Re: [PATCH RFC v7 net-next 00/28] BPF syscall

[Alexei Starovoitov]

On Tue, Aug 26, 2014 at 9:49 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Aug 26, 2014 at 9:35 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> On Tue, Aug 26, 2014 at 8:56 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Aug 26, 2014 7:29 PM, "Alexei Starovoitov" <ast@xxxxxxxxxxxx> wrote:
>>>>
>>>> Hi Ingo, David,
>>>>
>>>> posting whole thing again as RFC to get feedback on syscall only.
>>>> If syscall bpf(int cmd, union bpf_attr *attr, unsigned int size) is ok,
>>>> I'll split them into small chunks as requested and will repost without RFC.
>>>
>>> IMO it's much easier to review a syscall if we just look at a
>>> specification of what it does.  The code is, in some sense, secondary.
>>
>> 'specification of what it does'... hmm, you mean beyond what's
>> there in commit logs and in Documentation/networking/filter.txt ?
>> Aren't samples at the end give an idea on 'what it does'?
>> I'm happy to add 'specification', I just don't understand yet what
>> it suppose to talk about beyond what's already written.
>> I understand that the patches are missing explanation on 'why'
>> the syscall is being added, but I don't think it's what you're asking...
>
> I mean a hopefully short document that defines what the syscall does.
> It should be precise enough that one could, in principle, implement
> the syscall just by reading the document and that one could use the
> syscall just by reading the document.
>
> Given that there's a whole instruction set to go with it, it may end
> up being moderately complicated or saying things like "see this other
> thing for a description of the instruction set" and "there are some
> extensible sets of functions you can call with it".

I'm still lost.

Here is the quote from Documentation/networking/filter.txt
"
'maps' is a generic storage of different types for sharing data between kernel
and userspace.

The maps are accessed from user space via BPF syscall,
which has commands:
- create a map with given type and attributes
  map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size)
  using attr->map_type, attr->key_size, attr->value_size, attr->max_entries
  returns process-local file descriptor or negative error

- lookup key in a given map
  err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key, attr->value
  returns zero and stores found elem into value or negative error

- create or update key/value pair in a given map
  err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key, attr->value
  returns zero or negative error

- find and delete element by key in a given map
  err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key

- to delete map: close(fd)
  Exiting process will delete maps automatically

userspace programs uses this API to create/populate/read
maps that eBPF programs are concurrently updating.
"
and more in commit log:
"
- load eBPF program
  fd = bpf(BPF_PROG_LOAD, union bpf_attr *attr, u32 size)

  where 'attr' is
  struct {
      enum bpf_prog_type prog_type;
      __u32 insn_cnt;
      struct bpf_insn __user *insns;
      const char __user *license;
  };
  insns - array of eBPF instructions
  license - must be GPL compatible to call helper functions marked gpl_only

- unload eBPF program
  close(fd)
"

Isn't it short and describes what it does?
Do you want me to describe what eBPF program can do?
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