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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 4 Oct 2023 14:56:20 -0700
Message-ID: <CAEf4BzaftfOjHCy_tOnXeX7_ZioBwZtKfT-3yvcQ2M9XfzRM-w@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] bpftool: Align output skeleton ELF code
To:     Ian Rogers <irogers@google.com>
Cc:     Quentin Monnet <quentin@isovalent.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alan Maguire <alan.maguire@oracle.com>
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On Wed, Oct 4, 2023 at 1:43=E2=80=AFPM Ian Rogers <irogers@google.com> wrot=
e:
>
> 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 skel=
eton's embedded ELF data")
> Signed-off-by: Ian Rogers <irogers@google.com>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  tools/bpf/bpftool/gen.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
>

CI is not happy still, see [0]

  [0] https://github.com/kernel-patches/bpf/actions/runs/6411371996/job/174=
06696212

> 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 =3D (void *)%2$s__elf_bytes(&s->data_sz);=
     \n\
> +                       s->data =3D (void *)%1$s__elf_bytes(&s->data_sz);=
     \n\
>                                                                          =
   \n\
>                         obj->skeleton =3D 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 =3D %1$d;                                    =
     \n\
> -                       return (const void *)\"\\                        =
   \n\
> -               "
> -               , file_sz, obj_name);
> +                       static const char data[] __attribute__((__aligned=
__(8))) =3D \"\\\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 =3D sizeof(data) - 1;                        =
     \n\
> +                       return (const void *)data;                       =
   \n\
>                 }                                                        =
   \n\
>                                                                          =
   \n\
>                 #ifdef __cplusplus                                       =
   \n\
> --
> 2.42.0.609.gbb76f46606-goog
>