tools/build: Use SYSTEM_BPFTOOL for system bpftool

The feature test for system bpftool uses BPFTOOL as the variable to set
its path, defaulting to just "bpftool" if not set by the user.

This conflicts with selftests and a few other utilities, which expect
BPFTOOL to be set to the in-tree bpftool path by default. For example,
bpftool selftests fail to build:

$ make -C tools/testing/selftests/bpf/
make: Entering directory '/home/tglozar/dev/linux/tools/testing/selftests/bpf'

make: *** No rule to make target 'bpftool', needed by '/home/tglozar/dev/linux/tools/testing/selftests/bpf/tools/include/vmlinux.h'.  Stop.
make: Leaving directory '/home/tglozar/dev/linux/tools/testing/selftests/bpf'

Fix the problem by renaming the variable used for system bpftool from
BPFTOOL to SYSTEM_BPFTOOL, so that the new usage does not conflict with
the existing one of BPFTOOL.

Cc: John Kacur <[email protected]>
Cc: Luis Goncalves <[email protected]>
Link: https://lore.kernel.org/[email protected]
Fixes: 8a635c3 ("tools/build: Add bpftool-skeletons feature test")
Closes: https://lore.kernel.org/linux-kernel/[email protected]/
Reported-by: Venkat Rao Bagalkote <[email protected]>
Tested-by: Venkat Rao Bagalkote <[email protected]>
Suggested-by: Quentin Monnet <[email protected]>
Acked-by: Quentin Monnet <[email protected]>
Signed-off-by: Tomas Glozar <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: lenticularis39

tools/build/feature/Makefile

@@ -419,7 +419,7 @@ $(OUTPUT)test-libpfm4.bin:
 	$(BUILD) -lpfm
 
 $(OUTPUT)test-bpftool-skeletons.bin:
-	$(BPFTOOL) version | grep '^features:.*skeletons' \
+	$(SYSTEM_BPFTOOL) version | grep '^features:.*skeletons' \
 		> $(@:.bin=.make.output) 2>&1
 ###############################
 

tools/scripts/Makefile.include

@@ -92,7 +92,7 @@ LLVM_OBJCOPY	?= llvm-objcopy
 LLVM_STRIP	?= llvm-strip
 
 # Some tools require bpftool
-BPFTOOL		?= bpftool
+SYSTEM_BPFTOOL	?= bpftool
 
 ifeq ($(CC_NO_CLANG), 1)
 EXTRA_WARNINGS += -Wstrict-aliasing=3

tools/tracing/rtla/Makefile

@@ -78,7 +78,7 @@ src/timerlat.bpf.o: src/timerlat.bpf.c
 	$(QUIET_CLANG)$(CLANG) -g -O2 -target bpf -c $(filter %.c,$^) -o $@
 
 src/timerlat.skel.h: src/timerlat.bpf.o
-	$(QUIET_GENSKEL)$(BPFTOOL) gen skeleton $< > $@
+	$(QUIET_GENSKEL)$(SYSTEM_BPFTOOL) gen skeleton $< > $@
 else
 src/timerlat.skel.h:
 	$(Q)echo '/* BPF skeleton is disabled */' > src/timerlat.skel.h