netdev - Re: [PATCH bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

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Date:   Thu, 27 Jun 2019 01:00:03 +0000
From:   Song Liu <songliubraving@...com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "jannh@...gle.com" <jannh@...gle.com>
Subject: Re: [PATCH bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf



> On Jun 26, 2019, at 5:08 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> 
> On Wed, Jun 26, 2019 at 03:17:47PM +0000, Song Liu wrote:
>>>> +static struct miscdevice bpf_dev = {
>>>> +	.minor		= MISC_DYNAMIC_MINOR,
>>>> +	.name		= "bpf",
>>>> +	.fops		= &bpf_chardev_ops,
>>>> +	.mode		= 0440,
>>>> +	.nodename	= "bpf",
>>> 
>>> Here's what kvm does:
>>> 
>>> static struct miscdevice kvm_dev = {
>>>       KVM_MINOR,
>>>       "kvm",
>>>       &kvm_chardev_ops,
>>> };
> 
> Ick, I thought we converted all of these to named initializers a long
> time ago :)
> 
>>> Is there an actual reason that mode is not 0 by default in bpf case? Why
>>> we need to define nodename?
>> 
>> Based on my understanding, mode of 0440 is what we want. If we leave it 
>> as 0, it will use default value of 0600. I guess we can just set it to 
>> 0440, as user space can change it later anyway. 
> 
> Don't rely on userspace changing it, set it to what you want the
> permissions to be in the kernel here, otherwise you have to create a new
> udev rule and get it merged into all of the distros.  Just do it right
> the first time and there is no need for it.
> 
> What is wrong with 0600 for this?  Why 0440?

We would like root to own the device, and let users in a certain group 
to be able to open it. So 0440 is what we need. 

Thanks,
Song