Re: [RFC PATCH bpf-next 05/16] bpf: create file or anonymous dumpers

[Yonghong Song <yhs@fb.com>]

On 4/10/20 3:53 PM, Andrii Nakryiko wrote:
> On Fri, Apr 10, 2020 at 3:43 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 4/9/20 8:00 PM, Alexei Starovoitov wrote:
>>> On Wed, Apr 08, 2020 at 04:25:26PM -0700, Yonghong Song wrote:
>>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>>> index 0f1cbed446c1..b51d56fc77f9 100644
>>>> --- a/include/uapi/linux/bpf.h
>>>> +++ b/include/uapi/linux/bpf.h
>>>> @@ -354,6 +354,7 @@ enum {
>>>>    /* Flags for accessing BPF object from syscall side. */
>>>>       BPF_F_RDONLY            = (1U << 3),
>>>>       BPF_F_WRONLY            = (1U << 4),
>>>> +    BPF_F_DUMP              = (1U << 5),
>>> ...
>>>>    static int bpf_obj_pin(const union bpf_attr *attr)
>>>>    {
>>>> -    if (CHECK_ATTR(BPF_OBJ) || attr->file_flags != 0)
>>>> +    if (CHECK_ATTR(BPF_OBJ) || attr->file_flags & ~BPF_F_DUMP)
>>>>               return -EINVAL;
>>>>
>>>> +    if (attr->file_flags == BPF_F_DUMP)
>>>> +            return bpf_dump_create(attr->bpf_fd,
>>>> +                                   u64_to_user_ptr(attr->dumper_name));
>>>> +
>>>>       return bpf_obj_pin_user(attr->bpf_fd, u64_to_user_ptr(attr->pathname));
>>>>    }
>>>
>>> I think kernel can be a bit smarter here. There is no need for user space
>>> to pass BPF_F_DUMP flag to kernel just to differentiate the pinning.
>>> Can prog attach type be used instead?
>>
>> Think again. I think a flag is still useful.
>> Suppose that we have the following scenario:
>>     - the current directory /sys/fs/bpf/
>>     - user says pin a tracing/dump (target task) prog to "p1"
>>
>> It is not really clear whether user wants to pin to
>>      /sys/fs/bpf/p1
>> or user wants to pin to
>>      /sys/kernel/bpfdump/task/p1
>>
>> unless we say that a tracing/dump program cannot pin
>> to /sys/fs/bpf which seems unnecessary restriction.
>>
>> What do you think?
> 
> Instead of special-casing dumper_name, can we require specifying full
> path, and then check whether it is in BPF FS vs BPFDUMP FS? If the
> latter, additionally check that it is in the right sub-directory
> matching its intended target type.

We could. I just think specifying full path for bpfdump is not necessary 
since it is a single user mount...

> 
> But honestly, just doing everything within BPF FS starts to seem
> cleaner at this point...

bpffs is multi mount, which is not a perfect fit for bpfdump,
considering mounting inside namespace, etc, all dumpers are gone.