为什么 strace 在 Docker 中不起作用？
strace 容器
![](/data/attachment/album/202005/26/101909a5wsvedzw5svawa2.jpg)


在编辑“容器如何工作”爱好者杂志的能力页面时，我想试着解释一下为什么 `strace` 在 Docker 容器中无法工作。


这里的问题是 —— 如果我在笔记本上的 Docker 容器中运行 `strace`，就会出现这种情况：



```
$ docker run  -it ubuntu:18.04 /bin/bash
$ # ... install strace ...
[email protected]:/# strace ls
strace: ptrace(PTRACE_TRACEME, ...): Operation not permitted
```

`strace` 通过 `ptrace` 系统调用起作用，所以如果不允许使用 `ptrace`，它肯定是不能工作的! 这个问题很容易解决 —— 在我的机器上，是这样解决的：



```
docker run --cap-add=SYS_PTRACE  -it ubuntu:18.04 /bin/bash
```

但我对如何修复它不感兴趣，我想知道为什么会出现这种情况。为什么 `strace` 不能工作，为什么`--cap-add=SYS_PTRACE` 可以解决这个问题？


### 假设 1：容器进程缺少 `CAP_SYS_PTRACE` 能力。


我一直以为原因是 Docker 容器进程默认不具备 `CAP_SYS_PTRACE` 能力。这和它可以被 `--cap-add=SYS_PTRACE` 修复是一回事，是吧？


但这实际上是不合理的，原因有两个。


原因 1：在实验中，作为一个普通用户，我可以对我的用户运行的任何进程进行 `strace`。但如果我检查我的当前进程是否有 `CAP_SYS_PTRACE` 能力，则没有：



```
$ getpcaps $$
Capabilities for `11589': =
```

原因 2：`capabilities` 的手册页对 `CAP_SYS_PTRACE` 的介绍是：



```
CAP_SYS_PTRACE
       * Trace arbitrary processes using ptrace(2);
```

所以，`CAP_SYS_PTRACE` 的作用是让你像 root 一样，可以对任何用户拥有的**任意**进程进行 `ptrace`。你不需要用它来对一个只是由你的用户拥有的普通进程进行 `ptrace` 。


我用第三种方法测试了一下（LCTT 译注：此处可能原文有误） —— 我用 `docker run --cap-add=SYS_PTRACE -it ubuntu:18.04 /bin/bash` 运行了一个 Docker 容器，去掉了 `CAP_SYS_PTRACE` 能力，但我仍然可以跟踪进程，虽然我已经没有这个能力了。什么？为什么？！


### 假设 2：关于用户命名空间的事情？


我的下一个（没有那么充分的依据的）假设是“嗯，也许这个过程是在不同的用户命名空间里，而 `strace` 不能工作，因为某种原因而行不通？”这个问题其实并不相关，但这是我观察时想到的。


容器进程是否在不同的用户命名空间中？嗯，在容器中：



```
root@e27f594da870:/# ls /proc/$$/ns/user -l
... /proc/1/ns/user -> 'user:[4026531837]'
```

在宿主机：



```
bork@kiwi:~$ ls /proc/$$/ns/user -l
... /proc/12177/ns/user -> 'user:[4026531837]'
```

因为用户命名空间 ID（`4026531837`）是相同的，所以容器中的 root 用户和主机上的 root 用户是完全相同的用户。所以，绝对没有理由不能够对它创建的进程进行 `strace`!


这个假设并没有什么意义，但我（之前）没有意识到 Docker 容器中的 root 用户和主机上的 root 用户同一个，所以我觉得这很有意思。


### 假设 3：ptrace 系统的调用被 seccomp-bpf 规则阻止了


我也知道 Docker 使用 seccomp-bpf 来阻止容器进程运行许多系统调用。而 `ptrace` 在[被 Docker 默认的 seccomp 配置文件阻止的系统调用列表](https://docs.docker.com/engine/security/seccomp/)中!（实际上，允许的系统调用列表是一个白名单，所以只是`ptrace` 不在默认的白名单中。但得出的结果是一样的。）


这很容易解释为什么 `strace` 在 Docker 容器中不能工作 —— 如果 `ptrace` 系统调用完全被屏蔽了，那么你当然不能调用它，`strace` 就会失败。


让我们来验证一下这个假设 —— 如果我们禁用了所有的 seccomp 规则，`strace` 能在 Docker 容器中工作吗？



```
$ docker run --security-opt seccomp=unconfined -it ubuntu:18.04  /bin/bash
$ strace ls
execve("/bin/ls", ["ls"], 0x7ffc69a65580 /* 8 vars */) = 0
... it works fine ...
```

是的，很好用！很好。谜底解开了，除了…..


### 为什么 `--cap-add=SYS_PTRACE` 能解决问题？


我们还没有解释的是：为什么 `--cap-add=SYS_PTRACE` 可以解决这个问题？


`docker run` 的手册页是这样解释 `--cap-add` 参数的。



```
--cap-add=[]
   Add Linux capabilities
```

这跟 seccomp 规则没有任何关系! 怎么回事？


### 我们来看看 Docker 源码


当文档没有帮助的时候，唯一要做的就是去看源码。


Go 语言的好处是，因为依赖关系通常是在一个 Go 仓库里，你可以通过 `grep` 来找出做某件事的代码在哪里。所以我克隆了 `github.com/moby/moby`，然后对一些东西进行 `grep`，比如 `rg CAP_SYS_PTRACE`。


我认为是这样的。在 `containerd` 的 seccomp 实现中，在 [contrib/seccomp/seccomp/seccomp\_default.go](https://github.com/containerd/containerd/blob/4be98fa28b62e8a012491d655a4d6818ef87b080/contrib/seccomp/seccomp_default.go#L527-L537) 中，有一堆代码来确保如果一个进程有一个能力，那么它也会（通过 seccomp 规则）获得访问权限，以使用与该能力相关的系统调用。



```
case "CAP_SYS_PTRACE":
       s.Syscalls = append(s.Syscalls, specs.LinuxSyscall{
           Names: []string{
               "kcmp",
               "process_vm_readv",
               "process_vm_writev",
               "ptrace",
           },
           Action: specs.ActAllow,
           Args:   []specs.LinuxSeccompArg{},
       })
```

在 moby 中的 [profile/seccomp/seccomp.go](https://github.com/moby/moby/blob/cc0dfb6e7b22ad120c60a9ce770ea15415767cf9/profiles/seccomp/seccomp.go#L126-L132) 和 [默认的 seccomp 配置文件](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json#L723-L739)中，也有一些其他的代码似乎做了一些非常类似的事情，所以有可能就是这个代码在做这个事情。


所以我想我们有答案了!


### Docker 中的 `--cap-add` 做的事情比它说的要多


结果似乎是，`--cap-add` 并不像手册页里说的那样，它更像是 `--cap-add-and-also-whiteelist-some-extra-system-calls-if-required`。这很有意义! 如果你具有一个像 `--CAP_SYS_PTRACE` 这样的能力，可以让你使用 `process_vm_readv` 系统调用，但是该系统调用被 seccomp 配置文件阻止了，那对你没有什么帮助！


所以当你给容器 `CAP_SYS_PTRACE` 能力时，允许使用 `process_vm_readv` 和 `ptrace` 系统调用似乎是一个合理的选择。


### 就这样！


这是个有趣的小事情，我认为这是一个很好的例子，说明了容器是由许多移动的部件组成的，它们以不完全显而易见的方式一起工作。




---


via: <https://jvns.ca/blog/2020/04/29/why-strace-doesnt-work-in-docker/>


作者：[Julia Evans](https://jvns.ca/) 选题：[lujun9972](https://github.com/lujun9972) 译者：[wxy](https://github.com/wxy) 校对：[wxy](https://github.com/wxy)


本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出


OK
While editing the capabilities page of the [how containers work](https://wizardzines.com/zines/containers) zine, I found myself
trying to explain why `strace`

doesn’t work in a Docker container.

The problem here is – if I run `strace`

in a Docker container on my laptop, this happens:

```
$ docker run -it ubuntu:18.04 /bin/bash
$ # ... install strace ...
root@e27f594da870:/# strace ls
strace: ptrace(PTRACE_TRACEME, ...): Operation not permitted
```


strace works using the `ptrace`

system call, so if `ptrace`

isn’t
allowed, it’s definitely not gonna work! This is pretty easy to fix – on
my machine, this fixes it:

```
docker run --cap-add=SYS_PTRACE -it ubuntu:18.04 /bin/bash
```


But I wasn’t interested in fixing it, I wanted to know why it happens. So
why does strace not work, and why does `--cap-add=SYS_PTRACE`

fix it?

### hypothesis 1: container processes are missing the `CAP_SYS_PTRACE`

capability

I always thought the reason was that Docker container processes by
default didn’t have the `CAP_SYS_PTRACE`

capability. This is consistent
with it being fixed by `--cap-add=SYS_PTRACE`

, right?

But this actually doesn’t make sense for 2 reasons.

**Reason 1**: Experimentally, as a regular user, I can strace on any process run by my
user. But if I check if my current process has the `CAP_SYS_PTRACE`

capability, I don’t:

```
$ getpcaps $$
Capabilities for `11589': =
```


**Reason 2**: `man capabilities`

says this about `CAP_SYS_PTRACE`

:

```
CAP_SYS_PTRACE
* Trace arbitrary processes using ptrace(2);
```


So the point of `CAP_SYS_PTRACE`

is to let you ptrace **arbitrary**
processes owned by any user, the way that root usually can. You shouldn’t
need it to just ptrace a regular process owned by your user.

And I tested this a third way – I ran a Docker container with `docker run --cap-add=SYS_PTRACE -it ubuntu:18.04 /bin/bash`

, dropped the
`CAP_SYS_PTRACE`

capability, and I could still strace processes even
though I didn’t have that capability anymore. What? Why?

### hypothesis 2: something about user namespaces???

My next (much less well-founded) hypothesis was something along the lines of “um, maybe the process is in a different user namespace and strace doesn’t work because of… reasons?” This isn’t really coherent but here’s what happened when I looked into it.

Is the container process in a different user namespace? Well, in the container:

```
root@e27f594da870:/# ls /proc/$$/ns/user -l
... /proc/1/ns/user -> 'user:[4026531837]'
```


On the host:

```
bork@kiwi:~$ ls /proc/$$/ns/user -l
... /proc/12177/ns/user -> 'user:[4026531837]'
```


Because the user namespace ID (`4026531837`

) is the same, the root user
in the container is the exact same user as the root user on the host. So
there’s definitely no reason it shouldn’t be able to strace processes
that it created!

This hypothesis doesn’t make much sense but I hadn’t realized that the root user in a Docker container is the same as the root user on the host, so I thought that was interesting.

### hypothesis 3: the ptrace system call is being blocked by a seccomp-bpf rule

I also knew that Docker uses seccomp-bpf to stop container processes from
running a lot of system calls. And ptrace is in the [list of system calls
blocked by Docker’s default seccomp
profile](https://docs.docker.com/engine/security/seccomp/)! (actually the
list of allowed system calls is a whitelist, so it’s just that ptrace is
not in the default whitelist. But it comes out to the same thing.)

That easily explains why strace wouldn’t work in a Docker container – if
the `ptrace`

system call is totally blocked, then of course you can’t
call it at all and strace would fail.

Let’s verify this hypothesis – if we disable all seccomp rules, can we strace in a Docker container?

```
$ docker run --security-opt seccomp=unconfined -it ubuntu:18.04 /bin/bash
$ strace ls
execve("/bin/ls", ["ls"], 0x7ffc69a65580 /* 8 vars */) = 0
... it works fine ...
```


Yes! It works! Great. Mystery solved, except…

### why does `--cap-add=SYS_PTRACE`

fix the problem?

What we still haven’t explained is: why does `--cap-add=SYS_PTRACE`

would
fix the problem?

The man page for `docker run`

explains the `--cap-add`

argument this way:

```
--cap-add=[]
Add Linux capabilities
```


That doesn’t have anything to do with seccomp rules! What’s going on?

### let’s look at the Docker source code.

When the documentation doesn’t help, the only thing to do is go look at the source.

The nice thing about Go is, because dependencies are often vendored in a
Go repository, you can just grep the repository to figure out where the
code that does a thing is. So I cloned `github.com/moby/moby`

and grepped
for some things, like `rg CAP_SYS_PTRACE`

.

Here’s what I think is going on. In containerd’s seccomp implementation, in
[contrib/seccomp/seccomp_default.go](https://github.com/containerd/containerd/blob/4be98fa28b62e8a012491d655a4d6818ef87b080/contrib/seccomp/seccomp_default.go#L527-L537),
there’s a bunch of code that makes sure that if a process has a
capability, then it’s also given access (through a seccomp rule) to use
the system calls that go with that capability.

```
case "CAP_SYS_PTRACE":
s.Syscalls = append(s.Syscalls, specs.LinuxSyscall{
Names: []string{
"kcmp",
"process_vm_readv",
"process_vm_writev",
"ptrace",
},
Action: specs.ActAllow,
Args: []specs.LinuxSeccompArg{},
})
```


There’s some other code that seems to do something very similar in
[profiles/seccomp/seccomp.go](https://github.com/moby/moby/blob/cc0dfb6e7b22ad120c60a9ce770ea15415767cf9/profiles/seccomp/seccomp.go#L126-L132)
in moby and the [default seccomp
profile](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json#L723-L739),
so it’s possible that that’s what’s doing it instead.

So I think we have our answer!

`--cap-add`

in Docker does a little more than what it says

The upshot seems to be that `--cap-add`

doesn’t do exactly what it says
it does in the man page, it’s more like
`--cap-add-and-also-whitelist-some-extra-system-calls-if-required`

. Which makes
sense! If you have a capability like `CAP_SYS_PTRACE`

which is supposed
to let you use the `process_vm_readv`

system call but that system call is
blocked by a seccomp profile, that’s not going to help you much!

So allowing the `process_vm_readv`

and `ptrace`

system calls when you
give the container `CAP_SYS_PTRACE`

seems like a reasonable choice.

### strace actually does work in newer versions of Docker

As of [this commit](https://github.com/moby/moby/commit/1124543ca8071074a537a15db251af46a5189907) (docker 19.03), Docker does actually allow the `ptrace`

system calls for kernel versions newer than 4.8.

But the Docker version on my laptop is 18.09.7, so it predates that commit.

### that’s all!

This was a fun small thing to investigate, and I think it’s a nice example of how containers are made of lots of moving pieces that work together in not-completely-obvious ways.

If you liked this, you might like my new zine called [How Containers Work](https://wizardzines.com/zines/containers) that explains the Linux kernel features that make containers work in 24 pages. You can read the pages on [capabilities](https://wizardzines.com/comics/capabilities/) and [seccomp-bpf](https://wizardzines.com/comics/seccomp-bpf/) from the zine.