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bpf: BPF-helper for MTU checking add length input

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The FIB lookup example[1] show how the IP-header field tot_len
(iph->tot_len) is used as input to perform the MTU check.

This patch extend the BPF-helper bpf_check_mtu() with the same ability
to provide the length as user parameter input, via mtu_len parameter.

This still needs to be done before the bpf_check_mtu() helper API
becomes frozen.

  [1] samples/bpf/xdp_fwd_kern.c

Fixes: 34b2021cc6 ("bpf: Add BPF-helper for MTU checking")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/161521555850.3515614.6533850861569774444.stgit@firesoul
This commit is contained in:
Jesper Dangaard Brouer 2021-03-08 15:59:18 +01:00 committed by Daniel Borkmann
parent e7fb6465d4
commit e5e35e754c
2 changed files with 21 additions and 7 deletions
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16
include/uapi/linux/bpf.h
View File

@ -3850,7 +3850,7 @@ union bpf_attr {
*
* long bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)
* Description
* Check ctx packet size against exceeding MTU of net device (based
* Check packet size against exceeding MTU of net device (based
* on *ifindex*). This helper will likely be used in combination
* with helpers that adjust/change the packet size.
*
@ -3867,6 +3867,14 @@ union bpf_attr {
* against the current net device. This is practical if this isn't
* used prior to redirect.
*
* On input *mtu_len* must be a valid pointer, else verifier will
* reject BPF program. If the value *mtu_len* is initialized to
* zero then the ctx packet size is use. When value *mtu_len* is
* provided as input this specify the L3 length that the MTU check
* is done against. Remember XDP and TC length operate at L2, but
* this value is L3 as this correlate to MTU and IP-header tot_len
* values which are L3 (similar behavior as bpf_fib_lookup).
*
* 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**\ ()
@ -3891,11 +3899,9 @@ union bpf_attr {
*
* 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.
* which is returned here and XDP and TC 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.
* MTU value in your BPF-code.
*
* Return
* * 0 on success, and populate MTU value in *mtu_len* pointer.

12
net/core/filter.c
View File

@ -5658,7 +5658,7 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
return -EINVAL;
if (unlikely(flags & BPF_MTU_CHK_SEGS && len_diff))
if (unlikely(flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len)))
return -EINVAL;
dev = __dev_via_ifindex(dev, ifindex);
@ -5668,7 +5668,11 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
mtu = READ_ONCE(dev->mtu);
dev_len = mtu + dev->hard_header_len;
skb_len = skb->len + len_diff; /* minus result pass check */
/* If set use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
skb_len = *mtu_len ? *mtu_len + 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;
@ -5713,6 +5717,10 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
/* Add L2-header as dev MTU is L3 size */
dev_len = mtu + dev->hard_header_len;
/* Use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
if (*mtu_len)
xdp_len = *mtu_len + dev->hard_header_len;
xdp_len += len_diff; /* minus result pass check */
if (xdp_len > dev_len)
ret = BPF_MTU_CHK_RET_FRAG_NEEDED;