From 6e8387a0a4a2c43b8d604c27b1a10d7901479ef6 Mon Sep 17 00:00:00 2001
From: Pim Kunis <pim@kunis.nl>
Date: Tue, 24 Dec 2024 21:34:28 +0100
Subject: [PATCH] Add tcpfilter example

---
 README.md             |  3 +++
 dropworld/dropworld.c |  2 +-
 flake.nix             |  6 +++++-
 tcpfilter/default.nix | 16 ++++++++++++++++
 tcpfilter/tcpfilter.c | 33 +++++++++++++++++++++++++++++++++
 5 files changed, 58 insertions(+), 2 deletions(-)
 create mode 100644 README.md
 create mode 100644 tcpfilter/default.nix
 create mode 100644 tcpfilter/tcpfilter.c

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..2143087
--- /dev/null
+++ b/README.md
@@ -0,0 +1,3 @@
+# ebpf-sandbox
+
+[ebpf-tutorial](https://github.com/dio/ebpf-tutorial)
diff --git a/dropworld/dropworld.c b/dropworld/dropworld.c
index 1b98ec7..ce391a9 100644
--- a/dropworld/dropworld.c
+++ b/dropworld/dropworld.c
@@ -3,7 +3,7 @@
 
 SEC("xdp")
 int xdp_drop(struct xdp_md *ctx) {
-    return XDP_DROP; // Drop all packets
+    return XDP_PASS; // Drop all packets
 }
 
 char LICENSE[] SEC("license") = "GPL";
diff --git a/flake.nix b/flake.nix
index 4815e96..3ad43a4 100644
--- a/flake.nix
+++ b/flake.nix
@@ -11,7 +11,11 @@
       pkgs = inputs.nixpkgs.legacyPackages.${system};
     in
     {
-      packages.dropworld = pkgs.callPackage ./dropworld {};
+      packages = {
+        dropworld = pkgs.callPackage ./dropworld {};
+        tcpfilter = pkgs.callPackage ./tcpfilter {};
+      };
+
       devShells.default = pkgs.mkShell {
         buildInputs = with pkgs; [bpftools bpftop];
       };
diff --git a/tcpfilter/default.nix b/tcpfilter/default.nix
new file mode 100644
index 0000000..93140ea
--- /dev/null
+++ b/tcpfilter/default.nix
@@ -0,0 +1,16 @@
+{clangStdenv, libbpf, ...}: clangStdenv.mkDerivation {
+  name = "tcpfilter";
+  src = ./.;
+  hardeningDisable = [ "stackprotector" "zerocallusedregs" ];
+  dontFixup = true;
+  buildInputs = [ libbpf ];
+
+  buildPhase = ''
+    clang -O2 -target bpf -g -c tcpfilter.c -o tcpfilter.o
+  '';
+
+  installPhase = ''
+    mkdir $out
+    cp tcpfilter.o $out
+  '';
+}
diff --git a/tcpfilter/tcpfilter.c b/tcpfilter/tcpfilter.c
new file mode 100644
index 0000000..28562c7
--- /dev/null
+++ b/tcpfilter/tcpfilter.c
@@ -0,0 +1,33 @@
+#include <linux/bpf.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <linux/tcp.h>
+#include <linux/in.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+SEC("xdp")
+int isTCP( struct xdp_md *ctx ) {
+    void *data_end = (void *)(long) ctx->data_end;
+    void *data_begin = (void *)(long) ctx->data;
+    struct ethhdr* eth = data_begin;
+    
+    // Check packet's size
+    if(eth + 1 > data_end)
+      return XDP_PASS;
+    
+    // Check if Ethernet frame has IPv4 packet
+    if (eth->h_proto == bpf_htons( ETH_P_IP )) { 
+        struct iphdr *ipv4 = (struct iphdr *)( ((void*)eth) + ETH_HLEN );
+
+        if(ipv4 + 1 > data_end)
+          return XDP_PASS;
+
+        // Check if IPv4 packet contains a TCP segment
+        if (ipv4->protocol == IPPROTO_TCP) 
+          return XDP_PASS;
+    } 
+    return XDP_DROP;
+}
+
+char LICENSE[] SEC("license") = "GPL";