[PATCH v1] tools build: Fix feature checks to touch target files on success

[Ian Rogers <irogers@google.com>]

In tools/build/feature/Makefile, test-clang-bpf-co-re.bin and
test-bpftool-skeletons.bin redirected grep output but never touched or
created the $@ target file upon success.

Because the target file was never created on disk, Kbuild could never cache
the result of the check. Consequently, Make treated the prerequisite as
missing and continuously re-executed the Clang BPF backend and bpftool
feature checks on every single sub-make evaluation during build startup, or
on every incremental build.

Refactor both feature check recipes to touch $@ on success. For
test-clang-bpf-co-re.bin, group the shell pipeline within curly braces
and redirect both stdout and stderr to .make.output to allow errors to
be inspected and not appear in build output. List
test-clang-bpf-co-re.bin's input C file as a dependency so
modification triggers a rebuild. For test-bpftool-skeletons.bin, add
it to the FILES list so that it will be cleaned.

Tested-by: James Clark <james.clark@linaro.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
---
This patch originally appeared in the now merged perf patch series:
https://lore.kernel.org/linux-perf-users/20260518154638.2798789-1-irogers@google.com/
---
 tools/build/feature/Makefile | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index 704c687ed3ad..5b4a984973c4 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -75,7 +75,8 @@ FILES=                                          \
          test-file-handle.bin			\
          test-libpfm4.bin			\
          test-rust.bin				\
-         test-libopenssl.bin
+         test-libopenssl.bin			\
+         test-bpftool-skeletons.bin
 
 FILES := $(addprefix $(OUTPUT),$(FILES))
 
@@ -383,9 +384,9 @@ $(OUTPUT)test-libaio.bin:
 $(OUTPUT)test-libzstd.bin:
 	$(BUILD) -lzstd
 
-$(OUTPUT)test-clang-bpf-co-re.bin:
-	$(CLANG) -S -g --target=bpf -o - $(patsubst %.bin,%.c,$(@F)) |	\
-		grep BTF_KIND_VAR
+$(OUTPUT)test-clang-bpf-co-re.bin: test-clang-bpf-co-re.c
+	{ $(CLANG) -S -g --target=bpf -o - $< |	\
+		grep BTF_KIND_VAR; } > $(@:.bin=.make.output) 2>&1 && touch $@
 
 $(OUTPUT)test-file-handle.bin:
 	$(BUILD)
@@ -397,8 +398,8 @@ $(OUTPUT)test-libopenssl.bin:
 	$(BUILD) $(shell $(PKG_CONFIG) --libs --cflags openssl 2>/dev/null)
 
 $(OUTPUT)test-bpftool-skeletons.bin:
-	$(SYSTEM_BPFTOOL) version | grep '^features:.*skeletons' \
-		> $(@:.bin=.make.output) 2>&1
+	{ $(SYSTEM_BPFTOOL) version | grep '^features:.*skeletons'; } \
+		> $(@:.bin=.make.output) 2>&1 && touch $@
 
 # Testing Rust is special: we don't compile anything, it's enough to check the
 # compiler presence. Compiling a test code for this purposes is problematic,
-- 
2.54.0.823.g6e5bcc1fc9-goog