On 2020-03-16, Stanislav Fomichev wrote:
Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
BTF") switched from --dump-section to
--only-section/--change-section-address for BTF export assuming
those ("legacy") options should cover all objcopy versions.
Turns out llvm-objcopy doesn't implement --change-section-address [1],
but it does support --dump-section. Let's partially roll back and
try to use --dump-section first and fall back to
--only-section/--change-section-address for the older binutils.
1. https://bugs.llvm.org/show_bug.cgi?id=45217
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
scripts/link-vmlinux.sh | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..8ddf57cbc439 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -127,6 +127,16 @@ gen_btf()
cut -d, -f1 | cut -d' ' -f2)
bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
awk '{print $4}')
+
+ # Compatibility issues:
+ # - pre-2.25 binutils objcopy doesn't support --dump-section
+ # - llvm-objcopy doesn't support --change-section-address, but
+ # does support --dump-section
+ #
+ # Try to use --dump-section which should cover both recent
+ # binutils and llvm-objcopy and fall back to --only-section
+ # for pre-2.25 binutils.
+ ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
${OBJCOPY} --change-section-address .BTF=0 \
--set-section-flags .BTF=alloc -O binary \
--only-section=.BTF ${1} .btf.vmlinux.bin
--
2.25.1.481.gfbce0eb801-goog
So let me take advantage of this email to ask some questions about
commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").
Does .BTF have the SHF_ALLOC flag?
Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?
Regarding
Turns out llvm-objcopy doesn't implement --change-section-address [1],
This option will be difficult to implement in llvm-objcopy if we intend
it to have a GNU objcopy compatible behavior.
Without --only-section, it is not very clear how
--change-section-{address,vma,lma} will affect program headers.
There will be a debate even if we decide to implement them in llvm-objcopy.
Some PT_LOAD rewriting examples:
objcopy --change-section-address .plt=0 a b
objcopy --change-section-address .text=0 a b
There is another bug related to -B
(https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):
+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
-O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
+ objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
objcopy: architecture x86_64 unknown
+ echo 'Failed to generate BTF for vmlinux'
It should be i386:x86_64.
