On Mon, Feb 12, 2024 at 07:05:10PM -0700, Nathan Chancellor wrote:
> Commit 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> changed the ELF type of .btf.vmlinux.bin.o to ET_REL via dd, which works
> fine for little endian platforms:
>
> 00000000 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 |.ELF............|
> -00000010 03 00 b7 00 01 00 00 00 00 00 00 80 00 80 ff ff |................|
> +00000010 01 00 b7 00 01 00 00 00 00 00 00 80 00 80 ff ff |................|
>
> However, for big endian platforms, it changes the wrong byte, resulting
> in an invalid ELF file type, which ld.lld rejects:
>
> 00000000 7f 45 4c 46 02 02 01 00 00 00 00 00 00 00 00 00 |.ELF............|
> -00000010 00 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
> +00000010 01 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
>
> Type: <unknown>: 103
>
> ld.lld: error: .btf.vmlinux.bin.o: unknown file type
>
> Fix this by updating the entire 16-bit e_type field rather than just a
> single byte, so that everything works correctly for all platforms and
> linkers.
>
> 00000000 7f 45 4c 46 02 02 01 00 00 00 00 00 00 00 00 00 |.ELF............|
> -00000010 00 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
> +00000010 00 01 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
>
> Type: REL (Relocatable file)
>
> While in the area, update the comment to mention that binutils 2.35+
> matches LLD's behavior of rejecting an ET_EXEC input, which occurred
> after the comment was added.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> Link: https://github.com/llvm/llvm-project/pull/75643
> Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Writing the u16 looks fine to me.
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
--
Kees Cook
