netdev - Re: [PATCH v2] bpf: fix uapi bpf_prog

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Date:   Tue, 25 Jun 2019 07:16:55 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Baruch Siach <baruch@...s.co.il>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, "Dmitry V . Levin" <ldv@...linux.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] bpf: fix uapi bpf_prog_info fields alignment

On Tue, Jun 25, 2019 at 4:07 AM Baruch Siach <baruch@...s.co.il> wrote:
>
> Merge commit 1c8c5a9d38f60 ("Merge
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next") undid the
> fix from commit 36f9814a494 ("bpf: fix uapi hole for 32 bit compat
> applications") by taking the gpl_compatible 1-bit field definition from
> commit b85fab0e67b162 ("bpf: Add gpl_compatible flag to struct
> bpf_prog_info") as is. That breaks architectures with 16-bit alignment
> like m68k. Embed gpl_compatible into an anonymous union with 32-bit pad
> member to restore alignment of following fields.
>
> Thanks to Dmitry V. Levin his analysis of this bug history.
>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Baruch Siach <baruch@...s.co.il>
> ---
> v2:
> Use anonymous union with pad to make it less likely to break again in
> the future.
> ---
>  include/uapi/linux/bpf.h       | 5 ++++-
>  tools/include/uapi/linux/bpf.h | 5 ++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index a8b823c30b43..766eae02d7ae 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3142,7 +3142,10 @@ struct bpf_prog_info {
>         __aligned_u64 map_ids;
>         char name[BPF_OBJ_NAME_LEN];
>         __u32 ifindex;
> -       __u32 gpl_compatible:1;
> +       union {
> +               __u32 gpl_compatible:1;
> +               __u32 pad;
> +       };

Nack for the reasons explained in the previous thread
on the same subject.
Why cannot you go with earlier suggestion of _u32 :31; ?