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<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">eBPF 实践教程：基于 libbpf 和 CO-RE</li><li class="chapter-item expanded "><a href="../0-introduce/index.html"><strong aria-hidden="true">1.</strong> eBPF 入门开发实践教程一：介绍 eBPF 的基本概念、常见的开发工具</a></li><li class="chapter-item expanded "><a href="../1-helloworld/index.html"><strong aria-hidden="true">2.</strong> eBPF 入门开发实践教程二：Hello World，基本框架和开发流程</a></li><li class="chapter-item expanded "><a href="../2-kprobe-unlink/index.html"><strong aria-hidden="true">3.</strong> eBPF 入门开发实践教程二：在 eBPF 中使用 kprobe 监测捕获 unlink 系统调用</a></li><li class="chapter-item expanded "><a href="../3-fentry-unlink/index.html"><strong aria-hidden="true">4.</strong> eBPF 入门开发实践教程三：在 eBPF 中使用 fentry 监测捕获 unlink 系统调用</a></li><li class="chapter-item expanded "><a href="../4-opensnoop/index.html"><strong aria-hidden="true">5.</strong> eBPF 入门开发实践教程四：在 eBPF 中捕获进程打开文件的系统调用集合，使用全局变量过滤进程 pid</a></li><li class="chapter-item expanded "><a href="../5-uprobe-bashreadline/index.html"><strong aria-hidden="true">6.</strong> eBPF 入门开发实践教程五：在 eBPF 中使用 uprobe 捕获 bash 的 readline 函数调用</a></li><li class="chapter-item expanded "><a href="../6-sigsnoop/index.html"><strong aria-hidden="true">7.</strong> eBPF 入门开发实践教程六：捕获进程发送信号的系统调用集合，使用 hash map 保存状态</a></li><li class="chapter-item expanded "><a href="../7-execsnoop/index.html"><strong aria-hidden="true">8.</strong> eBPF 入门实践教程七：捕获进程执行/退出时间，通过 perf event array 向用户态打印输出</a></li><li class="chapter-item expanded "><a href="../8-exitsnoop/index.html"><strong aria-hidden="true">9.</strong> eBPF 入门开发实践教程八：在 eBPF 中使用 exitsnoop 监控进程退出事件，使用 ring buffer 向用户态打印输出</a></li><li class="chapter-item expanded "><a href="../9-runqlat/index.html"><strong aria-hidden="true">10.</strong> eBPF 入门开发实践教程九：一个 Linux 内核 BPF 程序，通过柱状图来总结调度程序运行队列延迟，显示任务等待运行在 CPU 上的时间长度</a></li><li class="chapter-item expanded "><a href="../10-hardirqs/index.html"><strong aria-hidden="true">11.</strong> eBPF 入门开发实践教程十：在 eBPF 中使用 hardirqs 或 softirqs 捕获中断事件</a></li><li class="chapter-item expanded "><a href="../11-bootstrap/index.html"><strong aria-hidden="true">12.</strong> eBPF 入门开发实践教程十一：在 eBPF 中使用 bootstrap 开发用户态程序并跟踪 exec() 和 exit() 系统调用</a></li><li class="chapter-item expanded "><a href="../13-tcpconnlat/index.html"><strong aria-hidden="true">13.</strong> eBPF入门实践教程：使用 libbpf-bootstrap 开发程序统计 TCP 连接延时</a></li><li class="chapter-item expanded "><a href="../13-tcpconnlat/tcpconnlat.html"><strong aria-hidden="true">14.</strong> eBPF 入门实践教程：编写 eBPF 程序 tcpconnlat 测量 tcp 连接延时</a></li><li class="chapter-item expanded "><a href="../14-tcpstates/index.html"><strong aria-hidden="true">15.</strong> eBPF入门实践教程：使用 libbpf-bootstrap 开发程序统计 TCP 连接延时</a></li><li class="chapter-item expanded "><a href="../15-tcprtt/index.html"><strong aria-hidden="true">16.</strong> eBPF 入门实践教程：编写 eBPF 程序 Tcprtt 测量 TCP 连接的往返时间</a></li><li class="chapter-item expanded "><a href="../16-memleak/index.html"><strong aria-hidden="true">17.</strong> eBPF 入门实践教程：编写 eBPF 程序 Memleak 监控内存泄漏</a></li><li class="chapter-item expanded "><a href="../17-biopattern/index.html"><strong aria-hidden="true">18.</strong> eBPF 入门实践教程：编写 eBPF 程序 Biopattern: 统计随机/顺序磁盘 I/O</a></li><li class="chapter-item expanded "><a href="../18-further-reading/index.html"><strong aria-hidden="true">19.</strong> 更多的参考资料</a></li><li class="chapter-item expanded "><a href="../19-lsm-connect/index.html" class="active"><strong aria-hidden="true">20.</strong> eBPF 入门实践教程：使用 LSM 进行安全检测防御</a></li><li class="chapter-item expanded "><a href="../20-tc/index.html"><strong aria-hidden="true">21.</strong> eBPF 入门实践教程：使用 eBPF 进行 tc 流量控制</a></li><li class="chapter-item expanded affix "><li class="part-title">bcc 开发者教程</li><li class="chapter-item expanded "><a href="../bcc-documents/kernel-versions.html"><strong aria-hidden="true">22.</strong> BPF Features by Linux Kernel Version</a></li><li class="chapter-item expanded "><a href="../bcc-documents/kernel_config.html"><strong aria-hidden="true">23.</strong> Kernel Configuration for BPF Features</a></li><li class="chapter-item expanded "><a href="../bcc-documents/reference_guide.html"><strong aria-hidden="true">24.</strong> bcc Reference Guide</a></li><li class="chapter-item expanded "><a href="../bcc-documents/special_filtering.html"><strong aria-hidden="true">25.</strong> Special Filtering</a></li><li class="chapter-item expanded "><a href="../bcc-documents/tutorial.html"><strong aria-hidden="true">26.</strong> bcc Tutorial</a></li><li class="chapter-item expanded "><a href="../bcc-documents/tutorial_bcc_python_developer.html"><strong aria-hidden="true">27.</strong> bcc Python Developer Tutorial</a></li></ol>
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<h1 id="ebpf-入门实践教程使用-lsm-进行安全检测防御"><a class="header" href="#ebpf-入门实践教程使用-lsm-进行安全检测防御">eBPF 入门实践教程：使用 LSM 进行安全检测防御</a></h1>
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<h2 id="背景"><a class="header" href="#背景">背景</a></h2>
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<p>LSM 从 Linux 2.6 开始成为官方内核的一个安全框架，基于此的安全实现包括 SELinux 和 AppArmor 等。在 Linux 5.7 引入 BPF LSM 后，系统开发人员已经能够自由地实现函数粒度的安全检查能力，本文就提供了这样一个案例：限制通过 socket connect 函数对特定 IPv4 地址进行访问的 BPF LSM 程序。（可见其控制精度是很高的）</p>
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<h2 id="lsm-概述"><a class="header" href="#lsm-概述">LSM 概述</a></h2>
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<p>LSM（Linux Security Modules）是 Linux 内核中用于支持各种计算机安全模型的框架。LSM 在 Linux 内核安全相关的关键路径上预置了一批 hook 点，从而实现了内核和安全模块的解耦，使不同的安全模块可以自由地在内核中加载/卸载，无需修改原有的内核代码就可以加入安全检查功能。</p>
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<p>在过去，使用 LSM 主要通过配置已有的安全模块（如 SELinux 和 AppArmor）或编写自己的内核模块；而在 Linux 5.7 引入 BPF LSM 机制后，一切都变得不同了：现在，开发人员可以通过 eBPF 编写自定义的安全策略，并将其动态加载到内核中的 LSM 挂载点，而无需配置或编写内核模块。</p>
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<p>现在 LSM 支持的 hook 点包括但不限于：</p>
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<ul>
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<li>对文件的打开、创建、删除和移动等；</li>
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<li>文件系统的挂载；</li>
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<li>对 task 和 process 的操作；</li>
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<li>对 socket 的操作（创建、绑定 socket，发送和接收消息等）；</li>
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</ul>
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<p>更多 hook 点可以参考 <a href="https://github.com/torvalds/linux/blob/master/include/linux/lsm_hooks.h">lsm_hooks.h</a>。</p>
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<h2 id="确认-bpf-lsm-是否可用"><a class="header" href="#确认-bpf-lsm-是否可用">确认 BPF LSM 是否可用</a></h2>
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<p>首先，请确认内核版本高于 5.7。接下来，可以通过</p>
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<pre><code class="language-console">$ cat /boot/config-$(uname -r) | grep BPF_LSM
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CONFIG_BPF_LSM=y
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</code></pre>
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<p>判断是否内核是否支持 BPF LSM。上述条件都满足的情况下，可以通过</p>
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<pre><code class="language-console">$ cat /sys/kernel/security/lsm
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ndlock,lockdown,yama,integrity,apparmor
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</code></pre>
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<p>查看输出是否包含 bpf 选项，如果输出不包含（像上面的例子），可以通过修改 <code>/etc/default/grub</code>：</p>
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<pre><code class="language-conf">GRUB_CMDLINE_LINUX="lsm=ndlock,lockdown,yama,integrity,apparmor,bpf"
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</code></pre>
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<p>并通过 <code>update-grub2</code> 命令更新 grub 配置（不同系统的对应命令可能不同），然后重启系统。</p>
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<h2 id="编写-ebpf-程序"><a class="header" href="#编写-ebpf-程序">编写 eBPF 程序</a></h2>
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<pre><code class="language-C">// lsm-connect.bpf.c
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#include "vmlinux.h"
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#include <bpf/bpf_core_read.h>
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#include <bpf/bpf_helpers.h>
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#include <bpf/bpf_tracing.h>
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char LICENSE[] SEC("license") = "GPL";
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#define EPERM 1
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#define AF_INET 2
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const __u32 blockme = 16843009; // 1.1.1.1 -> int
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SEC("lsm/socket_connect")
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int BPF_PROG(restrict_connect, struct socket *sock, struct sockaddr *address, int addrlen, int ret)
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{
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// Satisfying "cannot override a denial" rule
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if (ret != 0)
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{
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return ret;
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}
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// Only IPv4 in this example
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if (address->sa_family != AF_INET)
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{
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return 0;
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}
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// Cast the address to an IPv4 socket address
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struct sockaddr_in *addr = (struct sockaddr_in *)address;
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// Where do you want to go?
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__u32 dest = addr->sin_addr.s_addr;
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bpf_printk("lsm: found connect to %d", dest);
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if (dest == blockme)
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{
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bpf_printk("lsm: blocking %d", dest);
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return -EPERM;
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}
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return 0;
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}
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</code></pre>
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<p>这是一段 C 实现的 eBPF 内核侧代码，它会阻碍所有试图通过 socket 对 1.1.1.1 的连接操作，其中：</p>
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<ul>
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<li><code>SEC("lsm/socket_connect")</code> 宏指出该程序期望的挂载点；</li>
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<li>程序通过 <code>BPF_PROG</code> 宏定义（详情可查看 <a href="https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/lib/bpf/bpf_tracing.h">tools/lib/bpf/bpf_tracing.h</a>）；</li>
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<li><code>restrict_connect</code> 是 <code>BPF_PROG</code> 宏要求的程序名；</li>
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<li><code>ret</code> 是该挂载点上（潜在的）当前函数之前的 LSM 检查程序的返回值；</li>
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</ul>
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<p>整个程序的思路不难理解：</p>
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<ul>
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<li>首先，若其他安全检查函数返回值不为 0（不通过），则无需检查，直接返回不通过；</li>
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<li>接下来，判断是否为 IPV4 的连接请求，并比较试图连接的地址是否为 1.1.1.1；</li>
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<li>若请求地址为 1.1.1.1 则拒绝连接，否则允许连接；</li>
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</ul>
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<p>在程序运行期间，所有通过 socket 的连接操作都会被输出到 <code>/sys/kernel/debug/tracing/trace_pipe</code>。</p>
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<h2 id="编译运行"><a class="header" href="#编译运行">编译运行</a></h2>
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<p>通过容器编译：</p>
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<pre><code class="language-console">docker run -it -v `pwd`/:/src/ yunwei37/ebpm:latest
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</code></pre>
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<p>或是通过 <code>ecc</code> 编译：</p>
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<pre><code class="language-console">$ ecc lsm-connect.bpf.c
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Compiling bpf object...
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Packing ebpf object and config into package.json...
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</code></pre>
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<p>并通过 <code>ecli</code> 运行：</p>
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<pre><code class="language-shell">sudo ecli run package.json
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</code></pre>
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<p>接下来，可以打开另一个 terminal，并尝试访问 1.1.1.1：</p>
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<pre><code class="language-console">$ ping 1.1.1.1
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ping: connect: Operation not permitted
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$ curl 1.1.1.1
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curl: (7) Couldn't connect to server
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$ wget 1.1.1.1
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--2023-04-23 08:41:18-- (try: 2) http://1.1.1.1/
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Connecting to 1.1.1.1:80... failed: Operation not permitted.
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Retrying.
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</code></pre>
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<p>同时，我们可以查看 <code>bpf_printk</code> 的输出：</p>
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<pre><code class="language-console">$ sudo cat /sys/kernel/debug/tracing/trace_pipe
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ping-7054 [000] d...1 6313.430872: bpf_trace_printk: lsm: found connect to 16843009
|
||
ping-7054 [000] d...1 6313.430874: bpf_trace_printk: lsm: blocking 16843009
|
||
curl-7058 [000] d...1 6316.346582: bpf_trace_printk: lsm: found connect to 16843009
|
||
curl-7058 [000] d...1 6316.346584: bpf_trace_printk: lsm: blocking 16843009
|
||
wget-7061 [000] d...1 6318.800698: bpf_trace_printk: lsm: found connect to 16843009
|
||
wget-7061 [000] d...1 6318.800700: bpf_trace_printk: lsm: blocking 16843009
|
||
</code></pre>
|
||
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
||
<p>本文介绍了如何使用 BPF LSM 来限制通过 socket 对特定 IPv4 地址的访问。我们可以通过修改 GRUB 配置文件来开启 LSM 的 BPF 挂载点。在 eBPF 程序中，我们通过 <code>BPF_PROG</code> 宏定义函数，并通过 <code>SEC</code> 宏指定挂载点；在函数实现上，遵循 LSM 安全检查模块中 "cannot override a denial" 的原则，并根据 socket 连接请求的目的地址对该请求进行限制。</p>
|
||
<p>更多的例子和详细的开发指南，请参考 eunomia-bpf 的官方文档：<a href="https://github.com/eunomia-bpf/eunomia-bpf">https://github.com/eunomia-bpf/eunomia-bpf</a></p>
|
||
<p>完整的教程和源代码已经全部开源，可以在 <a href="https://github.com/eunomia-bpf/bpf-developer-tutorial">https://github.com/eunomia-bpf/bpf-developer-tutorial</a> 中查看。</p>
|
||
<h2 id="参考"><a class="header" href="#参考">参考</a></h2>
|
||
<ul>
|
||
<li><a href="https://github.com/leodido/demo-cloud-native-ebpf-day">https://github.com/leodido/demo-cloud-native-ebpf-day</a></li>
|
||
<li><a href="https://aya-rs.dev/book/programs/lsm/#writing-lsm-bpf-program">https://aya-rs.dev/book/programs/lsm/#writing-lsm-bpf-program</a></li>
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