Please remove all bit fields in bpf uapi

Please remove all bit fields in bpf uapi

[Meng Zhuo]

Hi, all

I'm tring to port bpf.h to Go, however it's very hard to make it right
with cgo since bit fields some fields didn't match any type of Go.

i.e.
struct bpf_prog_info {
        /* .... */
        __u32 gpl_compatible:1;             <-- boolean ?
        __u32 :31; /* alignment pad */   <--- padding with 31 ?

UAPI(User application interface) not just for GCC only right? If it's
true, I think remove all bit fields is more appropriate.

Thanks. Regards
Meng Zhuo

Re: Please remove all bit fields in bpf uapi

[Greg KH]

On Fri, Dec 18, 2020 at 04:52:15PM +0800, Meng Zhuo wrote:
> Hi, all
> 
> I'm tring to port bpf.h to Go, however it's very hard to make it right
> with cgo since bit fields some fields didn't match any type of Go.
> 
> i.e.
> struct bpf_prog_info {
>         /* .... */
>         __u32 gpl_compatible:1;             <-- boolean ?
>         __u32 :31; /* alignment pad */   <--- padding with 31 ?
> 
> UAPI(User application interface) not just for GCC only right? If it's
> true, I think remove all bit fields is more appropriate.

It's a bit late to change a user-visable api, sorry.  Go has to have
some way of properly handling bit fields, this isn't a new thing :)

good luck!

greg k-h

Re: Please remove all bit fields in bpf uapi

[Meng Zhuo]

Hi, Greg

Thank you for your reply
It's fine to do compile bit fields "by hand".
However is it possible to setup a guideline that forrbid "bit fields"
in uapi in the future?

Thanks

Greg KH <gregkh@linuxfoundation.org> 于2020年12月18日周五 下午4:57写道：
>
> On Fri, Dec 18, 2020 at 04:52:15PM +0800, Meng Zhuo wrote:
> > Hi, all
> >
> > I'm tring to port bpf.h to Go, however it's very hard to make it right
> > with cgo since bit fields some fields didn't match any type of Go.
> >
> > i.e.
> > struct bpf_prog_info {
> >         /* .... */
> >         __u32 gpl_compatible:1;             <-- boolean ?
> >         __u32 :31; /* alignment pad */   <--- padding with 31 ?
> >
> > UAPI(User application interface) not just for GCC only right? If it's
> > true, I think remove all bit fields is more appropriate.
>
> It's a bit late to change a user-visable api, sorry.  Go has to have
> some way of properly handling bit fields, this isn't a new thing :)
>
> good luck!
>
> greg k-h

Re: Please remove all bit fields in bpf uapi

[Greg KH]

On Fri, Dec 18, 2020 at 05:09:58PM +0800, Meng Zhuo wrote:
> Hi, Greg
> 
> Thank you for your reply
> It's fine to do compile bit fields "by hand".

Surely Go has something like "if (field & 0x01)", right?  That's all you
need for a bitfield.

Look at the most common syscall, open(2)?  It uses bitfields, why
can't Go handle that?

> However is it possible to setup a guideline that forrbid "bit fields"
> in uapi in the future?

Where would that guideline go and who would enforce it?  :)

thanks,

greg k-h

Re: Please remove all bit fields in bpf uapi

[Florian Weimer]

* Greg KH:

> On Fri, Dec 18, 2020 at 05:09:58PM +0800, Meng Zhuo wrote:
>> Hi, Greg
>> 
>> Thank you for your reply
>> It's fine to do compile bit fields "by hand".
>
> Surely Go has something like "if (field & 0x01)", right?  That's all you
> need for a bitfield.
>
> Look at the most common syscall, open(2)?  It uses bitfields, why
> can't Go handle that?

Flag arguments are very different from bit-fields in struct types.

Structs with bit-fields are not part of the highly portable C subset.
Even C compilers differ in their interpretation.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill

Re: Please remove all bit fields in bpf uapi

[Daniel Borkmann]

On 12/18/20 12:04 PM, Florian Weimer wrote:
>> On Fri, Dec 18, 2020 at 05:09:58PM +0800, Meng Zhuo wrote:
>>> Hi, Greg
>>>
>>> Thank you for your reply
>>> It's fine to do compile bit fields "by hand".
>>
>> Surely Go has something like "if (field & 0x01)", right?  That's all you
>> need for a bitfield.
>>
>> Look at the most common syscall, open(2)?  It uses bitfields, why
>> can't Go handle that?
> 
> Flag arguments are very different from bit-fields in struct types.
> 
> Structs with bit-fields are not part of the highly portable C subset.
> Even C compilers differ in their interpretation.

Bitfields are still quite common though, not special to BPF in any way ...
how do you deal with all these cases then?

git grep -P -n "[a-z]+:[1-9]+" include/uapi/ | wc -l
272