On 2020-03-16, Stanislav Fomichev wrote:
On 03/16, Fangrui Song wrote:
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?
No, that's why we manually do '--set-section-flags .BTF=alloc' to
make --only-section work.
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?
I think that '--set-section-flags .BTF=alloc' causes objcopy to put
some non-zero (valid) sh_addr, that's why we need to reset it to 0.
(it's not clear if it's a feature or a bug and man isn't helpful)
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.
Here is what I get:
+ bin_arch=i386:x86-64
+ bin_format=elf64-x86-64
+ 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 i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.
Can you try to see where your x86_64 is coming from?
llvm-objdump -f does not print bfdarch (ARCH= in binutils-gdb/ld/emulparams/*.sh).
% objdump -f .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: file format elf64-x86-64
architecture: i386:x86-64, flags 0x00000010:
HAS_SYMS
start address 0x0000000000000000
% llvm-objdump -f .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: file format elf64-x86-64
architecture: x86_64
start address: 0x0000000000000000
% objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
# happy
% objcopy -I binary -O elf64-x86-64 -B x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
objcopy: architecture x86-64 unknown
As a non-x86 example, elf64-powerpcle / powerpc:common64:
% powerpc64le-linux-gnu-objdump -f meow.btf.vmlinux.bin.o
meow.btf.vmlinux.bin.o: file format elf64-powerpcle
architecture: powerpc:common64, flags 0x00000010:
Unfortunately, GNU objcopy<2.34 (before I complained about the redundant -B https://sourceware.org/bugzilla/show_bug.cgi?id=24968)
could not infer -B from -O elf* .
% objcopy -I binary -O elf64-x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o #<2.34
% file .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: ELF 64-bit LSB relocatable, no machine, version 1 (SYSV), not stripped
objcopy: architecture x86-64 unknown
GNU ld and lld will error for e_machine==0.
I will be a bit nervous to make llvm-objdump behave more BFD like.
Adding i386:x86-64, powerpc:common64, etc does not look particularly clean.
Fortunately, looking at the code, it seems that we only want to retain .BTF
The following scheme may be simpler:
objcopy --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin.o && printf '\1' | dd of=.btf.vmlinux.bin.o conv=notrunc bs=1 seek=16
The command after && is to change e_type from ET_EXEC to ET_REL. GNU ld has an extremely rare feature that allows ET_EXEC to be linked,
but lld is more rigid and will reject such an input file.
https://mail.coreboot.org/hyperkitty/list/seabios@xxxxxxxxxxx/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/
