On Thu, 14 Jan 2021 00:07:14 +0100
Daniel Borkmann [off-list ref] wrote:

On 1/12/21 6:45 PM, Jesper Dangaard Brouer wrote:

quoted

This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.  

[...]

quoted

+ * 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) {

Maybe I'm misunderstanding you below?  Do you just want the above two
lines moved from the patch? (sure I can do that... as it is just an
extra check of the "head"/first segment of the packet, and only done if
BPF_MTU_CHK_SEGS is set)

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. 

That is why it is a flag, that need to be explicitly set.

Do you have a particular use case for the BPF_MTU_CHK_SEGS?

The complaint from Maze (and others) were that when skb_is_gso then all
the MTU checks are bypassed.  This flag enables checking the GSO part
via skb_gso_validate_network_len().  We cannot enable it per default,
as you say, it is universally correct in all cases.

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'm using the flag in the bpf-examples code[1], this is how I've tested
the code path.

I've not found a way to generate GSO packet via the selftests
infrastructure via bpf_prog_test_run_xattr().  I'm 

[1] https://github.com/xdp-project/bpf-examples/blob/master/MTU-tests/tc_mtu_enforce.c

I would just drop the flag altogether for the tc helper..

As explain I cannot drop the flag altogether, I would also have to
remove the code then.  Sorry, but I don't 100% understand the change
you are requesting.

quoted

+			ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+			goto out;
+		}
+	}
+out:
+	/* BPF verifier guarantees valid pointer */
+	*mtu_len = mtu;
+
+	return ret;
+}  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer