Re: [PATCH v1] bpftool: Align output skeleton ELF code

[Ian Rogers]

On Mon, Oct 2, 2023 at 3:32 PM Ian Rogers <irogers@xxxxxxxxxx> 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.
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---

Perhaps this could have a fixes tag:
Fixes: d510296d331a ("bpftool: Use syscall/loader program in "prog
load" and "gen skeleton" command.")

The unaligned problem was seen in perf's offcpu code as well as bcc's
libbpf_tools. I didn't see problems with map data and opts data, but
inspection of the code shows they likely have the same issue. I was
testing with -fsanitize=alignment and
-fsanitize-undefined-trap-on-error.

Thanks,
Ian

>  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\
>                                                                             \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\
> --
> 2.42.0.582.g8ccd20d70d-goog
>