Hi BTF folks, I have discovered some problems with pahole -J. Its usage in kernel scripts/link-vmlinux.sh is like (make LLVM=1 bzImage): ld.lld -m elf_x86_64 --emit-relocs --discard-none -z max-page-size=0x200000 --build-id=sha1 --orphan-handling=warn -o .tmp_vmlinux.btf -T ./arch/x86/kernel/vmlinux.lds --whole-archive arch/x86/kernel/head_64.o arch/x86/kernel/head64.o arch/x86/kernel/ebda.o arch/x86/kernel/platform-quirks.o init/built-in.a usr/built-in.a arch/x86/built-in.a kernel/built-in.a certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a security/built-in.a crypto/built-in.a block/built-in.a lib/built-in.a arch/x86/lib/built-in.a lib/lib.a arch/x86/lib/lib.a drivers/built-in.a sound/built-in.a net/built-in.a virt/built-in.a arch/x86/pci/built-in.a arch/x86/power/built-in.a arch/x86/video/built-in.a --no-whole-archive --start-group --end-group pahole -J .tmp_vmlinux.btf llvm-objcopy --only-section=.BTF --set-section-flags .BTF=alloc,readonly --strip-all .tmp_vmlinux.btf .btf.vmlinux.bin.o pahole -J adds .BTF and rewrites .tmp_vmlinux.btf, then llvm-objcopy produces .btf.vmlinux.bin.o of just one section. Why doesn't pahole provide a command generating an object file with just the .BTF section? When I contributed https://git.kernel.org/linus/90ceddcb495008ac8ba7a3dce297841efcd7d584 , I remember pahole at that time added a non-SHF_ALLOC .BTF , now (1.20) .BTF becomes SHF_ALLOC. This is problematic if pahole does not have full-fledged binary manipulation ability (objcopy,llvm-objcopy). In particular, there are two bugs: * pahole does not respect max-page-size (p_align of PT_LOAD). See the .text section, its sh_offset != sh_addr (mod max-page-size) Section Headers: [Nr] Name Type Address Off Size ES Flg Lk Inf Al [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 - [ 1] .text PROGBITS ffffffff81000000 200000 1003917 00 AX 0 0 4096 + [ 1] .text PROGBITS ffffffff81000000 001000 e0169a 00 AX 0 0 4096 * pahole does not rewrite p_offset/p_filesz of PT_LOAD segments. Because of the first bug, pahole -J rewritten object file generally has small offsets. If p_offset/p_filesz of PT_LOAD segments are not rewritten, the file offset range of .symtab may be within a PT_LOAD range. llvm-objcopy --strip-all considers .symtab as part of the PT_LOAD and refuses --strip-all: error: '.tmp_vmlinux.btf': string table '.strtab' cannot be removed because it is referenced by the symbol table '.symtab' This is very rare, though. So I suggest: * pahole -J: restore the previous non-SHF_ALLOC behavior. Don't rewrite sh_offset of existing sections. Place the new non-SHF_ALLOC .BTF section at the end. Then not rewriting PT_LOAD will be justified. * Add a new option incorporating the current pahole -J + llvm-objcopy
