Re: [RFC] kbuild: bpf: Do not run pahole with -j on 32bit userspace

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 20, 2024 at 10:59:50AM +0200, Jiri Slaby (SUSE) wrote:
> From: Jiri Slaby <jslaby@suse.cz>
> 
> == WARNING ==
> This is only a PoC. There are deficiencies like CROSS_COMPILE or LLVM
> are completely unhandled.
> 
> The simple version is just do there:
>   ifeq ($(CONFIG_64BIT,y)
> but it has its own deficiencies, of course.
> 
> So any ideas, inputs?
> == WARNING ==
> 
> When pahole is run with -j on 32bit userspace (32bit pahole in
> particular), it randomly fails with OOM:
> > btf_encoder__tag_kfuncs: Failed to get ELF section(62) data: out of memory.
> > btf_encoder__encode: failed to tag kfuncs!
> 
> or simply SIGSEGV (failed to allocate the btf encoder).
> 
> It very depends on how many threads are created.
> 
> So do not invoke pahole with -j on 32bit.

could you share more details about your setup?

does it need to run on pure 32bit to reproduce? I can't reproduce when
doing cross build and running 32 bit pahole on x86_64.. I do see some
errors though

  [667939] STRUCT bpf_prog_aux Error emitting BTF type
  Encountered error while encoding BTF.

thanks,
jirka

> 
> Signed-off-by: Jiri Slaby <jslaby@suse.cz>
> Fixes: b4f72786429c ("scripts/pahole-flags.sh: Parse DWARF and generate BTF with multithreading.")
> Closes: https://bugzilla.suse.com/show_bug.cgi?id=1229450
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nicolas Schier <nicolas@fjasle.eu>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yonghong.song@linux.dev>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Stanislav Fomichev <sdf@fomichev.me>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-kbuild@vger.kernel.org
> Cc: bpf@vger.kernel.org
> Cc: shung-hsi.yu@suse.com
> Cc: msuchanek@suse.com
> ---
>  init/Kconfig            |  4 ++++
>  scripts/Makefile.btf    |  2 ++
>  scripts/pahole-class.sh | 21 +++++++++++++++++++++
>  3 files changed, 27 insertions(+)
>  create mode 100644 scripts/pahole-class.sh
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index f36ca8a0e209..f5e80497eef0 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -113,6 +113,10 @@ config PAHOLE_VERSION
>  	int
>  	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
>  
> +config PAHOLE_CLASS
> +	string
> +	default $(shell,$(srctree)/scripts/pahole-class.sh $(PAHOLE))
> +
>  config CONSTRUCTORS
>  	bool
>  
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index b75f09f3f424..f7de8e922bce 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -12,7 +12,9 @@ endif
>  
>  pahole-flags-$(call test-ge, $(pahole-ver), 121)	+= --btf_gen_floats
>  
> +ifeq ($(CONFIG_PAHOLE_CLASS),ELF64)
>  pahole-flags-$(call test-ge, $(pahole-ver), 122)	+= -j
> +endif
>  
>  pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
>  
> diff --git a/scripts/pahole-class.sh b/scripts/pahole-class.sh
> new file mode 100644
> index 000000000000..d15a92077f76
> --- /dev/null
> +++ b/scripts/pahole-class.sh
> @@ -0,0 +1,21 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# Usage: $ ./pahole-class.sh pahole
> +#
> +# Prints pahole's ELF class, such as ELF64
> +
> +if [ ! -x "$(command -v "$@")" ]; then
> +	echo 0
> +	exit 1
> +fi
> +
> +PAHOLE="$(which "$@")"
> +CLASS="$(readelf -h "$PAHOLE" 2>/dev/null | sed -n 's/.*Class: *// p')"
> +
> +# Scripts like scripts/dummy-tools/pahole
> +if [ -n "$CLASS" ]; then
> +	echo "$CLASS"
> +else
> +	echo ELF64
> +fi
> -- 
> 2.46.0
>