Hi Stanislav,

On Tue, Feb 28, 2023 at 11:37:16AM -0800, Stanislav Fomichev wrote:

On 02/27, Daniel Xu wrote:

quoted

This kfunc is used to defragment IPv4 packets. The idea is that if you
see a fragmented packet, you call this kfunc. If the kfunc returns 0,
then the skb has been updated to contain the entire reassembled packet.

quoted

If the kfunc returns an error (most likely -EINPROGRESS), then it means
the skb is part of a yet-incomplete original packet. A reasonable
response to -EINPROGRESS is to drop the packet, as the ip defrag
infrastructure is already hanging onto the frag for future reassembly.

quoted

Care has been taken to ensure the prog skb remains valid no matter what
the underlying ip_check_defrag() call does. This is in contrast to
ip_defrag(), which may consume the skb if the skb is part of a
yet-incomplete original packet.

quoted

So far this kfunc is only callable from TC clsact progs.

quoted

Signed-off-by: Daniel Xu <redacted>
---
  include/net/ip.h           | 11 +++++
  net/ipv4/Makefile          |  1 +
  net/ipv4/ip_fragment.c     |  2 +
  net/ipv4/ip_fragment_bpf.c | 98 ++++++++++++++++++++++++++++++++++++++
  4 files changed, 112 insertions(+)
  create mode 100644 net/ipv4/ip_fragment_bpf.c

quoted

diff --git a/include/net/ip.h b/include/net/ip.h
index c3fffaa92d6e..f3796b1b5cac 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h

@@ -680,6 +680,7 @@ enum ip_defrag_users {
  	IP_DEFRAG_VS_FWD,
  	IP_DEFRAG_AF_PACKET,
  	IP_DEFRAG_MACVLAN,
+	IP_DEFRAG_BPF,
  };

quoted

  /* Return true if the value of 'user' is between 'lower_bond'

@@ -693,6 +694,16 @@ static inline bool ip_defrag_user_in_between(u32

user,
  }

quoted

  int ip_defrag(struct net *net, struct sk_buff *skb, u32 user);
+
+#ifdef CONFIG_DEBUG_INFO_BTF
+int register_ip_frag_bpf(void);
+#else
+static inline int register_ip_frag_bpf(void)
+{
+	return 0;
+}
+#endif
+
  #ifdef CONFIG_INET
  struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb,
u32 user);
  #else

diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index 880277c9fd07..950efb166d37 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile

@@ -65,6 +65,7 @@ obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
  obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
  obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o
  obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
+obj-$(CONFIG_DEBUG_INFO_BTF) += ip_fragment_bpf.o

quoted

  obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
  		      xfrm4_output.o xfrm4_protocol.o

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 959d2c4260ea..e3fda5203f09 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c

@@ -759,5 +759,7 @@ void __init ipfrag_init(void)
  	if (inet_frags_init(&ip4_frags))
  		panic("IP: failed to allocate ip4_frags cache\n");
  	ip4_frags_ctl_register();
+	if (register_ip_frag_bpf())
+		panic("IP: bpf: failed to register ip_frag_bpf\n");
  	register_pernet_subsys(&ip4_frags_ops);
  }

diff --git a/net/ipv4/ip_fragment_bpf.c b/net/ipv4/ip_fragment_bpf.c
new file mode 100644
index 000000000000..a9e5908ed216
--- /dev/null
+++ b/net/ipv4/ip_fragment_bpf.c

@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Unstable ipv4 fragmentation helpers for TC-BPF hook
+ *
+ * These are called from SCHED_CLS BPF programs. Note that it is

allowed to
+ * break compatibility for these functions since the interface they are
exposed
+ * through to BPF programs is explicitly unstable.
+ */
+
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/ip.h>
+#include <linux/filter.h>
+#include <linux/netdevice.h>
+#include <net/ip.h>
+#include <net/sock.h>
+
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in ip_fragment BTF");
+
+/* bpf_ip_check_defrag - Defragment an ipv4 packet
+ *
+ * This helper takes an skb as input. If this skb successfully
reassembles
+ * the original packet, the skb is updated to contain the original,
reassembled
+ * packet.
+ *
+ * Otherwise (on error or incomplete reassembly), the input skb remains
+ * unmodified.
+ *
+ * Parameters:
+ * @ctx		- Pointer to program context (skb)
+ * @netns	- Child network namespace id. If value is a negative signed
+ *		  32-bit integer, the netns of the device in the skb is used.
+ *
+ * Return:
+ * 0 on successfully reassembly or non-fragmented packet. Negative
value on
+ * error or incomplete reassembly.
+ */
+int bpf_ip_check_defrag(struct __sk_buff *ctx, u64 netns)

Needs a __bpf_kfunc tag as well?

Ack.

quoted

+{
+	struct sk_buff *skb = (struct sk_buff *)ctx;
+	struct sk_buff *skb_cpy, *skb_out;
+	struct net *caller_net;
+	struct net *net;
+	int mac_len;
+	void *mac;
+

[..]

quoted

+	if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
+		return -EINVAL;

Can you explain what it does? Is it checking for -1 explicitly? Not sure
it works :-/

Maybe we can spell out the cases explicitly?
if (unlikely(
	     ((s32)netns < 0 && netns != S32_MAX) || /* -1 */
	     netns > U32_MAX /* higher 4 bytes */
	    )
	return -EINVAL;

I copied this from net/core/filter.c:__bpf_skc_lookup:

	if (unlikely(flags || !((s32)netns_id < 0 || netns_id <= S32_MAX)))
		goto out;

The semantics are a bit odd, but I thought it'd be good to maintain
consistency. I believe the code correctly checks what the docs describe:

        @netns	- Child network namespace id. If value is a negative signed
                  32-bit integer, the netns of the device in the skb is used.

I can pull out the logic into a helper for v3.

[...]


Thanks,
Daniel