Re: [PATCH net-next 1/7] bpf: add 'flags' attribute to BPF_MAP_UPDATE_ELEM command

Alexei Starovoitov
2014-11-06T17:39:42Z

On Wed, Nov 5, 2014 at 6:57 AM, Daniel Borkmann <dborkman@redhat.com> wrote:
> On 11/05/2014 12:04 AM, Alexei Starovoitov wrote:
>>
>> On Tue, Nov 4, 2014 at 1:25 AM, Daniel Borkmann <dborkman@redhat.com>
>> wrote:
>>>
>>> On 11/04/2014 03:54 AM, Alexei Starovoitov wrote:
>>>>
>>>>
>>>> the current meaning of BPF_MAP_UPDATE_ELEM syscall command is:
>>>> either update existing map element or create a new one.
>>>> Initially the plan was to add a new command to handle the case of
>>>> 'create new element if it didn't exist', but 'flags' style looks
>>>> cleaner and overall diff is much smaller (more code reused), so add
>>>> 'flags'
>>>> attribute to BPF_MAP_UPDATE_ELEM command with the following meaning:
>>>> enum {
>>>> BPF_MAP_UPDATE_OR_CREATE = 0, /* add new element or update existing
>>>> */
>>>> BPF_MAP_CREATE_ONLY, /* add new element if it didn't exist
>>>> */
>>>> BPF_MAP_UPDATE_ONLY /* update existing element */
>>>> };
>>>
>>>
>>> From you commit message/code I currently don't see an explanation why
>>> it cannot be done in typical ``flags style'' as various syscalls do,
>>> i.e. BPF_MAP_UPDATE_OR_CREATE rather represented as ...
>>>
>>> BPF_MAP_CREATE | BPF_MAP_UPDATE
>>>
>>> Do you expect more than 64 different flags to be passed from user space
>>> for BPF_MAP?
>>
>>
>> several reasons:
>> - preserve flags==0 as default behavior
>> - avoid holes and extra checks for invalid combinations, so
>> if (flags > BPF_MAP_UPDATE_ONLY) goto err, is enough.
>> - it looks much neater when user space uses
>> BPF_MAP_UPDATE_OR_CREATE instead of ORing bits.
>>
>> Note this choice doesn't prevent adding bit-like flags
>> in the future. Today I cannot think of any new flags
>> for the update() command, but if somebody comes up with
>> a new selector that can apply to all three combinations,
>> we can add it as 3rd bit that can be ORed.
>
>
> Hm, mixing enums together with bitfield-like flags seems
> kind of hacky ... :/ Or, do you mean to say you're adding
> a 2nd flag field, i.e. splitting the 64bits into a 32bit
> ``cmd enum'' and 32bit ``flag section''?

something like this.
or splitting 64-bit into 2 and 62. We'll see.
First two encode this 'type' of update and the rest -
whatever else.

> Hm, my concern is that we start to add many *_OR_* enum
> elements once we find that a flag might be a useful in
> combination with many other flags ... even though if we
> only can think of 3 flags /right now/.

Agree. Adding many *_OR_* would look bad, that's
why I said that future additions can be bits. Like in
paragraph above.

Also, we don't have 3 flags now. In this patch I'm
showing 3 types and you're suggesting to treat
them as 2 flags. To me that's incorrect, since 'no flags'
becomes invalid combination, which logically incorrect.
Therefore I cannot see them as 'flags'. This is a 'type'
or 'style' of update() command.

I think it actually matches how open() defines things
in similar situation:
#define O_RDONLY 00000000
#define O_WRONLY 00000001
#define O_RDWR 00000002
We used to think of them as flags, but they're not
bit flags, though the rest of open() flags are bit-like.
If we apply your argument to open() then open()
should have defined O_RD as 1 and OR_WR as 2
and force everyone to mix and match them, but
then zero would be invalid. So I still think that
what I have is a cleaner API :)