[PATCH bpf-next V3 3/6] bpf: add BPF-helper for MTU checking

[Jesper Dangaard Brouer <brouer@redhat.com>]

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

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.

V3: Take L2/ETH_HLEN header size into account and document it.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h       |   63 +++++++++++++++++++++
 net/core/filter.c              |  119 ++++++++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |   63 +++++++++++++++++++++
 3 files changed, 245 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 4a46a1de6d16..1dcf5d8195f4 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3718,6 +3718,56 @@ union bpf_attr {
  *		never return NULL.
  *	Return
  *		A pointer pointing to the kernel percpu variable on this cpu.
+ *
+ * int bpf_mtu_check(void *ctx, u32 ifindex, u32 *mtu_result, 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.
+ *
+ *		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_RELAX**
+ *			This flag relax or increase the MTU with room for one
+ *			VLAN header (4 bytes) and take into account net device
+ *			hard_header_len.  This relaxation is also used by the
+ *			kernels own forwarding MTU checks.
+ *
+ *		**BPF_MTU_CHK_GSO**
+ *			This flag will only works for *ctx* **struct sk_buff**.
+ *			If packet context contains extra packet segment buffers
+ *			(often knows as frags), then those are also checked
+ *			against the MTU size.
+ *
+ *		The *mtu_result* pointer contains the MTU value of the net
+ *		device including the L2 header size (usually 14 bytes Ethernet
+ *		header). The net device configured MTU is the L3 size, but as
+ *		XDP and TX length operate at L2 this helper include L2 header
+ *		size in reported MTU.
+ *
+ *	Return
+ *		* 0 on success, and populate MTU value in *mtu_result* pointer.
+ *
+ *		* < 0 if any input argument is invalid (*mtu_result* not updated)
+ *
+ *		MTU violations return positive values, but also populate MTU
+ *		value in *mtu_result* pointer, as this can be needed for
+ *		implemeting PMTU handing:
+ *
+ *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
+ *		* **BPF_MTU_CHK_RET_GSO_TOOBIG**
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3875,6 +3925,7 @@ union bpf_attr {
 	FN(redirect_neigh),		\
 	FN(bpf_per_cpu_ptr),            \
 	FN(bpf_this_cpu_ptr),		\
+	FN(mtu_check),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4889,6 +4940,18 @@ struct bpf_fib_lookup {
 	__u8	dmac[6];     /* ETH_ALEN */
 };
 
+/* bpf_mtu_check flags*/
+enum  bpf_mtu_check_flags {
+	BPF_MTU_CHK_RELAX = (1U << 0),
+	BPF_MTU_CHK_GSO   = (1U << 1),
+};
+
+enum bpf_mtu_check_ret {
+	BPF_MTU_CHK_RET_SUCCESS,      /* check and lookup successful */
+	BPF_MTU_CHK_RET_FRAG_NEEDED,  /* fragmentation required to fwd */
+	BPF_MTU_CHK_RET_GSO_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 da74d6ddc4d7..5986156e700e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5513,6 +5513,121 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
 	.arg4_type	= ARG_ANYTHING,
 };
 
+static int bpf_mtu_lookup(struct net *netns, u32 ifindex, u64 flags)
+{
+	struct net_device *dev;
+	int mtu;
+
+	dev = dev_get_by_index_rcu(netns, ifindex);
+	if (!dev)
+		return -ENODEV;
+
+	/* XDP+TC len is L2: Add L2-header as dev MTU is L3 size */
+	mtu = dev->mtu + dev->hard_header_len;
+
+	/*  Same relax as xdp_ok_fwd_dev() and is_skb_forwardable() */
+	if (flags & BPF_MTU_CHK_RELAX)
+		mtu += VLAN_HLEN;
+
+	return mtu;
+}
+
+static unsigned int __bpf_len_adjust_positive(unsigned int len, int len_diff)
+{
+	int len_new = len + len_diff; /* notice len_diff can be negative */
+
+	if (len_new > 0)
+		return len_new;
+
+	return 0;
+}
+
+BPF_CALL_5(bpf_skb_mtu_check, struct sk_buff *, skb,
+	   u32, ifindex, u32 *, mtu_result, s32, len_diff, u64, flags)
+{
+	struct net *netns = dev_net(skb->dev);
+	int ret = BPF_MTU_CHK_RET_SUCCESS;
+	unsigned int len = skb->len;
+	int mtu;
+
+	if (flags & ~(BPF_MTU_CHK_RELAX | BPF_MTU_CHK_GSO))
+		return -EINVAL;
+
+	mtu = bpf_mtu_lookup(netns, ifindex, flags);
+	if (unlikely(mtu < 0))
+		return mtu; /* errno */
+
+	len = __bpf_len_adjust_positive(len, len_diff);
+	if (len > mtu) {
+		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+		goto out;
+	}
+
+	if (flags & BPF_MTU_CHK_GSO &&
+	    skb_is_gso(skb) &&
+	    skb_gso_validate_network_len(skb, mtu)) {
+		ret = BPF_MTU_CHK_RET_GSO_TOOBIG;
+		goto out;
+	}
+
+out:
+	if (mtu_result)
+		*mtu_result = mtu;
+
+	return ret;
+}
+
+BPF_CALL_5(bpf_xdp_mtu_check, struct xdp_buff *, xdp,
+	   u32, ifindex, u32 *, mtu_result, s32, len_diff, u64, flags)
+{
+	unsigned int len = xdp->data_end - xdp->data;
+	struct net_device *dev = xdp->rxq->dev;
+	struct net *netns = dev_net(dev);
+	int ret = BPF_MTU_CHK_RET_SUCCESS;
+	int mtu;
+
+	/* XDP variant doesn't support multi-buffer segment check (yet) */
+	if (flags & ~BPF_MTU_CHK_RELAX)
+		return -EINVAL;
+
+	mtu = bpf_mtu_lookup(netns, ifindex, flags);
+	if (unlikely(mtu < 0))
+		return mtu; /* errno */
+
+	len = __bpf_len_adjust_positive(len, len_diff);
+	if (len > mtu) {
+		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+		goto out;
+	}
+out:
+	if (mtu_result)
+		*mtu_result = mtu;
+
+	return ret;
+}
+
+static const struct bpf_func_proto bpf_skb_mtu_check_proto = {
+	.func		= bpf_skb_mtu_check,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_ANYTHING,
+	.arg3_type      = ARG_PTR_TO_MEM,
+	.arg4_type      = ARG_ANYTHING,
+	.arg5_type      = ARG_ANYTHING,
+};
+
+static const struct bpf_func_proto bpf_xdp_mtu_check_proto = {
+	.func		= bpf_xdp_mtu_check,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_ANYTHING,
+	.arg3_type      = ARG_PTR_TO_MEM,
+	.arg4_type      = ARG_ANYTHING,
+	.arg5_type      = ARG_ANYTHING,
+};
+
 #if IS_ENABLED(CONFIG_IPV6_SEG6_BPF)
 static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len)
 {
@@ -7076,6 +7191,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_socket_uid_proto;
 	case BPF_FUNC_fib_lookup:
 		return &bpf_skb_fib_lookup_proto;
+	case BPF_FUNC_mtu_check:
+		return &bpf_skb_mtu_check_proto;
 	case BPF_FUNC_sk_fullsock:
 		return &bpf_sk_fullsock_proto;
 	case BPF_FUNC_sk_storage_get:
@@ -7145,6 +7262,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_xdp_adjust_tail_proto;
 	case BPF_FUNC_fib_lookup:
 		return &bpf_xdp_fib_lookup_proto;
+	case BPF_FUNC_mtu_check:
+		return &bpf_xdp_mtu_check_proto;
 #ifdef CONFIG_INET
 	case BPF_FUNC_sk_lookup_udp:
 		return &bpf_xdp_sk_lookup_udp_proto;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 4a46a1de6d16..1dcf5d8195f4 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3718,6 +3718,56 @@ union bpf_attr {
  *		never return NULL.
  *	Return
  *		A pointer pointing to the kernel percpu variable on this cpu.
+ *
+ * int bpf_mtu_check(void *ctx, u32 ifindex, u32 *mtu_result, 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.
+ *
+ *		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_RELAX**
+ *			This flag relax or increase the MTU with room for one
+ *			VLAN header (4 bytes) and take into account net device
+ *			hard_header_len.  This relaxation is also used by the
+ *			kernels own forwarding MTU checks.
+ *
+ *		**BPF_MTU_CHK_GSO**
+ *			This flag will only works for *ctx* **struct sk_buff**.
+ *			If packet context contains extra packet segment buffers
+ *			(often knows as frags), then those are also checked
+ *			against the MTU size.
+ *
+ *		The *mtu_result* pointer contains the MTU value of the net
+ *		device including the L2 header size (usually 14 bytes Ethernet
+ *		header). The net device configured MTU is the L3 size, but as
+ *		XDP and TX length operate at L2 this helper include L2 header
+ *		size in reported MTU.
+ *
+ *	Return
+ *		* 0 on success, and populate MTU value in *mtu_result* pointer.
+ *
+ *		* < 0 if any input argument is invalid (*mtu_result* not updated)
+ *
+ *		MTU violations return positive values, but also populate MTU
+ *		value in *mtu_result* pointer, as this can be needed for
+ *		implemeting PMTU handing:
+ *
+ *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
+ *		* **BPF_MTU_CHK_RET_GSO_TOOBIG**
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3875,6 +3925,7 @@ union bpf_attr {
 	FN(redirect_neigh),		\
 	FN(bpf_per_cpu_ptr),            \
 	FN(bpf_this_cpu_ptr),		\
+	FN(mtu_check),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4889,6 +4940,18 @@ struct bpf_fib_lookup {
 	__u8	dmac[6];     /* ETH_ALEN */
 };
 
+/* bpf_mtu_check flags*/
+enum  bpf_mtu_check_flags {
+	BPF_MTU_CHK_RELAX = (1U << 0),
+	BPF_MTU_CHK_GSO   = (1U << 1),
+};
+
+enum bpf_mtu_check_ret {
+	BPF_MTU_CHK_RET_SUCCESS,      /* check and lookup successful */
+	BPF_MTU_CHK_RET_FRAG_NEEDED,  /* fragmentation required to fwd */
+	BPF_MTU_CHK_RET_GSO_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 */