kbuild: avoid too many execution of scripts/pahole-flags.sh

scripts/pahole-flags.sh is executed so many times.

You can confirm it, as follows:

  $ cat <<EOF >> scripts/pahole-flags.sh
  > echo "scripts/pahole-flags.sh was executed" >&2
  > EOF

  $ make -s
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
  scripts/pahole-flags.sh was executed
    [ lots of repeated lines... ]

This scripts is executed more than 20 times during the kernel build
because PAHOLE_FLAGS is a recursively expanded variable and exported
to sub-processes.

With GNU Make >= 4.4, it is executed more than 60 times because
exported variables are also passed to other $(shell ) invocations.
Without careful coding, it is known to cause an exponential fork
explosion. [1]

The use of $(shell ) in an exported recursive variable is likely wrong
because $(shell ) is always evaluated due to the 'export' keyword, and
the evaluation can occur multiple times by the nature of recursive
variables.

Convert the shell script to a Makefile, which is included only when
CONFIG_DEBUG_INFO_BTF=y.

[1]: https://savannah.gnu.org/bugs/index.php?64746

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

2 files changed, 19 insertions, 30 deletions

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
new file mode 100644
index 000000000000..82377e470aed
--- /dev/null
+++ b/scripts/Makefile.btf
@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: GPL-2.0
+
+pahole-ver := $(CONFIG_PAHOLE_VERSION)
+pahole-flags-y :=
+
+# pahole 1.18 through 1.21 can't handle zero-sized per-CPU vars
+ifeq ($(call test-le, $(pahole-ver), 121),y)
+pahole-flags-$(call test-ge, $(pahole-ver), 118)	+= --skip_encoding_btf_vars
+endif
+
+pahole-flags-$(call test-ge, $(pahole-ver), 121)	+= --btf_gen_floats
+
+pahole-flags-$(call test-ge, $(pahole-ver), 122)	+= -j
+
+pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
+
+pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
+
+export PAHOLE_FLAGS := $(pahole-flags-y)
diff --git a/scripts/pahole-flags.sh b/scripts/pahole-flags.sh
deleted file mode 100755
index 728d55190d97..000000000000
--- a/scripts/pahole-flags.sh
+++ /dev/null
@@ -1,30 +0,0 @@
-#!/bin/sh
-# SPDX-License-Identifier: GPL-2.0
-
-extra_paholeopt=
-
-if ! [ -x "$(command -v ${PAHOLE})" ]; then
-	exit 0
-fi
-
-pahole_ver=$($(dirname $0)/pahole-version.sh ${PAHOLE})
-
-if [ "${pahole_ver}" -ge "118" ] && [ "${pahole_ver}" -le "121" ]; then
-	# pahole 1.18 through 1.21 can't handle zero-sized per-CPU vars
-	extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_vars"
-fi
-if [ "${pahole_ver}" -ge "121" ]; then
-	extra_paholeopt="${extra_paholeopt} --btf_gen_floats"
-fi
-if [ "${pahole_ver}" -ge "122" ]; then
-	extra_paholeopt="${extra_paholeopt} -j"
-fi
-if [ "${pahole_ver}" -ge "124" ]; then
-	# see PAHOLE_HAS_LANG_EXCLUDE
-	extra_paholeopt="${extra_paholeopt} --lang_exclude=rust"
-fi
-if [ "${pahole_ver}" -ge "125" ]; then
-	extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_inconsistent_proto --btf_gen_optimized"
-fi
-
-echo ${extra_paholeopt}