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

["Jiri Slaby (SUSE)" <jirislaby@kernel.org>]

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.

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