譯者注

筆者在MacBook M2上搭建Linux虛擬機上開發eBPF程序時，遇到一些LSM eBPF類型程序無法運行的問題，哪怕是5.15內核的ubuntu server，依舊無法正常運行。顯然，aarch64跟x86_64的內核功能有差異。在筆者嘗試定位這些差異時，看到這篇文章，可以讓大家更直觀地了解LSM eBPF在兩種CPU 內核上的差異。

原文本博客文章是我們在Linux中對于`aarch64`上`BPF LSM`支持的內部研究的摘要。如果你對內核代碼庫不熟悉，要開始查看內核源碼是非常困難的，因此我們決定發布這篇文章，展示我們的方法，因為這對于想要探索內核內部的任何人都可能有所幫助。

簡介

在x86_64上，我們已經在使用BPF LSM，而在aarch64上，我們依賴于Kprobes，因此我們想知道內核中缺少了哪些功能，才能讓這些功能在aarch64上可用。

我們曾多次深入研究內核源代碼，但通常我們搜索的是已經存在的東西，以了解其工作原理。但在這種情況下，我們在尋找的是不存在的東西，我們追尋的是那些因為未實現而返回錯誤的內容。

回想起Steven Rostedt關于如何開始學習Linux內核的講話，我們從ftrace（以及構建在跟蹤基礎設施上的工具）開始，以了解當我們將一個不受支持的BPF程序加載到內核時會發生什么。

問題

這是當我們嘗試將一個BPF LSM程序加載到aarch64 5.15 Linux內核時，使用我們的軟件pulsar[2]時的輸出：

root@pine64-1:/home/exein#./pulsar-enterprise-execpulsard
[2023-02-16T1445ZINFOpulsar::daemon]Startingmoduleprocess-monitor
[2023-02-16T1445ZINFOpulsar::daemon]Startingmodulefile-system-monitor
[2023-02-16T1446ZINFOpulsar::daemon]Startingmodulenetwork-monitor
[2023-02-16T1446ZINFOpulsar::daemon]Startingmodulelogger
[2023-02-16T1446ZINFOpulsar::daemon]Startingmodulerules-engine
[2023-02-16T1446ZINFOpulsar::daemon]Startingmoduledesktop-notifier
[2023-02-16T1446ZERRORpulsar::module_manager]Moduleerrorinfile-system-monitor:failedprogramattachlsmpath_mknod

Causedby:
0:`bpf_raw_tracepoint_open`failed
1:Noerrorinformation(oserror524)
[2023-02-16T1446ZINFOpulsar::daemon]Startingmoduleanomaly-detection
[2023-02-16T1446ZINFOpulsar::daemon]Startingmodulemalware-detection
[2023-02-16T1446ZERRORpulsar::module_manager]Moduleerrorinmalware-detection:/var/lib/pulsar/malware_detection/models/parameters.jsonnotfound
[2023-02-16T1446ZINFOpulsar::daemon]Startingmoduleplatform-connector
[2023-02-16T1446ZINFOplatform_connector::client]Connectedtohttps://platform-dev-instance.exein.io:8001/
[2023-02-16T1446ZINFOpulsar::daemon]Startingmodulethreat-response
[2023-02-16T1446ZERRORpulsar::module_manager]Moduleerrorinnetwork-monitor:failedprogramattachlsmsocket_bind

Causedby:
0:`bpf_raw_tracepoint_open`failed
1:Noerrorinformation(oserror524)

我們在嘗試加載與path_mknodLSM掛鉤相關的BPF程序時，pulsar出現了錯誤524或ENOTSUPP。讓我們嘗試深入研究這個問題。

注意： 在進行這項研究時，我們當時無法找到預先編譯為啟用BPF和BTF的aarch64，因此我們不得不編譯一個自定義內核。我們還啟用了跟蹤選項和function_graph插件，以使用下面的工具。
所有的實驗都是在一臺裝有定制Armbian[3]鏡像的Pine A64上進行的。
這些鏡像具有帶有標準Ubuntu 22.04 LTS Jammy用戶空間的自定義內核。

工具

為了調查這個問題，我們使用了以下工具：

bpftrace[4]：基于BPF的工具，使用自定義類C語言動態附加探針。

trace-cmd[5]：圍繞tracefs文件系統的包裝器，與ftrace基礎設施交互。

要使用這些工具，您需要在Linux內核中啟用一些選項，請查閱官方文檔獲取完整的要求。

注意： 也可以使用其他工具來完成相同的工作，例如perf-tools[6]中的funcgraph和kprobe。

Linux 5.15

現在我們開始使用這些工具來查看在內核5.15中嘗試加載我們的BPF程序時會發生什么。

從這一點開始到本文末尾，我們將使用probe二進制文件代替pulsar，因為它更簡單。為了簡要概括其工作原理，以下是命令行幫助：

exein@pine64-1:~$./probe
TestrunnerforeBPFprograms

Usage:probe[OPTIONS]

Commands:
file-system-monitorWatchfilecreations
process-monitorWatchprocessevents(fork/exec/exit)
network-monitorWatchnetworkevents
helpPrintthismessageorthehelpofthegivensubcommand(s)

Options:
-v,--verbose
-h,--helpPrinthelp
-V,--versionPrintversion

在這些示例中，我們將嘗試加載file-system-monitor探針。

通過運行以下命令，我們可以看到__sys_bpf函數的函數圖調用，這是BPF系統調用的入口點：

trace-cmdrecord-pfunction_graph-g__sys_bpf./probefile-system-monitor
trace-cmdreport

輸出是一個非常龐大的函數圖，太大了，無法在這里粘貼。由于我們遇到了錯誤，我們對程序停止前的最后幾個函數感興趣。以下是trace-cmd report輸出的最后幾行：

...
tokio-runtime-w-1666[003]1318.058019:funcgraph_entry:|bpf_trampoline_link_prog(){
tokio-runtime-w-1666[003]1318.058020:funcgraph_entry:2.292us|bpf_attach_type_to_tramp();
tokio-runtime-w-1666[003]1318.058024:funcgraph_entry:1.250us|mutex_lock();
tokio-runtime-w-1666[003]1318.058028:funcgraph_entry:|bpf_trampoline_update(){
tokio-runtime-w-1666[003]1318.058030:funcgraph_entry:|kmem_cache_alloc_trace(){
tokio-runtime-w-1666[003]1318.058031:funcgraph_entry:1.167us|should_failslab();
tokio-runtime-w-1666[003]1318.058036:funcgraph_exit:6.792us|}
tokio-runtime-w-1666[003]1318.058039:funcgraph_entry:|kmem_cache_alloc_trace(){
tokio-runtime-w-1666[003]1318.058042:funcgraph_entry:2.750us|should_failslab();
tokio-runtime-w-1666[003]1318.058046:funcgraph_exit:6.417us|}
tokio-runtime-w-1666[003]1318.058048:funcgraph_entry:2.708us|bpf_jit_charge_modmem();
tokio-runtime-w-1666[003]1318.058053:funcgraph_entry:|bpf_jit_alloc_exec_page(){
tokio-runtime-w-1666[003]1318.058055:funcgraph_entry:|bpf_jit_alloc_exec(){
tokio-runtime-w-1666[003]1318.058057:funcgraph_entry:|vmalloc(){
tokio-runtime-w-1666[003]1318.058059:funcgraph_entry:|__vmalloc_node(){
tokio-runtime-w-1666[003]1318.058061:funcgraph_entry:|__vmalloc_node_range(){
tokio-runtime-w-1666[003]1318.058064:funcgraph_entry:|__get_vm_area_node.constprop.64(){
tokio-runtime-w-1666[003]1318.058067:funcgraph_entry:|kmem_cache_alloc_node_trace(){
tokio-runtime-w-1666[003]1318.058069:funcgraph_entry:1.459us|should_failslab();
tokio-runtime-w-1666[003]1318.058073:funcgraph_exit:6.292us|}
tokio-runtime-w-1666[003]1318.058075:funcgraph_entry:|alloc_vmap_area(){
tokio-runtime-w-1666[003]1318.058077:funcgraph_entry:|kmem_cache_alloc_node(){
tokio-runtime-w-1666[003]1318.058079:funcgraph_entry:1.167us|should_failslab();
tokio-runtime-w-1666[003]1318.058085:funcgraph_exit:7.625us|}
tokio-runtime-w-1666[003]1318.058088:funcgraph_entry:|kmem_cache_alloc_node(){
tokio-runtime-w-1666[003]1318.058089:funcgraph_entry:1.208us|should_failslab();
tokio-runtime-w-1666[003]1318.058092:funcgraph_exit:4.584us|}
tokio-runtime-w-1666[003]1318.058104:funcgraph_entry:|kmem_cache_free(){
tokio-runtime-w-1666[003]1318.058107:funcgraph_entry:2.084us|__slab_free();
tokio-runtime-w-1666[003]1318.058110:funcgraph_exit:5.667us|}
tokio-runtime-w-1666[003]1318.058112:funcgraph_entry:6.375us|insert_vmap_area.constprop.74();
tokio-runtime-w-1666[003]1318.058119:funcgraph_exit:+44.667us|}
tokio-runtime-w-1666[003]1318.058122:funcgraph_exit:+58.250us|}
tokio-runtime-w-1666[003]1318.058124:funcgraph_entry:|__kmalloc_node(){
tokio-runtime-w-1666[003]1318.058125:funcgraph_entry:1.625us|kmalloc_slab();
tokio-runtime-w-1666[003]1318.058128:funcgraph_entry:1.167us|should_failslab();
tokio-runtime-w-1666[003]1318.058131:funcgraph_exit:7.208us|}
tokio-runtime-w-1666[003]1318.058133:funcgraph_entry:|alloc_pages(){
tokio-runtime-w-1666[003]1318.058135:funcgraph_entry:1.583us|get_task_policy.part.48();
tokio-runtime-w-1666[003]1318.058138:funcgraph_entry:1.500us|policy_node();
tokio-runtime-w-1666[003]1318.058141:funcgraph_entry:1.209us|policy_nodemask();
tokio-runtime-w-1666[003]1318.058143:funcgraph_entry:|__alloc_pages(){
tokio-runtime-w-1666[003]1318.058145:funcgraph_entry:1.458us|should_fail_alloc_page();
tokio-runtime-w-1666[003]1318.058147:funcgraph_entry:|get_page_from_freelist(){
tokio-runtime-w-1666[003]1318.058150:funcgraph_entry:1.583us|prep_new_page();
tokio-runtime-w-1666[003]1318.058153:funcgraph_exit:5.459us|}
tokio-runtime-w-1666[003]1318.058154:funcgraph_exit:+10.542us|}
tokio-runtime-w-1666[003]1318.058155:funcgraph_exit:+22.083us|}
tokio-runtime-w-1666[003]1318.058157:funcgraph_entry:|__cond_resched(){
tokio-runtime-w-1666[003]1318.058158:funcgraph_entry:1.833us|rcu_all_qs();
tokio-runtime-w-1666[003]1318.058161:funcgraph_exit:4.167us|}
tokio-runtime-w-1666[003]1318.058166:funcgraph_entry:5.542us|vmap_pages_range_noflush();
tokio-runtime-w-1666[003]1318.058173:funcgraph_exit:!112.375us|}
tokio-runtime-w-1666[003]1318.058175:funcgraph_exit:!116.000us|}
tokio-runtime-w-1666[003]1318.058176:funcgraph_exit:!119.292us|}
tokio-runtime-w-1666[003]1318.058177:funcgraph_exit:!122.542us|}
tokio-runtime-w-1666[003]1318.058179:funcgraph_entry:|find_vm_area(){
tokio-runtime-w-1666[003]1318.058180:funcgraph_entry:1.375us|find_vmap_area();
tokio-runtime-w-1666[003]1318.058183:funcgraph_exit:4.333us|}
tokio-runtime-w-1666[003]1318.058185:funcgraph_entry:|set_memory_x(){
tokio-runtime-w-1666[003]1318.058186:funcgraph_entry:|change_memory_common(){
tokio-runtime-w-1666[003]1318.058188:funcgraph_entry:|find_vm_area(){
tokio-runtime-w-1666[003]1318.058189:funcgraph_entry:1.333us|find_vmap_area();
tokio-runtime-w-1666[003]1318.058192:funcgraph_exit:3.875us|}
tokio-runtime-w-1666[003]1318.058193:funcgraph_entry:|vm_unmap_aliases(){
tokio-runtime-w-1666[003]1318.058194:funcgraph_entry:|_vm_unmap_aliases.part.58(){
tokio-runtime-w-1666[003]1318.058196:funcgraph_entry:1.542us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058199:funcgraph_entry:1.208us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058202:funcgraph_entry:1.166us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058205:funcgraph_entry:1.208us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058207:funcgraph_entry:1.208us|mutex_lock();
tokio-runtime-w-1666[003]1318.058210:funcgraph_entry:|purge_fragmented_blocks_allcpus(){
tokio-runtime-w-1666[003]1318.058212:funcgraph_entry:1.500us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058214:funcgraph_entry:1.500us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058217:funcgraph_entry:1.500us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058220:funcgraph_entry:1.167us|rcu_read_unlock_strict();
tokio-runtime-w-1666[003]1318.058222:funcgraph_exit:+11.917us|}
tokio-runtime-w-1666[003]1318.058224:funcgraph_entry:|__purge_vmap_area_lazy(){
tokio-runtime-w-1666[003]1318.058232:funcgraph_entry:|kmem_cache_free(){
tokio-runtime-w-1666[003]1318.058234:funcgraph_entry:1.250us|__slab_free();
tokio-runtime-w-1666[003]1318.058237:funcgraph_exit:4.791us|}
tokio-runtime-w-1666[003]1318.058241:funcgraph_entry:1.209us|__cond_resched_lock();
tokio-runtime-w-1666[003]1318.058244:funcgraph_exit:+19.625us|}
tokio-runtime-w-1666[003]1318.058245:funcgraph_entry:1.167us|mutex_unlock();
tokio-runtime-w-1666[003]1318.058247:funcgraph_exit:+53.042us|}
tokio-runtime-w-1666[003]1318.058248:funcgraph_exit:+55.625us|}
tokio-runtime-w-1666[003]1318.058250:funcgraph_entry:|__change_memory_common(){
tokio-runtime-w-1666[003]1318.058251:funcgraph_entry:|apply_to_page_range(){
tokio-runtime-w-1666[003]1318.058253:funcgraph_entry:|__apply_to_page_range(){
tokio-runtime-w-1666[003]1318.058255:funcgraph_entry:1.250us|pud_huge();
tokio-runtime-w-1666[003]1318.058258:funcgraph_entry:1.166us|pmd_huge();
tokio-runtime-w-1666[003]1318.058260:funcgraph_entry:1.208us|change_page_range();
tokio-runtime-w-1666[003]1318.058263:funcgraph_exit:9.834us|}
tokio-runtime-w-1666[003]1318.058264:funcgraph_exit:+12.709us|}
tokio-runtime-w-1666[003]1318.058266:funcgraph_exit:+15.459us|}
tokio-runtime-w-1666[003]1318.058268:funcgraph_exit:+80.791us|}
tokio-runtime-w-1666[003]1318.058270:funcgraph_exit:+84.834us|}
tokio-runtime-w-1666[003]1318.058272:funcgraph_exit:!218.500us|}
tokio-runtime-w-1666[003]1318.058274:funcgraph_entry:|__alloc_percpu_gfp(){
tokio-runtime-w-1666[003]1318.058276:funcgraph_entry:|pcpu_alloc(){
tokio-runtime-w-1666[003]1318.058281:funcgraph_entry:2.250us|mutex_lock_killable();
tokio-runtime-w-1666[003]1318.058290:funcgraph_entry:|pcpu_find_block_fit(){
tokio-runtime-w-1666[003]1318.058293:funcgraph_entry:2.833us|pcpu_next_fit_region.constprop.38();
tokio-runtime-w-1666[003]1318.058299:funcgraph_exit:9.084us|}
tokio-runtime-w-1666[003]1318.058301:funcgraph_entry:|pcpu_alloc_area(){
tokio-runtime-w-1666[003]1318.058315:funcgraph_entry:4.000us|pcpu_block_update_hint_alloc();
tokio-runtime-w-1666[003]1318.058320:funcgraph_entry:2.208us|pcpu_chunk_relocate();
tokio-runtime-w-1666[003]1318.058324:funcgraph_exit:+22.625us|}
tokio-runtime-w-1666[003]1318.058327:funcgraph_entry:1.208us|mutex_unlock();
tokio-runtime-w-1666[003]1318.058332:funcgraph_entry:1.584us|pcpu_memcg_post_alloc_hook();
tokio-runtime-w-1666[003]1318.058335:funcgraph_exit:+58.833us|}
tokio-runtime-w-1666[003]1318.058336:funcgraph_exit:+61.834us|}
tokio-runtime-w-1666[003]1318.058338:funcgraph_entry:|kmem_cache_alloc_trace(){
tokio-runtime-w-1666[003]1318.058339:funcgraph_entry:1.167us|should_failslab();
tokio-runtime-w-1666[003]1318.058342:funcgraph_exit:4.458us|}
tokio-runtime-w-1666[003]1318.058359:funcgraph_entry:|bpf_image_ksym_add(){
tokio-runtime-w-1666[003]1318.058360:funcgraph_entry:|bpf_ksym_add(){
tokio-runtime-w-1666[003]1318.058363:funcgraph_entry:1.583us|__local_bh_enable_ip();
tokio-runtime-w-1666[003]1318.058366:funcgraph_exit:5.750us|}
tokio-runtime-w-1666[003]1318.058369:funcgraph_exit:9.834us|}
tokio-runtime-w-1666[003]1318.058371:funcgraph_entry:1.250us|arch_prepare_bpf_trampoline();
tokio-runtime-w-1666[003]1318.058373:funcgraph_entry:2.292us|kfree();
tokio-runtime-w-1666[003]1318.058377:funcgraph_exit:!348.625us|}
tokio-runtime-w-1666[003]1318.058379:funcgraph_entry:1.250us|mutex_unlock();
tokio-runtime-w-1666[003]1318.058382:funcgraph_exit:!363.167us|}
tokio-runtime-w-1666[003]1318.058384:funcgraph_entry:|bpf_link_cleanup(){
tokio-runtime-w-1666[003]1318.058386:funcgraph_entry:|bpf_link_free_id.part.30(){
tokio-runtime-w-1666[003]1318.058392:funcgraph_entry:|call_rcu(){
tokio-runtime-w-1666[003]1318.058396:funcgraph_entry:1.834us|rcu_segcblist_enqueue();
tokio-runtime-w-1666[003]1318.058401:funcgraph_exit:9.333us|}
tokio-runtime-w-1666[003]1318.058403:funcgraph_entry:1.542us|__local_bh_enable_ip();
tokio-runtime-w-1666[003]1318.058406:funcgraph_exit:+19.542us|}
tokio-runtime-w-1666[003]1318.058408:funcgraph_entry:|fput(){
tokio-runtime-w-1666[003]1318.058409:funcgraph_entry:|fput_many(){
tokio-runtime-w-1666[003]1318.058411:funcgraph_entry:|task_work_add(){
tokio-runtime-w-1666[003]1318.058414:funcgraph_entry:1.625us|kick_process();
tokio-runtime-w-1666[003]1318.058418:funcgraph_exit:6.750us|}
tokio-runtime-w-1666[003]1318.058419:funcgraph_exit:+10.333us|}
tokio-runtime-w-1666[003]1318.058420:funcgraph_exit:+12.708us|}
tokio-runtime-w-1666[003]1318.058422:funcgraph_entry:2.250us|put_unused_fd();
tokio-runtime-w-1666[003]1318.058426:funcgraph_exit:+41.416us|}
tokio-runtime-w-1666[003]1318.058428:funcgraph_entry:1.292us|mutex_unlock();
tokio-runtime-w-1666[003]1318.058430:funcgraph_entry:1.250us|kfree();
tokio-runtime-w-1666[003]1318.058433:funcgraph_exit:!567.458us|}
tokio-runtime-w-1666[003]1318.058435:funcgraph_entry:2.125us|__bpf_prog_put.isra.47();
tokio-runtime-w-1666[003]1318.058438:funcgraph_exit:!602.291us|}
tokio-runtime-w-1666[003]1318.058439:funcgraph_exit:!631.791us|}
```shell
這是`kernel/bpf/trampoline.c`中與最后執行的函數`bpf_trampoline_update`對應的源代碼：
```c
staticintbpf_trampoline_update(structbpf_trampoline*tr)
{
structbpf_tramp_image*im;
structbpf_tramp_progs*tprogs;
u32flags=BPF_TRAMP_F_RESTORE_REGS;
boolip_arg=false;
interr,total;

tprogs=bpf_trampoline_get_progs(tr,&total,&ip_arg);
if(IS_ERR(tprogs))
returnPTR_ERR(tprogs);

if(total==0){
err=unregister_fentry(tr,tr->cur_image->image);
bpf_tramp_image_put(tr->cur_image);
tr->cur_image=NULL;
tr->selector=0;
gotoout;
}

im=bpf_tramp_image_alloc(tr->key,tr->selector);
if(IS_ERR(im)){
err=PTR_ERR(im);
gotoout;
}

if(tprogs[BPF_TRAMP_FEXIT].nr_progs||
tprogs[BPF_TRAMP_MODIFY_RETURN].nr_progs)
flags=BPF_TRAMP_F_CALL_ORIG|BPF_TRAMP_F_SKIP_FRAME;

if(ip_arg)
flags|=BPF_TRAMP_F_IP_ARG;

err=arch_prepare_bpf_trampoline(im,im->image,im->image+PAGE_SIZE,
&tr->func.model,flags,tprogs,
tr->func.addr);
if(errcur_image&&tr->selector==0);
WARN_ON(!tr->cur_image&&tr->selector);
if(tr->cur_image)
/*progsalreadyrunningatthisaddress*/
err=modify_fentry(tr,tr->cur_image->image,im->image);
else
/*firsttimeregistering*/
err=register_fentry(tr,im->image);
if(err)
gotoout;
if(tr->cur_image)
bpf_tramp_image_put(tr->cur_image);
tr->cur_image=im;
tr->selector++;
out:
kfree(tprogs);
returnerr;
}

根據先前的輸出，我們可以看到：

tokio-runtime-w-1666[003]1318.058371:funcgraph_entry:1.250us|arch_prepare_bpf_trampoline();
tokio-runtime-w-1666[003]1318.058373:funcgraph_entry:2.292us|kfree();

在arch_prepare_bpf_trampoline和kfree函數之間沒有其他函數調用，所以很可能第一個函數在err變量中返回了錯誤代碼。讓我們來驗證一下！

通過以下方式在shell中啟動bpftace，我們可以捕獲arch_prepare_bpf_trampoline函數的返回值并將其打印到控制臺上：

bpftrace-e'kretprobe:arch_prepare_bpf_trampoline{printf("retvallink:%d
",retval);}'

并且在另一個終端中啟動probe后，我們從bpftace得到了以下輸出：

root@pine64-1:/home/exein#bpftrace-e'kretprobe:arch_prepare_bpf_trampoline{printf("retvallink:%d
",retval);}'
Attaching1probe...
retvallink:-524

這是因為內核5.15缺乏對aarch64架構的arch_prepare_bpf_trampoline實現，并使用了默認的占位符實現。

int__weak
arch_prepare_bpf_trampoline(structbpf_tramp_image*tr,void*image,void*image_end,
conststructbtf_func_model*m,u32flags,
structbpf_tramp_links*tlinks,
void*orig_call)
{
return-ENOTSUPP;
}

因此，這個功能在這個內核版本上是不受支持的。好消息是，多虧了這個補丁[7]，它在6.x內核中得到了實現。

讓我們移步到6.x內核。

Linux 6.1

如果我們嘗試在內核 6.1 上運行 probe，我們會得到以下輸出：

root@pine64:/home/exein#./probefile-system-monitor
thread'main'panickedat'initializationfailed:ProgramAttachError{program:"lsmpath_mknod",program_error:SyscallError{call:"bpf_raw_tracepoint_open",io_error:Os{code:524,kind:Uncategorized,message:"Noerrorinformation"}}}',src/bin/probe.rs43
note:runwith`RUST_BACKTRACE=1`environmentvariabletodisplayabacktrace

對于內核版本6.1，我們仍然遇到了和5.15內核一樣的錯誤！！！讓我們找出其中的原因。

這次在arch_prepare_bpf_trampoline上運行bpftrace，我們得到了以下輸出：

root@pine64:/home/exein#bpftrace-e'kretprobe:arch_prepare_bpf_trampoline{printf("retvaltplink:%d
",retval);}'
Attaching1probe...
retvaltplink:284

所以問題不在這里，這個函數不再返回錯誤了。讓我們回到函數調用圖。

這次我們啟動trace-cmd，跳過一些函數以獲得更清晰的輸出：

trace-cmdrecord
-pfunction_graph
-gbpf_trampoline_link_prog
-nbpf_jit_alloc_exec
-nkmalloc_trace
-narch_prepare_bpf_trampoline
-ngeneric_handle_domain_irq
-ndo_interrupt_handler
-nirq_exit_rcu
./probefile-system-monitor

我們從trace-cmd report中獲得以下輸出：

root@pine64:/home/exein#trace-cmdreport
CPU0isempty
CPU1isempty
CPU3isempty
cpus=4
tokio-runtime-w-11886[002]193385.056283:funcgraph_entry:|bpf_trampoline_link_prog(){
tokio-runtime-w-11886[002]193385.056321:funcgraph_entry:+15.042us|mutex_lock();
tokio-runtime-w-11886[002]193385.056373:funcgraph_entry:|__bpf_trampoline_link_prog(){
tokio-runtime-w-11886[002]193385.056395:funcgraph_entry:+14.833us|bpf_attach_type_to_tramp();
tokio-runtime-w-11886[002]193385.056428:funcgraph_entry:|bpf_trampoline_update.isra.23(){
tokio-runtime-w-11886[002]193385.056459:funcgraph_entry:2.917us|bpf_jit_charge_modmem();
tokio-runtime-w-11886[002]193385.056531:funcgraph_entry:|find_vm_area(){
tokio-runtime-w-11886[002]193385.056540:funcgraph_entry:3.000us|find_vmap_area();
tokio-runtime-w-11886[002]193385.056547:funcgraph_exit:+16.208us|}
tokio-runtime-w-11886[002]193385.056554:funcgraph_entry:|__alloc_percpu_gfp(){
tokio-runtime-w-11886[002]193385.056563:funcgraph_entry:|pcpu_alloc(){
tokio-runtime-w-11886[002]193385.056568:funcgraph_entry:4.875us|mutex_lock_killable();
tokio-runtime-w-11886[002]193385.056591:funcgraph_entry:|pcpu_find_block_fit(){
tokio-runtime-w-11886[002]193385.056599:funcgraph_entry:8.625us|pcpu_next_fit_region.constprop.38();
tokio-runtime-w-11886[002]193385.056608:funcgraph_exit:+17.166us|}
tokio-runtime-w-11886[002]193385.056610:funcgraph_entry:|pcpu_alloc_area(){
tokio-runtime-w-11886[002]193385.056639:funcgraph_entry:9.167us|pcpu_block_update();
tokio-runtime-w-11886[002]193385.056656:funcgraph_entry:7.667us|pcpu_block_update_hint_alloc();
tokio-runtime-w-11886[002]193385.056671:funcgraph_entry:7.750us|pcpu_chunk_relocate();
tokio-runtime-w-11886[002]193385.056679:funcgraph_exit:+69.667us|}
tokio-runtime-w-11886[002]193385.056682:funcgraph_entry:7.042us|mutex_unlock();
tokio-runtime-w-11886[002]193385.056703:funcgraph_entry:2.792us|pcpu_memcg_post_alloc_hook();
tokio-runtime-w-11886[002]193385.056712:funcgraph_exit:!148.709us|}
tokio-runtime-w-11886[002]193385.056719:funcgraph_exit:!165.250us|}
tokio-runtime-w-11886[002]193385.056866:funcgraph_entry:|bpf_image_ksym_add(){
tokio-runtime-w-11886[002]193385.056873:funcgraph_entry:|bpf_ksym_add(){
tokio-runtime-w-11886[002]193385.056882:funcgraph_entry:2.750us|__local_bh_disable_ip();
tokio-runtime-w-11886[002]193385.056897:funcgraph_entry:4.625us|__local_bh_enable_ip();
tokio-runtime-w-11886[002]193385.056905:funcgraph_exit:+32.459us|}
tokio-runtime-w-11886[002]193385.056922:funcgraph_entry:7.584us|perf_event_ksymbol();
tokio-runtime-w-11886[002]193385.056944:funcgraph_exit:+78.417us|}
tokio-runtime-w-11886[002]193385.057492:funcgraph_entry:|set_memory_ro(){
tokio-runtime-w-11886[002]193385.057501:funcgraph_entry:|change_memory_common(){
tokio-runtime-w-11886[002]193385.057504:funcgraph_entry:|find_vm_area(){
tokio-runtime-w-11886[002]193385.057506:funcgraph_entry:8.875us|find_vmap_area();
tokio-runtime-w-11886[002]193385.057518:funcgraph_exit:+14.250us|}
tokio-runtime-w-11886[002]193385.057522:funcgraph_entry:|__change_memory_common(){
tokio-runtime-w-11886[002]193385.057531:funcgraph_entry:|apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057538:funcgraph_entry:|__apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057544:funcgraph_entry:+12.791us|pud_huge();
tokio-runtime-w-11886[002]193385.057559:funcgraph_entry:2.708us|pmd_huge();
tokio-runtime-w-11886[002]193385.057574:funcgraph_entry:+15.125us|change_page_range();
tokio-runtime-w-11886[002]193385.057591:funcgraph_exit:+53.792us|}
tokio-runtime-w-11886[002]193385.057597:funcgraph_exit:+66.083us|}
tokio-runtime-w-11886[002]193385.057610:funcgraph_exit:+88.125us|}
tokio-runtime-w-11886[002]193385.057619:funcgraph_entry:|vm_unmap_aliases(){
tokio-runtime-w-11886[002]193385.057622:funcgraph_entry:|_vm_unmap_aliases.part.77(){
tokio-runtime-w-11886[002]193385.057625:funcgraph_entry:9.125us|mutex_lock();
tokio-runtime-w-11886[002]193385.057637:funcgraph_entry:3.084us|purge_fragmented_blocks_allcpus();
tokio-runtime-w-11886[002]193385.057643:funcgraph_entry:|__purge_vmap_area_lazy(){
tokio-runtime-w-11886[002]193385.057687:funcgraph_entry:|kmem_cache_free(){
tokio-runtime-w-11886[002]193385.057693:funcgraph_entry:+13.250us|__slab_free();
tokio-runtime-w-11886[002]193385.057705:funcgraph_exit:+18.750us|}
tokio-runtime-w-11886[002]193385.057718:funcgraph_entry:7.416us|__cond_resched_lock();
tokio-runtime-w-11886[002]193385.057733:funcgraph_exit:+90.042us|}
tokio-runtime-w-11886[002]193385.057741:funcgraph_entry:2.792us|mutex_unlock();
tokio-runtime-w-11886[002]193385.057747:funcgraph_exit:!124.666us|}
tokio-runtime-w-11886[002]193385.057749:funcgraph_exit:!130.291us|}
tokio-runtime-w-11886[002]193385.057756:funcgraph_entry:|__change_memory_common(){
tokio-runtime-w-11886[002]193385.057759:funcgraph_entry:|apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057765:funcgraph_entry:|__apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057768:funcgraph_entry:4.125us|pud_huge();
tokio-runtime-w-11886[002]193385.057778:funcgraph_entry:8.750us|pmd_huge();
tokio-runtime-w-11886[002]193385.057790:funcgraph_entry:4.625us|change_page_range();
tokio-runtime-w-11886[002]193385.057797:funcgraph_exit:+31.958us|}
tokio-runtime-w-11886[002]193385.057803:funcgraph_exit:+44.375us|}
tokio-runtime-w-11886[002]193385.057817:funcgraph_exit:+61.208us|}
tokio-runtime-w-11886[002]193385.057820:funcgraph_exit:!319.292us|}
tokio-runtime-w-11886[002]193385.057826:funcgraph_exit:!333.667us|}
tokio-runtime-w-11886[002]193385.057840:funcgraph_entry:|set_memory_x(){
tokio-runtime-w-11886[002]193385.057847:funcgraph_entry:|change_memory_common(){
tokio-runtime-w-11886[002]193385.057855:funcgraph_entry:|find_vm_area(){
tokio-runtime-w-11886[002]193385.057858:funcgraph_entry:2.917us|find_vmap_area();
tokio-runtime-w-11886[002]193385.057870:funcgraph_exit:+14.375us|}
tokio-runtime-w-11886[002]193385.057876:funcgraph_entry:|vm_unmap_aliases(){
tokio-runtime-w-11886[002]193385.057879:funcgraph_entry:|_vm_unmap_aliases.part.77(){
tokio-runtime-w-11886[002]193385.057882:funcgraph_entry:3.959us|mutex_lock();
tokio-runtime-w-11886[002]193385.057893:funcgraph_entry:3.000us|purge_fragmented_blocks_allcpus();
tokio-runtime-w-11886[002]193385.057900:funcgraph_entry:2.791us|__purge_vmap_area_lazy();
tokio-runtime-w-11886[002]193385.057907:funcgraph_entry:2.709us|mutex_unlock();
tokio-runtime-w-11886[002]193385.057913:funcgraph_exit:+33.708us|}
tokio-runtime-w-11886[002]193385.057915:funcgraph_exit:+43.000us|}
tokio-runtime-w-11886[002]193385.057922:funcgraph_entry:|__change_memory_common(){
tokio-runtime-w-11886[002]193385.057925:funcgraph_entry:|apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057930:funcgraph_entry:|__apply_to_page_range(){
tokio-runtime-w-11886[002]193385.057933:funcgraph_entry:4.292us|pud_huge();
tokio-runtime-w-11886[002]193385.057945:funcgraph_entry:8.750us|pmd_huge();
tokio-runtime-w-11886[002]193385.057956:funcgraph_entry:3.958us|change_page_range();
tokio-runtime-w-11886[002]193385.058037:funcgraph_exit:+32.083us|}
tokio-runtime-w-11886[002]193385.058089:funcgraph_entry:7.667us|irq_enter_rcu();
tokio-runtime-w-11886[002]193385.058233:funcgraph_exit:!308.041us|}
tokio-runtime-w-11886[002]193385.058239:funcgraph_exit:!316.709us|}
tokio-runtime-w-11886[002]193385.058247:funcgraph_exit:!400.417us|}
tokio-runtime-w-11886[002]193385.058255:funcgraph_exit:!415.000us|}
tokio-runtime-w-11886[002]193385.058555:funcgraph_entry:8.250us|irq_enter_rcu();
tokio-runtime-w-11886[002]193385.058958:funcgraph_entry:|kallsyms_lookup_size_offset(){
tokio-runtime-w-11886[002]193385.058974:funcgraph_entry:+36.333us|get_symbol_pos();
tokio-runtime-w-11886[002]193385.059017:funcgraph_exit:+59.750us|}
tokio-runtime-w-11886[002]193385.059043:funcgraph_entry:|kfree(){
tokio-runtime-w-11886[002]193385.059057:funcgraph_entry:3.000us|__kmem_cache_free();
tokio-runtime-w-11886[002]193385.059065:funcgraph_exit:+22.833us|}
tokio-runtime-w-11886[002]193385.059073:funcgraph_exit:#2644.708us|}
tokio-runtime-w-11886[002]193385.059079:funcgraph_exit:#2706.292us|}
tokio-runtime-w-11886[002]193385.059095:funcgraph_entry:2.792us|mutex_unlock();
tokio-runtime-w-11886[002]193385.059101:funcgraph_exit:#2870.416us|}

這次程序已經通過了arch_prepare_bpf_trampoline、set_memory_ro和set_memory_x，我們看到的最后一個函數是kallsyms_lookup_size_offset。

正如我們在kernel/bpf/trampoline.c中的bpf_trampoline_update函數中所看到的，這里并沒有明確調用kallsyms_lookup_size_offset：

staticintbpf_trampoline_update(structbpf_trampoline*tr,boollock_direct_mutex)
{

//...OTHERCODE...

#ifdefCONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
again:
if((tr->flags&BPF_TRAMP_F_SHARE_IPMODIFY)&&
(tr->flags&BPF_TRAMP_F_CALL_ORIG))
tr->flags|=BPF_TRAMP_F_ORIG_STACK;
#endif

err=arch_prepare_bpf_trampoline(im,im->image,im->image+PAGE_SIZE,
&tr->func.model,tr->flags,tlinks,
tr->func.addr);
if(errimage,1);
set_memory_x((long)im->image,1);

WARN_ON(tr->cur_image&&tr->selector==0);
WARN_ON(!tr->cur_image&&tr->selector);
if(tr->cur_image)
/*progsalreadyrunningatthisaddress*/
err=modify_fentry(tr,tr->cur_image->image,im->image,lock_direct_mutex);
else
/*firsttimeregistering*/
err=register_fentry(tr,im->image);

#ifdefCONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
if(err==-EAGAIN){
/*-EAGAINfrombpf_tramp_ftrace_ops_func.Now
*BPF_TRAMP_F_SHARE_IPMODIFYisset,wecangeneratethe
*trampolineagain,andretryregister.
*/
/*resetfops->funcandfops->trampolineforre-register*/
tr->fops->func=NULL;
tr->fops->trampoline=0;

/*resetim->imagememoryattrforarch_prepare_bpf_trampoline*/
set_memory_nx((long)im->image,1);
set_memory_rw((long)im->image,1);
gotoagain;
}
#endif
if(err)
gotoout;

if(tr->cur_image)
bpf_tramp_image_put(tr->cur_image);
tr->cur_image=im;
tr->selector++;
out:
/*Ifanyerrorhappens,restorepreviousflags*/
if(err)
tr->flags=orig_flags;
kfree(tlinks);
returnerr;
}
```shell

>**注意：**`bpf_trampoline_update`的實現與之前的內核5.15稍有不同。

`kallsyms_lookup_size_offset`的調用被隱藏在另一個函數內部。我們在函數圖中看不到它，因為編譯器將其內聯了。

看起來`kallsyms_lookup_size_offset`是由`ftrace_location`調用的：
```c
unsignedlongftrace_location(unsignedlongip)
{
structdyn_ftrace*rec;
unsignedlongoffset;
unsignedlongsize;

rec=lookup_rec(ip,ip);
if(!rec){
if(!kallsyms_lookup_size_offset(ip,&size,&offset))
gotoout;

/*mapsym+0to__fentry__*/
if(!offset)
rec=lookup_rec(ip,ip+size-1);
}

if(rec)
returnrec->ip;

out:
return0;
}

ftrace_location被register_fentry調用，而register_fentry在調用ftrace_location之后，在struct bpf_trampoline *tr的fops字段上包含了一次檢查。

/*firsttimeregistering*/
staticintregister_fentry(structbpf_trampoline*tr,void*new_addr)
{
void*ip=tr->func.addr;
unsignedlongfaddr;
intret;

faddr=ftrace_location((unsignedlong)ip);
if(faddr){
if(!tr->fops)
return-ENOTSUPP;
tr->func.ftrace_managed=true;
}

if(bpf_trampoline_module_get(tr))
return-ENOENT;

if(tr->func.ftrace_managed){
ftrace_set_filter_ip(tr->fops,(unsignedlong)ip,0,1);
ret=register_ftrace_direct_multi(tr->fops,(long)new_addr);
}else{
ret=bpf_arch_text_poke(ip,BPF_MOD_CALL,NULL,new_addr);
}

if(ret)
bpf_trampoline_module_put(tr);
returnret;
}

確實，如果tr->fops為false，該函數將返回錯誤-ENOTSUPP。

讓我們找出tr->fops是在哪里初始化的。

如果我們是正確的，那么創建trampoline的地方應該在bpf_trampoline_lookup函數內部。

staticstructbpf_trampoline*bpf_trampoline_lookup(u64key)
{
structbpf_trampoline*tr;
structhlist_head*head;
inti;

mutex_lock(&trampoline_mutex);
head=&trampoline_table[hash_64(key,TRAMPOLINE_HASH_BITS)];
hlist_for_each_entry(tr,head,hlist){
if(tr->key==key){
refcount_inc(&tr->refcnt);
gotoout;
}
}
tr=kzalloc(sizeof(*tr),GFP_KERNEL);
if(!tr)
gotoout;
#ifdefCONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
tr->fops=kzalloc(sizeof(structftrace_ops),GFP_KERNEL);
if(!tr->fops){
kfree(tr);
tr=NULL;
gotoout;
}
tr->fops->private=tr;
tr->fops->ops_func=bpf_tramp_ftrace_ops_func;
#endif

tr->key=key;
INIT_HLIST_NODE(&tr->hlist);
hlist_add_head(&tr->hlist,head);
refcount_set(&tr->refcnt,1);
mutex_init(&tr->mutex);
for(i=0;iprogs_hlist[i]);
out:
mutex_unlock(&trampoline_mutex);
returntr;
}

在分配之后，只有在出現CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS標志時，才會填充trampoline的fops字段。這個標志依賴于HAVE_CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS標志，而這個標志在aarch64上不存在。

結論

當前情況下，由于缺少_ftrace直接調用_功能，無法在aarch64上使用BPF LSM。幸運的是，當前的mainline分支已經合并了一個補丁[8]，該補丁將在aarch64上啟用LSMs（以及其他功能）。

預計這些變化將會在下一個6.4版的Linux內核中發布。

審核編輯：湯梓紅