kbuild: Fix changing ELF file type for output of gen_btf for big endian
Commit90ceddcb49
("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@vger.kernel.org Fixes:90ceddcb49
("bpf: Support llvm-objcopy for vmlinux BTF") Link: https://github.com/llvm/llvm-project/pull/75643 Suggested-by: Masahiro Yamada <masahiroy@kernel.org> Signed-off-by: Nathan Chancellor <nathan@kernel.org> Reviewed-by: Fangrui Song <maskray@google.com> Reviewed-by: Nicolas Schier <nicolas@fjasle.eu> Reviewed-by: Kees Cook <keescook@chromium.org> Reviewed-by: Justin Stitt <justinstitt@google.com> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
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@ -135,8 +135,13 @@ gen_btf()
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${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
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--strip-all ${1} ${2} 2>/dev/null
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# Change e_type to ET_REL so that it can be used to link final vmlinux.
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# Unlike GNU ld, lld does not allow an ET_EXEC input.
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printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
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# GNU ld 2.35+ and lld do not allow an ET_EXEC input.
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if is_enabled CONFIG_CPU_BIG_ENDIAN; then
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et_rel='\0\1'
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else
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et_rel='\1\0'
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fi
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printf "${et_rel}" | dd of=${2} conv=notrunc bs=1 seek=16 status=none
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}
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# Create ${2} .S file with all symbols from the ${1} object file
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