On 1/12/21 6:45 PM, Jesper Dangaard Brouer wrote:
This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.
[...]
+ * int bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags) + * Description + * Check ctx packet size against MTU of net device (based on + * *ifindex*). This helper will likely be used in combination with + * helpers that adjust/change the packet size. The argument + * *len_diff* can be used for querying with a planned size + * change. This allows to check MTU prior to changing packet ctx. + * + * Specifying *ifindex* zero means the MTU check is performed + * against the current net device. This is practical if this isn't + * used prior to redirect. + * + * The Linux kernel route table can configure MTUs on a more + * specific per route level, which is not provided by this helper. + * For route level MTU checks use the **bpf_fib_lookup**\ () + * helper. + * + * *ctx* is either **struct xdp_md** for XDP programs or + * **struct sk_buff** for tc cls_act programs. + * + * The *flags* argument can be a combination of one or more of the + * following values: + * + * **BPF_MTU_CHK_SEGS** + * This flag will only works for *ctx* **struct sk_buff**. + * If packet context contains extra packet segment buffers + * (often knows as GSO skb), then MTU check is harder to + * check at this point, because in transmit path it is + * possible for the skb packet to get re-segmented + * (depending on net device features). This could still be + * a MTU violation, so this flag enables performing MTU + * check against segments, with a different violation + * return code to tell it apart. Check cannot use len_diff. + * + * On return *mtu_len* pointer contains the MTU value of the net + * device. Remember the net device configured MTU is the L3 size, + * which is returned here and XDP and TX length operate at L2. + * Helper take this into account for you, but remember when using + * MTU value in your BPF-code. On input *mtu_len* must be a valid + * pointer and be initialized (to zero), else verifier will reject + * BPF program. + * + * Return + * * 0 on success, and populate MTU value in *mtu_len* pointer. + * + * * < 0 if any input argument is invalid (*mtu_len* not updated) + * + * MTU violations return positive values, but also populate MTU + * value in *mtu_len* pointer, as this can be needed for + * implementing PMTU handing: + * + * * **BPF_MTU_CHK_RET_FRAG_NEEDED** + * * **BPF_MTU_CHK_RET_SEGS_TOOBIG** + * */ #define __BPF_FUNC_MAPPER(FN) \ FN(unspec), \ @@ -3998,6 +4053,7 @@ union bpf_attr { FN(ktime_get_coarse_ns), \ FN(ima_inode_hash), \ FN(sock_from_file), \ + FN(check_mtu), \ /* *//* integer value in 'imm' field of BPF_CALL instruction selects which helper@@ -5030,6 +5086,17 @@ struct bpf_redir_neigh { }; };+/* bpf_check_mtu flags*/+enum bpf_check_mtu_flags { + BPF_MTU_CHK_SEGS = (1U << 0), +}; + +enum bpf_check_mtu_ret { + BPF_MTU_CHK_RET_SUCCESS, /* check and lookup successful */ + BPF_MTU_CHK_RET_FRAG_NEEDED, /* fragmentation required to fwd */ + BPF_MTU_CHK_RET_SEGS_TOOBIG, /* GSO re-segmentation needed to fwd */ +}; + enum bpf_task_fd_type { BPF_FD_TYPE_RAW_TRACEPOINT, /* tp name */ BPF_FD_TYPE_TRACEPOINT, /* tp name */ diff --git a/net/core/filter.c b/net/core/filter.c index db59ab55572c..3f2e593244ca 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -5604,6 +5604,124 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = { .arg4_type = ARG_ANYTHING, };+static struct net_device *__dev_via_ifindex(struct net_device *dev_curr,+ u32 ifindex) +{ + struct net *netns = dev_net(dev_curr); + + /* Non-redirect use-cases can use ifindex=0 and save ifindex lookup */ + if (ifindex == 0) + return dev_curr; + + return dev_get_by_index_rcu(netns, ifindex); +} + +BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb, + u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags) +{ + int ret = BPF_MTU_CHK_RET_FRAG_NEEDED; + struct net_device *dev = skb->dev; + int skb_len, dev_len; + int mtu; + + if (unlikely(flags & ~(BPF_MTU_CHK_SEGS))) + return -EINVAL; + + dev = __dev_via_ifindex(dev, ifindex); + if (unlikely(!dev)) + return -ENODEV; + + mtu = READ_ONCE(dev->mtu); + + dev_len = mtu + dev->hard_header_len; + skb_len = skb->len + len_diff; /* minus result pass check */ + if (skb_len <= dev_len) { + ret = BPF_MTU_CHK_RET_SUCCESS; + goto out; + } + /* At this point, skb->len exceed MTU, but as it include length of all + * segments, it can still be below MTU. The SKB can possibly get + * re-segmented in transmit path (see validate_xmit_skb). Thus, user + * must choose if segs are to be MTU checked. Last SKB "headlen" is + * checked against MTU. + */ + if (skb_is_gso(skb)) { + ret = BPF_MTU_CHK_RET_SUCCESS; + + if (!(flags & BPF_MTU_CHK_SEGS)) + goto out; + + if (!skb_gso_validate_network_len(skb, mtu)) { + ret = BPF_MTU_CHK_RET_SEGS_TOOBIG; + goto out; + } + + skb_len = skb_headlen(skb) + len_diff; + if (skb_len > dev_len) {
This is still not universally correct given drivers could cook up non-linear skbs (e.g. page frags) on rx. So the result from BPF_MTU_CHK_SEGS flag cannot be relied on. Do you have a particular use case for the BPF_MTU_CHK_SEGS? I also don't see the flag being used anywhere in your selftests, so I presume not as otherwise you would have added an example there? I would just drop the flag altogether for the tc helper..
+ ret = BPF_MTU_CHK_RET_FRAG_NEEDED; + goto out; + } + } +out: + /* BPF verifier guarantees valid pointer */ + *mtu_len = mtu; + + return ret; +}
