Re: [PATCH bpf-next 1/3] libbpf: avoid use of __int128 in typed dump display

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Jul 20, 2021 at 2:14 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On Mon, 19 Jul 2021, Andrii Nakryiko wrote:
>
> > On Mon, Jul 19, 2021 at 2:41 PM Alan Maguire <alan.maguire@oracle.com> wrote:
> > >
> > > __int128 is not supported for some 32-bit platforms (arm and i386).
> > > __int128 was used in carrying out computations on bitfields which
> > > aid display, but the same calculations could be done with __u64
> > > with the small effect of not supporting 128-bit bitfields.
> > >
> > > With these changes, a big-endian issue with casting 128-bit integers
> > > to 64-bit for enum bitfields is solved also, as we now use 64-bit
> > > integers for bitfield calculations.
> > >
> > > Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> > > Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> > > Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> > > ---
> >
> > Changes look good to me, thanks. But they didn't appear in patchworks
> > yet so I can't easily test and apply them. It might be because of
> > patchworks delay or due to a very long CC list. Try trimming the cc
> > list down and re-submit?
> >
>
> Done, looks like the v2 with the trimmed cc list made it into patchwork
> this time.

v1 also made it to the list right after I wrote the email :)

>
> > Also, while I agree that supporting 128-bit bitfields isn't important,
> > I wonder if we should warn/error on that (instead of shifting by
> > negative amount and reporting some garbage value), what do you think?
> > Is there one place in the code where we can error out early if the
> > type actually has bitfield with > 64 bits? I'd prefer to keep
> > btf_dump_bitfield_get_data() itself non-failing though.
> >
>
> Sorry, I missed the last part and made that function fail since
> it's probably the easiest place to capture too-large bitfields.
> I renamed it to btf_dump_get_bitfield_value() to match
> btf_dump_get_enum_value() which as a similar function signature
> (return int, pass in a pointer to the value we want to retrieve).
>
> We can't localize bitfield size checking to
> btf_dump_type_data_check_zero() because - depending on flags -
> the associated checks might not be carried out.  So duplication
> of bitfield size checks between the zero checking and bitfield/enum
> bitfield display seems inevitable, and that being the case, the
> extra error checking required around btf_dump_get_bitfield_value()
> seems to be required.
>
> I might be missing a better approach here of course; let me know what you
> think. Thanks again!

Nah, that's fine. Looks good. Testing and pushing in a few minutes. Thanks.

>
> Alan