Re: [PATCH] bpf: Support llvm-objcopy for vmlinux BTF

[Greg KH <greg@kroah.com>]

On Mon, Jun 08, 2020 at 01:39:59PM +0000, Maria Teguiani wrote:
> From: Fangrui Song <maskray@google.com>
> 
> Simplify gen_btf logic to make it work with llvm-objcopy. The existing
> 'file format' and 'architecture' parsing logic is brittle and does not
> work with llvm-objcopy/llvm-objdump.
> 
> 'file format' output of llvm-objdump>=11 will match GNU objdump, but
> 'architecture' (bfdarch) may not.
> 
> .BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag
> because it is part of vmlinux image used for introspection. C code
> can reference the section via linker script defined __start_BTF and
> __stop_BTF. This fixes a small problem that previous .BTF had the
> SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).
> 
> Additionally, `objcopy -I binary` synthesized symbols
> _binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not
> used elsewhere) are replaced with more commonplace __start_BTF and
> __stop_BTF.
> 
> Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns
> "empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"
> 
> We use a dd command to change the e_type field in the ELF header from
> ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o.  Accepting
> ET_EXEC as an input file is an extremely rare GNU ld feature that lld
> does not intend to support, because this is error-prone.
> 
> The output section description .BTF in include/asm-generic/vmlinux.lds.h
> avoids potential subtle orphan section placement issues and suppresses
> --orphan-handling=warn warnings.
> 
> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section")
> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Tested-by: Stanislav Fomichev <sdf@google.com>
> Tested-by: Andrii Nakryiko <andriin@fb.com>
> Reviewed-by: Stanislav Fomichev <sdf@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
> Link: https://github.com/ClangBuiltLinux/linux/issues/871
> Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com
> (cherry picked from commit 90ceddcb495008ac8ba7a3dce297841efcd7d584)
> Cc: <stable@vger.kernel.org> # 5.4.x
> Signed-off-by: Maria Teguiani <teguiani@google.com>

I've also queued this up to the 5.6.y tree, thanks.

greg k-h