Re: [PATCH dwarves v3 5/5] btf_encoder: Align .BTF section to 8 bytes

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On Fri, Feb 5, 2021 at 5:42 AM Giuliano Procida <gprocida@xxxxxxxxxx> wrote:
>
> This is to avoid misaligned access to BTF type structs when
> memory-mapping ELF objects.
>
> Signed-off-by: Giuliano Procida <gprocida@xxxxxxxxxx>
> ---

I trust you did verify that it actually works in cases where
previously .BTF was mis-aligned?

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

>  libbtf.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/libbtf.c b/libbtf.c
> index 9f4abb3..6754a17 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -744,6 +744,14 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>                 goto out;
>         }
>
> +       /*
> +        * We'll align .BTF to 8 bytes to cater for all architectures. It'd be
> +        * nice if we could fetch this value from somewhere. The BTF
> +        * specification does not discuss alignment and its trailing string
> +        * table is not currently padded to any particular alignment.
> +        */
> +       const size_t btf_alignment = 8;
> +
>         /*
>          * First we check if there is already a .BTF section present.
>          */
> @@ -821,6 +829,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>                 elf_error("elf_getshdr(btf_scn) failed");
>                 goto out;
>         }
> +       btf_shdr.sh_addralign = btf_alignment;
>         btf_shdr.sh_entsize = 0;
>         btf_shdr.sh_flags = 0;
>         if (dot_btf_offset)
> --
> 2.30.0.478.g8a0d178c01-goog
>



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