kbuild: Fix changing ELF file type for output of gen_btf for big endian

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: 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>

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index a432b171be826a9c4ff362c518f97024ab90a811..7862a81017477daec1f702fdf46166e381bd396d 100755 (executable)
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -135,8 +135,13 @@ 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 is_enabled 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} .S file with all symbols from the ${1} object file