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

[Daniel Borkmann]

On 11/05/2014 12:04 AM, Alexei Starovoitov wrote:
> On Tue, Nov 4, 2014 at 1:25 AM, Daniel Borkmann <dborkman@xxxxxxxxxx> 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''?

I see the point with flags == 0 as default behavior though,
but at this point in time we won't get burnt by it since
the API is not yet in a usable state and defaults to be
compiled-out.

> Default will stay zero and 'if >' check in older
> kernels will seamlessly work with new userspace.
> I don't like holes in flags and combinatorial
> explosion of bits and checks for them unless
> absolutely necessary.

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/.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/