Subject: Re: [PATCH v2] bpf: fix uapi bpf_prog_info fields alignment

From: "Dmitry V. Levin" <[email-masked]>
Date: Tue, 25 Jun 2019 18:36:29 +0300

On Tue, Jun 25, 2019 at 08:19:35AM -0700, Alexei Starovoitov wrote:
> On Tue, Jun 25, 2019 at 8:08 AM Dmitry V. Levin <[email-masked]> wrote:
> > On Tue, Jun 25, 2019 at 07:16:55AM -0700, Alexei Starovoitov wrote:
> > > On Tue, Jun 25, 2019 at 4:07 AM Baruch Siach <[email-masked]> 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 <[email-masked]>
> > > > Cc: Daniel Borkmann <[email-masked]>
> > > > Cc: Geert Uytterhoeven <[email-masked]>
> > > > Cc: Linus Torvalds <[email-masked]>
> > > > Signed-off-by: Baruch Siach <[email-masked]>
> > > > ---
> > > > 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; ?
> >
> > By the way, why not use aligned types as suggested by Geert?
> > They are already used for other members of struct bpf_prog_info anyway.
> >
> > FWIW, we use aligned types for bpf in strace and that approach
> > proved to be more robust than manual padding.
> 
> because __aligned_u64 is used for pointers.

Does the fact that __aligned_u64 is used for pointers mean that
__aligned_u64 should not be used for anything but pointers?

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