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

[Yonghong Song <yhs@fb.com>]

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?