Subject: Re: [PATCH bpf-next] kbuild: add ability to generate BTF type info for vmlinux

From: Jiri Olsa <[email-masked]>
Date: Wed, 3 Apr 2019 10:46:19 +0200

On Tue, Apr 02, 2019 at 09:49:50AM -0700, [email-masked] wrote:
> From: Andrii Nakryiko <[email-masked]>
> 
> This patch adds new config option to trigger generation of BTF type
> information from DWARF debuginfo for vmlinux and kernel modules through
> pahole, which in turn relies on libbpf for btf_dedup() algorithm.
> 
> The intent is to record compact type information of all types used
> inside kernel, including all the structs/unions/typedefs/etc. This
> enables BPF's compile-once-run-everywhere ([0]) approach, in which
> tracing programs that are inspecting kernel's internal data (e.g.,
> struct task_struct) can be compiled on a system running some kernel
> version, but would be possible to run on other kernel versions (and
> configurations) without recompilation, even if the layout of structs
> changed and/or some of the fields were added, removed, or renamed.
> 
> This is only possible if BPF loader can get kernel type info to adjust
> all the offsets correctly. This patch is a first time in this direction,
> making sure that BTF type info is part of Linux kernel image in
> non-loadable ELF section.
> 
> BTF deduplication ([1]) algorithm typically provides 100x savings
> compared to DWARF data, so resulting .BTF section is not big as is
> typically about 2MB in size.

hi,
I'm using the latest pahole from git tree:
  https://github.com/acmel/dwarves.git

and getting pahole crash:

  LD      vmlinux
  BTF     vmlinux
die__process_inline_expansion: DW_TAG_label (0xa) @ <0x3eef8> not handled!
scripts/link-vmlinux.sh: line 96: 31222 Segmentation fault      (core dumped) LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
make[1]: *** [/home/jolsa/linux/Makefile:1025: vmlinux] Error 139
make: *** [Makefile:170: sub-make] Error 2

is there some other source/dependency I'm missing?

thanks,
jirka

> 
> [0] http://vger.kernel.org/lpc-bpf2018.html#session-2
> [1] https://facebookmicrosites.github.io/bpf/blog/2018/11/14/btf-enhancement.html
> 
> Cc: Masahiro Yamada <[email-masked]>
> Cc: Arnaldo Carvalho de Melo <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: Alexei Starovoitov <[email-masked]>
> Cc: Yonghong Song <[email-masked]>
> Cc: Martin KaFai Lau <[email-masked]>
> Signed-off-by: Andrii Nakryiko <[email-masked]>
> ---
>  Makefile                |  3 ++-
>  lib/Kconfig.debug       |  8 ++++++++
>  scripts/link-vmlinux.sh | 20 +++++++++++++++++++-
>  3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index d5713e7b1e50..a55308147a09 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -387,6 +387,7 @@ NM		= $(CROSS_COMPILE)nm
>  STRIP		= $(CROSS_COMPILE)strip
>  OBJCOPY		= $(CROSS_COMPILE)objcopy
>  OBJDUMP		= $(CROSS_COMPILE)objdump
> +PAHOLE		= pahole
>  LEX		= flex
>  YACC		= bison
>  AWK		= awk
> @@ -442,7 +443,7 @@ KBUILD_LDFLAGS :=
>  GCC_PLUGINS_CFLAGS :=
>  
>  export ARCH SRCARCH CONFIG_SHELL HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE AS LD CC
> -export CPP AR NM STRIP OBJCOPY OBJDUMP KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS
> +export CPP AR NM STRIP OBJCOPY OBJDUMP PAHOLE KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS
>  export MAKE LEX YACC AWK GENKSYMS INSTALLKERNEL PERL PYTHON PYTHON2 PYTHON3 UTS_MACHINE
>  export HOSTCXX KBUILD_HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
>  
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index d4df5b24d75e..cce78dcd19a2 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -200,6 +200,14 @@ config DEBUG_INFO_DWARF4
>  	  But it significantly improves the success of resolving
>  	  variables in gdb on optimized code.
>  
> +config DEBUG_INFO_BTF
> +	bool "Generate BTF typeinfo"
> +	depends on DEBUG_INFO
> +	help
> +	  Generate deduplicated BTF type information from DWARF debug info.
> +	  Turning this on expects presence of pahole tool, which will convert
> +	  DWARF type info into equivalent deduplicated BTF type info.
> +
>  config GDB_SCRIPTS
>  	bool "Provide GDB scripts for kernel debugging"
>  	depends on DEBUG_INFO
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index c8cf45362bd6..73bb7dfdc2c9 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -40,7 +40,7 @@ set -e
>  info()
>  {
>  	if [ "${quiet}" != "silent_" ]; then
> -		printf "  %-7s %s\n" ${1} ${2}
> +		printf "  %-7s %s\n" "${1}" "${2}"
>  	fi
>  }
>  
> @@ -114,6 +114,20 @@ vmlinux_link()
>  	fi
>  }
>  
> +# generate .BTF typeinfo from DWARF debuginfo
> +gen_btf()
> +{
> +	local pahole_ver;
> +
> +	pahole_ver=$(${PAHOLE} --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/')
> +	if [ "${pahole_ver}" -lt "113" ]; then
> +		info "BTF" "${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.13"
> +		exit 0
> +	fi
> +
> +	info "BTF" ${1}
> +	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> +}
>  
>  # Create ${2} .o file with all symbols from the ${1} object file
>  kallsyms()
> @@ -281,6 +295,10 @@ fi
>  info LD vmlinux
>  vmlinux_link "${kallsymso}" vmlinux
>  
> +if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
> +	gen_btf vmlinux
> +fi
> +
>  if [ -n "${CONFIG_BUILDTIME_EXTABLE_SORT}" ]; then
>  	info SORTEX vmlinux
>  	sortextable vmlinux
> -- 
> 2.17.1
>