Re: [PATCH bpf-next v1 2/3] bpf: btf: Ensure natural alignment of .BTF_ids section

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On Fri, 2024-09-20 at 00:49 -0700, Tony Ambardar wrote:
> While building of vmlinux employs a linker script to align the .BTF_ids
> section to 4 bytes, other usage leaves .BTF_ids unaligned and may lead to
> problems (e.g. [1]). Post-processing and libelf-based endian translation by
> resolve_btfids may also potentially suffer from misalignment.
> 
> Update encoding macros in btf_ids.h to always align BTF ID data to 4 bytes.
> 
> [1]: 3effc06a4dde ("selftests/bpf: Fix alignment of .BTF_ids")
> 
> Signed-off-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
> ---
>  include/linux/btf_ids.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> index c0e3e1426a82..c10b163dc340 100644
> --- a/include/linux/btf_ids.h
> +++ b/include/linux/btf_ids.h
> @@ -89,6 +89,7 @@ word							\
>  #define __BTF_ID_LIST(name, scope)			\
>  asm(							\
>  ".pushsection " BTF_IDS_SECTION ",\"a\";       \n"	\
> +".balign 4, 0;                                 \n"	\
>  "." #scope " " #name ";                        \n"	\
>  #name ":;                                      \n"	\
>  ".popsection;                                  \n");

This forces all id list symbols to be aligned on 4 bytes.
Should the same be done for __BTF_SET_START?

Also, is it guaranteed that all btf ids are organized in lists and sets?
Grepping through the code it seems they are, but it looks like resolve_btfids
does not really enforce this, simply looking for symbols matching a special name
__BTF_ID__<type>__<symbol>[__<id>] .





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