[bpf-next PATCH v2 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh

Masahiro Yamada
2023-10-18T15:20:21Z

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

You can check how many times it is invoked during the build, 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 suppressed... ]

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 the 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>
---

Changes in v2:
- Fix a typo in commit description
- Update MAINTAINERS

MAINTAINERS | 2 +-
Makefile | 4 +---
scripts/Makefile.btf | 19 +++++++++++++++++++
scripts/pahole-flags.sh | 30 ------------------------------
4 files changed, 21 insertions(+), 34 deletions(-)
create mode 100644 scripts/Makefile.btf
delete mode 100755 scripts/pahole-flags.sh

diff --git a/MAINTAINERS b/MAINTAINERS
index 35977b269d5e..a08d558b1aaa 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3742,7 +3742,7 @@ F: net/sched/act_bpf.c
F: net/sched/cls_bpf.c
F: samples/bpf/
F: scripts/bpf_doc.py
-F: scripts/pahole-flags.sh
+F: scripts/Makefile.btf
F: scripts/pahole-version.sh
F: tools/bpf/
F: tools/lib/bpf/
diff --git a/Makefile b/Makefile
index fed9a6cc3665..eaddec67e5e1 100644
--- a/Makefile
+++ b/Makefile
@@ -513,8 +513,6 @@ LZ4 = lz4c
XZ = xz
ZSTD = zstd

-PAHOLE_FLAGS = $(shell PAHOLE=$(PAHOLE) $(srctree)/scripts/pahole-flags.sh)
-
CHECKFLAGS := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
-Wbitwise -Wno-return-void -Wno-unknown-attribute $(CF)
NOSTDINC_FLAGS :=
@@ -605,7 +603,6 @@ export KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE
export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_RUSTFLAGS_MODULE KBUILD_LDFLAGS_MODULE
export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL KBUILD_RUSTFLAGS_KERNEL
-export PAHOLE_FLAGS

# Files to ignore in find ... statements

@@ -1002,6 +999,7 @@ KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
# include additional Makefiles when needed
include-y := scripts/Makefile.extrawarn
include-$(CONFIG_DEBUG_INFO) += scripts/Makefile.debug
+include-$(CONFIG_DEBUG_INFO_BTF)+= scripts/Makefile.btf
include-$(CONFIG_KASAN) += scripts/Makefile.kasan
include-$(CONFIG_KCSAN) += scripts/Makefile.kcsan
include-$(CONFIG_KMSAN) += scripts/Makefile.kmsan
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}
--
2.40.1