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

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Message-ID: <CAEf4Bza0YGVW0G-oO3h1j0L0ytiKsn-pRbuqU39C2wO0VP__BA@mail.gmail.com>
Date:   Fri, 6 Oct 2023 13:20:11 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Ian Rogers <irogers@...gle.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 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!


>                                                                             \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
>