netdev - Re: [PATCH v6 bpf-next 1/3] libbpf: BTF dumper support for typed data

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Message-ID: <alpine.LRH.2.23.451.2107191506130.18107@localhost>
Date:   Mon, 19 Jul 2021 15:15:06 +0100 (IST)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Naresh Kamboju <naresh.kamboju@...aro.org>, andrii@...nel.org
cc:     Alan Maguire <alan.maguire@...cle.com>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john.fastabend@...il.com, kpsingh@...nel.org, morbo@...gle.com,
        Shuah Khan <shuah@...nel.org>, bpf <bpf@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        lkft-triage@...ts.linaro.org
Subject: Re: [PATCH v6 bpf-next 1/3] libbpf: BTF dumper support for typed
 data


On Mon, 19 Jul 2021, Naresh Kamboju wrote:

> On Thu, 15 Jul 2021 at 20:46, Alan Maguire <alan.maguire@...cle.com> wrote:
> >
> > Add a BTF dumper for typed data, so that the user can dump a typed
> > version of the data provided.
> 
> <trim>
> 
> > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > index 5dc6b517..929cf93 100644
> > --- a/tools/lib/bpf/btf_dump.c
> > +++ b/tools/lib/bpf/btf_dump.c
> 
> 
> Following perf build errors noticed on i386 and arm 32-bit architectures on
> linux next 20210719 tag with gcc-11.
> 
> metadata:
> --------------
>    git_repo: https://gitlab.com/Linaro/lkft/mirrors/next/linux-next
>    git_short_log: 08076eab6fef ( Add linux-next specific files for 20210719 )
>    toolchain: gcc-11
>    target_arch: arm and i386
> 
> 
> > +static void btf_dump_int128(struct btf_dump *d,
> > +                           const struct btf_type *t,
> > +                           const void *data)
> > +{
> > +       __int128 num = *(__int128 *)data;
> 
> 
> btf_dump.c: In function 'btf_dump_int128':
> btf_dump.c:1559:9: error: expected expression before '__int128'
>  1559 |         __int128 num = *(__int128 *)data;
>       |         ^~~~~~~~
> btf_dump.c:1561:14: error: 'num' undeclared (first use in this function)
>  1561 |         if ((num >> 64) == 0)
>       |              ^~~
> btf_dump.c:1561:14: note: each undeclared identifier is reported only
> once for each function it appears in
> btf_dump.c: At top level:
> btf_dump.c:1568:17: error: '__int128' is not supported on this target
>  1568 | static unsigned __int128 btf_dump_bitfield_get_data(struct btf_dump *d,
>       |                 ^~~~~~~~
> btf_dump.c: In function 'btf_dump_bitfield_get_data':
> btf_dump.c:1576:18: error: '__int128' is not supported on this target
>  1576 |         unsigned __int128 num = 0, ret;
>       |                  ^~~~~~~~
> btf_dump.c: In function 'btf_dump_bitfield_check_zero':
> btf_dump.c:1608:9: error: expected expression before '__int128'
>  1608 |         __int128 check_num;
>       |         ^~~~~~~~
> btf_dump.c:1610:9: error: 'check_num' undeclared (first use in this function)
>  1610 |         check_num = btf_dump_bitfield_get_data(d, t, data,
> bits_offset, bit_sz);
>       |         ^~~~~~~~~
> btf_dump.c: In function 'btf_dump_bitfield_data':
> btf_dump.c:1622:18: error: '__int128' is not supported on this target
>  1622 |         unsigned __int128 print_num;
>       |                  ^~~~~~~~
> btf_dump.c: In function 'btf_dump_dump_type_data':
> btf_dump.c:2212:34: error: '__int128' is not supported on this target
>  2212 |                         unsigned __int128 print_num;
>       |                                  ^~~~~~~~
> 
>

Thanks for the report Naresh! Andrii, I'm thinking the best
approach might be to remove use of int128 and have the bitfield
computations operate on a __u64 representation instead.  With
that change, we would only lose the ability to handle int128
bitfields; what do you think? I hope to have something ready
shortly covering that, the non-propogation of return values
and the endianness issues with enum handling - in fact the
latter goes away if the bitfield computations are done for
64-bit values.

Thanks!

Alan