR4vm+XiaLC9v6WC0u75rDZtHwlsvi2AIxi5+H zzBbLP65AahqyQxGB36PLStvMnlMbH7H7rFz1l12j5Yjb1k9um5cYvbYtKqTzePzJrkA9qgc m4zUxJTUIoXUvOT8lMy8dFsl7+B453hTMwNDXUNLC3MlhbzE3FRbJRefAF23zBygs5UUyhJz SoFCAYnFxUr6djZF+aUlqQoZ+cUltkqpBSk5BYaGBXrFibnFpXnpesn5uVaGBgZGpkCVCTkZ +xf+YC+Y4FlxvusSSwNjg3kXIyeHhICJxIHTV1i6GLk4hAR2MEpcvHSVDcL5xCgxd841dgjn G6NEa88NNpiWzUfus0Ik9jJKNB07wASSEBJ4wShxcDcPiM0moCXxZlY7K4gtIpAu8bu7B2wS s8A8FomGhW+ZQRKcAoESk6fOB5sqLBAs8XRSE9ggFgEVidY309lBbF4BS4me3/NYIWxBiZMz n7CA2MwC8hLb385hhrhIQeLn02VQy8ok9nbsZISoEZGY3dnGDLJYQuAOh8SGaa+ZIBpcJP7t v84IYQtLvDq+hR3ClpJ42d8GZHMA2fUSrd0xEL09jBJX9kEslhAwlpj1rJ0RpIZZQFNi/S59 iHJliSO3oE7jk+g4/BdqCq9ER5sQhKkkMfFLPMQMCYkXJyezTGBUmoXkr1lI/pqF5P5ZCKsW MLKsYhRLLSjOTU8tNiowQY7rTYzgFK3lsYNx9tsPeocYmTgYDzFKcDArifCeXjQ5QYg3JbGy KrUoP76oNCe1+BCjKTCkJzJLiSbnA7NEXkm8oamRmZmBpamFqZmRhZI478/UugQhgfTEktTs 1NSC1CKYPiYOTqkGppg9Lw7OcHVvXXTk3YF0znLuCM20uluHJ4pERPhtVmRYYpha0P9iwjGL HlarT9HVR0PvZlQ/CW7/528rFW089+KpEnn5K9/Onawts92boVXLsclgzs2vWivkjs5/7qe4 +bZVdq5fXOXdfxZuIl8YjkdGp/R5qz3VlNTK2/X4qt9yr1ypn8v/rUk78H9BoOWi2gXnNu7p Odtu02jqlbNtw7sP53/38h0T8/gefiGz9pm3h82VWsu5O6Lj5EqTF4eI3HZV/qKntXnithD+ ySeDlmzQOX5RWapnxWGtjE7PZa1XRZeqRqhY/vNv73/yvIFjT8gcqbZ1QmYnA2ewMV5Y+9bU xpyFa4oN0/cdfL6zlFiKMxINtZiLihMBe5UHLFoEAAA= X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFtrEIsWRmVeSWpSXmKPExsWy7bCSvG7vzykJBnMemFl8/z2b2eLLz9vs Fp+PHGezWLzwG7PFnPMtLBZNO1YwWbz48ITR4vm+XiaLC9v6WC0u75rDZtHwlsvi2AIxi5+H zzBbLP65AahqyQxGB36PLStvMnlMbH7H7rFz1l12j5Yjb1k9um5cYvbYtKqTzePzJrkA9igu m5TUnMyy1CJ9uwSujP0Lf7AXTPCsON91iaWBscG8i5GTQ0LARGLzkfusXYxcHEICuxkl7p3s ZOli5ABKSEjs2uwKUSMscb/lCCuILSTwjFFi87l8EJtNQEvizax2sLiIQLrEnrNf2EDmMAus YJFomvaNDaJhFqvEyS0SIDanQKDE5KnzweLCQHb/v1uMIDaLgIpE65vp7CA2r4ClRM/veawQ tqDEyZlPWEBsZgFtid6HrYwQtrzE9rdzmCGOU5D4+XQZ1BFlEns7dkLViEjM7mxjnsAoPAvJ qFlIRs1CMmoWkpYFjCyrGCVTC4pz03OLDQsM81LL9YoTc4tL89L1kvNzNzGCo1VLcwfj9lUf 9A4xMnEwHmKU4GBWEuE9vWhyghBvSmJlVWpRfnxRaU5q8SFGaQ4WJXHeC10n44UE0hNLUrNT UwtSi2CyTBycUg1MTYdzFPodtbbNnx3w30BNoizijKrSft7M5Q5v2Ba17+U/ePfyqXs6i54Z nQktmrl+54u7KjeXtGqKN+TVSzd/37SzJsmg2TJ20pybc46WHpl5hmHGnydpr+dJPN8TMftf tEptj6UEy9fWI7sm3Un75xrScG/N+mvLBO6/+bDS77VpyvRHSRce7jquIr16zY81yjrZDO5x X5T2mqt3/N9vV7aUXfIpm3/+MSHF9TM/MbnLntutk5SuZqWvN3X67E26U2O/7lK7pNL/YNN0 4Rzm4xZPtTrLr28wn/c2SeB81qms2jwhGw23xz3pvtNWqlzgUt4kUHeyUIkjjv/9jPRH8uLX 17YyZV7NSa77XOOvrcRSnJFoqMVcVJwIADMIND1FAwAA X-CMS-MailID: 20210507082550epcas2p2d08871891e12b5b0c22281344e53afcf X-Msg-Generator: CA Content-Type: text/plain; charset="utf-8" X-Sendblock-Type: AUTO_CONFIDENTIAL CMS-TYPE: 102P DLP-Filter: Pass X-CFilter-Loop: Reflected X-CMS-RootMailID: 20210429102143epcas2p4c8747c09a9de28f003c20389c050394a References: <1619690903-1138-1-git-send-email-dseok.yi@samsung.com> <8c2ea41a-3fc5-d560-16e5-bf706949d857@iogearbox.net> <02bf01d74211$0ff4aed0$2fde0c70$@samsung.com> <02c801d7421f$65287a90$2f796fb0$@samsung.com> <001801d742db$68ab8060$3a028120$@samsung.com> <436dbc62-451b-9b29-178d-9da28f47ef24@huawei.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 06, 2021 at 09:53:45PM -0400, Willem de Bruijn wrote: > On Thu, May 6, 2021 at 9:45 PM Yunsheng Lin wrote: > > > > On 2021/5/7 9:25, Willem de Bruijn wrote: > > >>>> head_skb's data_len is the sum of skb_gro_len for each skb of the frags. > > >>>> data_len could be 8 if server sent a small size packet and it is GROed > > >>>> to head_skb. > > >>>> > > >>>> Please let me know if I am missing something. > > >>> > > >>> This is my understanding of the data path. This is a forwarding path > > >>> for TCP traffic. > > >>> > > >>> GRO is enabled and will coalesce multiple segments into a single large > > >>> packet. In bad cases, the coalesced packet payload is > MSS, but < MSS > > >>> + 20. > > >>> > > >>> Somewhere between GRO and GSO you have a BPF program that converts the > > >>> IPv6 address to IPv4. > > >> > > >> Your understanding is right. The data path is GRO -> BPF 6 to 4 -> > > >> GSO. > > >> > > >>> > > >>> There is no concept of head_skb at the time of this BPF program. It is > > >>> a single SKB, with an skb linear part and multiple data items in the > > >>> frags (no frag_list). > > >> > > >> Sorry for the confusion. head_skb what I mentioned was a skb linear > > >> part. I'm considering a single SKB with frags too. > > >> > > >>> > > >>> When entering the GSO stack, this single skb now has a payload length > > >>> < MSS. So it would just make a valid TCP packet on its own? > > >>> > > >>> skb_gro_len is only relevant inside the GRO stack. It internally casts > > >>> the skb->cb[] to NAPI_GRO_CB. This field is a scratch area that may be > > >>> reused for other purposes later by other layers of the datapath. It is > > >>> not safe to read this inside bpf_skb_proto_6_to_4. > > >> > > >> The condition what I made uses skb->data_len not skb_gro_len. Does > > >> skb->data_len have a different meaning on each layer? As I know, > > >> data_len indicates the amount of frags or frag_list. skb->data_len > > >> should be > 20 in the sample case because the payload size of the skb > > >> linear part is the same with mss. > > > > > > Ah, got it. > > > > > > data_len is the length of the skb minus the length in the skb linear > > > section (as seen in skb_headlen). > > > > > > So this gso skb consists of two segments, the first one entirely > > > linear, the payload of the second is in skb_shinfo(skb)->frags[0]. > > > > > > It is not guaranteed that gso skbs built from two individual skbs end > > > up looking like that. Only protocol headers in the linear segment and > > > the payload of both in frags is common. > > > > > >> We can modify netif_needs_gso as another option to hit > > >> skb_needs_linearize in validate_xmit_skb. But I think we should compare > > >> skb->gso_size and skb->data_len too to check if mss exceed a payload > > >> size. > > > > > > The rest of the stack does not build such gso packets with payload len > > > < mss, so we should not have to add workarounds in the gso hot path > > > for this. > > > > > > Also no need to linearize this skb. I think that if the bpf program > > > would just clear the gso type, the packet would be sent correctly. > > > Unless I'm missing something. > > > > Does the checksum/len field in ip and tcp/udp header need adjusting > > before clearing gso type as the packet has became bigger? > > gro takes care of this. see for instance inet_gro_complete for updates > to the ip header. I think clearing the gso type will get an error at tcp4_gso_segment because netif_needs_gso returns true in validate_xmit_skb. > > > Also, instead of testing skb->data_len, may test the skb->len? > > > > skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff > > Yes. Essentially doing the same calculation as the gso code that is > causing the packet to be dropped. BPF program is usually out of control. Can we take a general approach? The below 2 cases has no issue when mss upgrading. 1) skb->data_len > mss + 20 2) skb->data_len < mss && skb->data_len > 20 The corner case is when 3) skb->data_len > mss && skb->data_len < mss + 20 But to cover #3 case, we should check the condition Yunsheng Lin said. What if we do mss upgrading for both #1 and #2 cases only? + unsigned short off_len = skb->data_len > shinfo->gso_size ? + shinfo->gso_size : 0; [...] /* Due to IPv4 header, MSS can be upgraded. */ - skb_increase_gso_size(shinfo, len_diff); + if (skb->data_len - off_len > len_diff) + skb_increase_gso_size(shinfo, len_diff); > > > > > > > But I don't mean to argue that it should do that in production. > > > Instead, not playing mss games would solve this and stay close to the > > > original datapath if no bpf program had been present. Including > > > maintaining the GSO invariant of sending out the same chain of packets > > > as received (bar the IPv6 to IPv4 change). > > > > > > This could be achieved by adding support for the flag > > > BPF_F_ADJ_ROOM_FIXED_GSO in the flags field of bpf_skb_change_proto. > > > And similar to bpf_skb_net_shrink: > > > > > > /* Due to header shrink, MSS can be upgraded. */ > > > if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) > > > skb_increase_gso_size(shinfo, len_diff); > > > > > > The other case, from IPv4 to IPv6 is more difficult to address, as not > > > reducing the MSS will result in packets exceeding MTU. That calls for > > > workarounds like MSS clamping. Anyway, that is out of scope here. > > > > > > > > > > > >>> > > >>> > > >>>>> > > >>>>> One simple solution if this packet no longer needs to be segmented > > >>>>> might be to reset the gso_type completely. > > >>>> > > >>>> I am not sure gso_type can be cleared even when GSO is needed. > > >>>> > > >>>>> > > >>>>> In general, I would advocate using BPF_F_ADJ_ROOM_FIXED_GSO. When > > >>>>> converting from IPv6 to IPv4, fixed gso will end up building packets > > >>>>> that are slightly below the MTU. That opportunity cost is negligible > > >>>>> (especially with TSO). Unfortunately, I see that that flag is > > >>>>> available for bpf_skb_adjust_room but not for bpf_skb_proto_6_to_4. > > >>>>> > > >>>>> > > >>>>>>>> would increse the gso_size to 1392. tcp_gso_segment will get an error > > >>>>>>>> with 1380 <= 1392. > > >>>>>>>> > > >>>>>>>> Check for the size of GROed payload if it is really bigger than target > > >>>>>>>> mss when increase mss. > > >>>>>>>> > > >>>>>>>> Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper) > > >>>>>>>> Signed-off-by: Dongseok Yi > > >>>>>>>> --- > > >>>>>>>> net/core/filter.c | 4 +++- > > >>>>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) > > >>>>>>>> > > >>>>>>>> diff --git a/net/core/filter.c b/net/core/filter.c > > >>>>>>>> index 9323d34..3f79e3c 100644 > > >>>>>>>> --- a/net/core/filter.c > > >>>>>>>> +++ b/net/core/filter.c > > >>>>>>>> @@ -3308,7 +3308,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb) > > >>>>>>>> } > > >>>>>>>> > > >>>>>>>> /* Due to IPv4 header, MSS can be upgraded. */ > > >>>>>>>> - skb_increase_gso_size(shinfo, len_diff); > > >>>>>>>> + if (skb->data_len > len_diff) > > >>>>>>> > > >>>>>>> Could you elaborate some more on what this has to do with data_len specifically > > >>>>>>> here? I'm not sure I follow exactly your above commit description. Are you saying > > >>>>>>> that you're hitting in tcp_gso_segment(): > > >>>>>>> > > >>>>>>> [...] > > >>>>>>> mss = skb_shinfo(skb)->gso_size; > > >>>>>>> if (unlikely(skb->len <= mss)) > > >>>>>>> goto out; > > >>>>>>> [...] > > >>>>>> > > >>>>>> Yes, right > > >>>>>> > > >>>>>>> > > >>>>>>> Please provide more context on the bug, thanks! > > >>>>>> > > >>>>>> tcp_gso_segment(): > > >>>>>> [...] > > >>>>>> __skb_pull(skb, thlen); > > >>>>>> > > >>>>>> mss = skb_shinfo(skb)->gso_size; > > >>>>>> if (unlikely(skb->len <= mss)) > > >>>>>> [...] > > >>>>>> > > >>>>>> skb->len will have total GROed TCP payload size after __skb_pull. > > >>>>>> skb->len <= mss will not be happened in a normal GROed situation. But > > >>>>>> bpf_skb_proto_6_to_4 would upgrade MSS by increasing gso_size, it can > > >>>>>> hit an error condition. > > >>>>>> > > >>>>>> We should ensure the following condition. > > >>>>>> total GROed TCP payload > the original mss + (IPv6 size - IPv4 size) > > >>>>>> > > >>>>>> Due to > > >>>>>> total GROed TCP payload = the original mss + skb->data_len > > >>>>>> IPv6 size - IPv4 size = len_diff > > >>>>>> > > >>>>>> Finally, we can get the condition. > > >>>>>> skb->data_len > len_diff > > >>>>>> > > >>>>>>> > > >>>>>>>> + skb_increase_gso_size(shinfo, len_diff); > > >>>>>>>> + > > >>>>>>>> /* Header must be checked, and gso_segs recomputed. */ > > >>>>>>>> shinfo->gso_type |= SKB_GSO_DODGY; > > >>>>>>>> shinfo->gso_segs = 0; > > >>>>>>>> > > >>>>>> > > >>>>>> > > >>>> > > >> > > > > > > . > > > > >