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

From: Alan Maguire
Date: Tue Jul 20 2021 - 05:18:01 EST

On Mon, 19 Jul 2021, Andrii Nakryiko wrote:

> On Mon, Jul 19, 2021 at 2:41 PM Alan Maguire <alan.maguire@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> > Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > ---
> 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.

> 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!