On 1/18/21 5:54 PM, Jesper Dangaard Brouer wrote:
This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.
The SKB object is complex and the skb->len value (accessible from
BPF-prog) also include the length of any extra GRO/GSO segments, but
without taking into account that these GRO/GSO segments get added
transport (L4) and network (L3) headers before being transmitted. Thus,
this BPF-helper is created such that the BPF-programmer don't need to
handle these details in the BPF-prog.
The API is designed to help the BPF-programmer, that want to do packet
context size changes, which involves other helpers. These other helpers
usually does a delta size adjustment. This helper also support a delta
size (len_diff), which allow BPF-programmer to reuse arguments needed by
these other helpers, and perform the MTU check prior to doing any actual
size adjustment of the packet context.
It is on purpose, that we allow the len adjustment to become a negative
result, that will pass the MTU check. This might seem weird, but it's not
this helpers responsibility to "catch" wrong len_diff adjustments. Other
helpers will take care of these checks, if BPF-programmer chooses to do
actual size adjustment.
V12:
- Simplify segment check that calls skb_gso_validate_network_len.
- Helpers should return long
V9:
- Use dev->hard_header_len (instead of ETH_HLEN)
- Annotate with unlikely req from Daniel
- Fix logic error using skb_gso_validate_network_len from Daniel
V6:
- Took John's advice and dropped BPF_MTU_CHK_RELAX
- Returned MTU is kept at L3-level (like fib_lookup)
V4: Lot of changes
- ifindex 0 now use current netdev for MTU lookup
- rename helper from bpf_mtu_check to bpf_check_mtu
- fix bug for GSO pkt length (as skb->len is total len)
- remove __bpf_len_adj_positive, simply allow negative len adj
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
include/uapi/linux/bpf.h | 67 ++++++++++++++++++++++++
net/core/filter.c | 111 ++++++++++++++++++++++++++++++++++++++++
tools/include/uapi/linux/bpf.h | 67 ++++++++++++++++++++++++
3 files changed, 245 insertions(+)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 05bfc8c843dc..f17381a337ec 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3839,6 +3839,61 @@ union bpf_attr {
* Return
* A pointer to a struct socket on success or NULL if the file is
* not a socket.
+ *
+ * long 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**
+ *
*/
[...]
+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.
+ */
+ if (skb_is_gso(skb)) {
+ ret = BPF_MTU_CHK_RET_SUCCESS;
+
+ if (flags & BPF_MTU_CHK_SEGS &&
+ !skb_gso_validate_network_len(skb, mtu))
+ ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
I think that looks okay overall now. One thing that will easily slip through
is that in the helper description you mentioned 'Check cannot use len_diff.'
for BPF_MTU_CHK_SEGS flag. So right now for non-zero len_diff the user
will still get BPF_MTU_CHK_RET_SUCCESS if the current length check via
skb_gso_validate_network_len(skb, mtu) passes. If it cannot be checked,
maybe enforce len_diff == 0 for gso skbs on BPF_MTU_CHK_SEGS?
+ }
+out:
+ /* BPF verifier guarantees valid pointer */
+ *mtu_len = mtu;
+
+ return ret;
+}
