> On Jun 27, 2019, at 9:37 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 27, 2019 at 01:00:03AM +0000, Song Liu wrote:
>>
>>
>>> On Jun 26, 2019, at 5:08 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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.
>
> But you are doing a "write" ioctl here, right? So don't you really need
By "write", you meant that we are modifying a bit in task_struct, right?
In that sense, we probably need 0220?
> 0660 at the least? And if you "know" the group id, I think you can
> specify it too so udev doesn't have to do a ton of work, but that only
> works for groups that all distros number the same.
I don't think we know the group id yet.
>
> And why again is this an ioctl instead of a syscall? What is so magic
> about the file descriptor here?
We want to control the permission of this operation via this device.
Users that can open the device would be able to run the ioctl. I think
syscall cannot achieve control like this, unless we introduce something
like CAP_BPF_ADMIN?
Thanks,
Song
