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Commit 4ae191ef authored by Nathan Chancellor's avatar Nathan Chancellor Committed by Greg Kroah-Hartman
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kbuild: Fix changing ELF file type for output of gen_btf for big endian 

commit e3a9ee963ad8ba677ca925149812c5932b49af69 upstream.

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@vger.kernel.org
Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
Link: https://github.com/llvm/llvm-project/pull/75643


Suggested-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
Signed-off-by: default avatarNathan Chancellor <nathan@kernel.org>
Reviewed-by: default avatarFangrui Song <maskray@google.com>
Reviewed-by: default avatarNicolas Schier <nicolas@fjasle.eu>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarJustin Stitt <justinstitt@google.com>
Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
[nathan: Fix silent conflict due to lack of 7d153696e5db in older trees]
Signed-off-by: default avatarNathan Chancellor <nathan@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 750a4e59

scripts/link-vmlinux.sh

+7 −2
Original line number Diff line number Diff line
@@ -138,8 +138,13 @@ gen_btf() 
	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \

		--strip-all ${1} ${2} 2>/dev/null

	# Change e_type to ET_REL so that it can be used to link final vmlinux.

	# Unlike GNU ld, lld does not allow an ET_EXEC input.

	printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none

	# GNU ld 2.35+ and lld do not allow an ET_EXEC input.

	if [ -n "${CONFIG_CPU_BIG_ENDIAN}" ]; then

		et_rel='\0\1'

	else

		et_rel='\1\0'

	fi

	printf "${et_rel}" | dd of=${2} conv=notrunc bs=1 seek=16 status=none

}



# Create ${2} .o file with all symbols from the ${1} object file