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Date: Mon, 16 Mar 2020 17:56:51 -0700 From: Fangrui Song <maskray@...gle.com> To: Stanislav Fomichev <sdf@...ichev.me> Cc: Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net, ast@...nel.org, daniel@...earbox.net, Nick Desaulniers <ndesaulniers@...gle.com>, Nathan Chancellor <natechancellor@...il.com>, clang-built-linux@...glegroups.com Subject: Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF 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@...gle.com> >> > Reported-by: Nathan Chancellor <natechancellor@...il.com> >> > Link: https://github.com/ClangBuiltLinux/linux/issues/871 >> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com> >> > --- >> > 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@seabios.org/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/
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