Re: [RFC][PATCH] net/bpfilter: Remove this broken and apparently unmantained

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On 2020/06/10 7:32, Alexei Starovoitov wrote:
>> You can't start a usermode helper which requires access to filesystems (e.g. ELF loaders,
>> shared libraries) before call_usermodehelper() can start a usermode helper which requires
>> access to filesystems. Under such a restricted condition, what is nice with starting a
>> usermode helper? Programs which can be started under such condition will be quite limited.
>> My question is: why you can't use existing call_usermodehelper() (if you need to call
>> a usermode helper) ?
> 
> I think the confusion comes from assumption that usermode blob is a dynamic file that
> needs ld.so, shared libs and rootfs.

Yes, I assume that usermode blob needs to be able to access the rootfs.

>                                      This mode is supported by the blob loading
> logic, but it's not a primary use case. It's nice to be able to compile
> that blob with -g and be able to 'gdb -p' into it.

Where can the gdb come from when the rootfs is not accessible?

>                                                    That works and very
> convenient when it comes to debugging. Compare that to debugging a kernel module!

Userspace is convenient for debugging, at the cost of robustness (e.g. being killed
by SIGKILL).

> 
> The main mode of bpfilter operation was envisioned as rootfs-less.
> It must work with any init= including busybox. For production the bpfilter
> user mode blob was compiled as static binary with no dependencies.

I still can't imagine. Compiling a user mode blob as a static binary is possible.
But what does 'It must work with any init=' mean? The use of init= depends on
the rootfs being ready.

> So there is no path to point to. It should be ready before pid 1
> will do its first iptables sys_setsockopt.

There has to be at least the root directory in order to use init= parameter.

What does the "pid 1" mean? Why you can't specify your "user mode blob" using init=
parameter and then transfer the control of "pid 1" from your "user mode blob" to
"some program which will do its first iptables sys_setsockopt()" ?

>                                            If user reboots the kernel
> with different init= cmdline the bpfilter should still be doing its job.
> Like builtin kernel module.

Even when rebooting the kernel with different init= cmdline, you have a space for
running your "user mode blob" (e.g.

  init=/path/to/your/user/mode/blob init_after_blob=/path/to/some/program/which/will/do/something/else

), don't you?

There is no need to use call_usermodehelper(), let alone fork_usermode_blob()...



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