linux-kernel - Re: [PATCH v5 1/2] bpftool: Align output skeleton ELF code

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Message-ID: <CAP-5=fW=v9oFr1+B_JcEe+kG60wB2W0dtKkD7Fmkh8KSKdu7Aw@mail.gmail.com>
Date:   Fri, 6 Oct 2023 13:22:05 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH v5 1/2] bpftool: Align output skeleton ELF code

On Fri, Oct 6, 2023 at 1:20 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Oct 5, 2023 at 3:03 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > libbpf accesses the ELF data requiring at least 8 byte alignment,
> > however, the data is generated into a C string that doesn't guarantee
> > alignment. Fix this by assigning to an aligned char array. Use sizeof
> > on the array, less one for the \0 terminator, rather than generating a
> > constant.
> >
> > Fixes: a6cc6b34b93e ("bpftool: Provide a helper method for accessing skeleton's embedded ELF data")
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > Acked-by: Quentin Monnet <quentin@...valent.com>
> > Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
> > ---
> >  tools/bpf/bpftool/gen.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> > index 2883660d6b67..b8ebcee9bc56 100644
> > --- a/tools/bpf/bpftool/gen.c
> > +++ b/tools/bpf/bpftool/gen.c
> > @@ -1209,7 +1209,7 @@ static int do_skeleton(int argc, char **argv)
> >         codegen("\
> >                 \n\
> >                                                                             \n\
> > -                       s->data = (void *)%2$s__elf_bytes(&s->data_sz);     \n\
> > +                       s->data = (void *)%1$s__elf_bytes(&s->data_sz);     \n\
>
> Seems like you based this on top of bpf tree, can you please rebase
> onto bpf-next, it has a small change here and I can't apply it cleanly
> anymore. Other than that it looks good. Thanks!

Sure thing, sorry for so many V-s, we'll get there in the end.

Thanks,
Ian

> >                                                                             \n\
> >                         obj->skeleton = s;                                  \n\
> >                         return 0;                                           \n\
> > @@ -1218,12 +1218,12 @@ static int do_skeleton(int argc, char **argv)
> >                         return err;                                         \n\
> >                 }                                                           \n\
> >                                                                             \n\
> > -               static inline const void *%2$s__elf_bytes(size_t *sz)       \n\
> > +               static inline const void *%1$s__elf_bytes(size_t *sz)       \n\
> >                 {                                                           \n\
> > -                       *sz = %1$d;                                         \n\
> > -                       return (const void *)\"\\                           \n\
> > -               "
> > -               , file_sz, obj_name);
> > +                       static const char data[] __attribute__((__aligned__(8))) = \"\\\n\
> > +               ",
> > +               obj_name
> > +       );
> >
> >         /* embed contents of BPF object file */
> >         print_hex(obj_data, file_sz);
> > @@ -1231,6 +1231,9 @@ static int do_skeleton(int argc, char **argv)
> >         codegen("\
> >                 \n\
> >                 \";                                                         \n\
> > +                                                                           \n\
> > +                       *sz = sizeof(data) - 1;                             \n\
> > +                       return (const void *)data;                          \n\
> >                 }                                                           \n\
> >                                                                             \n\
> >                 #ifdef __cplusplus                                          \n\
> > --
> > 2.42.0.609.gbb76f46606-goog
> >