netdev - Re: [PATCH bpf-next 1/3] libbpf: avoid use of _

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Date:   Tue, 20 Jul 2021 10:13:48 +0100 (IST)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
cc:     Alan Maguire <alan.maguire@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        Shuah Khan <shuah@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/3] libbpf: avoid use of __int128 in typed dump
 display

On Mon, 19 Jul 2021, Andrii Nakryiko wrote:

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

Alan