tools: bpftool: ignore make built-in rules for getting kernel version

Bpftool calls the toplevel Makefile to get the kernel version for the
sources it is built from. But when the utility is built from the top of
the kernel repository, it may dump the following error message for
certain architectures (including x86):

    $ make tools/bpf
    [...]
    make[3]: *** [checkbin] Error 1
    [...]

This does not prevent bpftool compilation, but may feel disconcerting.
The "checkbin" arch-dependent target is not supposed to be called for
target "kernelversion", which is a simple "echo" of the version number.

It turns out this is caused by the make invocation in tools/bpf/bpftool,
which attempts to find implicit rules to apply. Extract from debug
output:

    Reading makefiles...
    Reading makefile 'Makefile'...
    Reading makefile 'scripts/Kbuild.include' (search path) (no ~ expansion)...
    Reading makefile 'scripts/subarch.include' (search path) (no ~ expansion)...
    Reading makefile 'arch/x86/Makefile' (search path) (no ~ expansion)...
    Reading makefile 'scripts/Makefile.kcov' (search path) (no ~ expansion)...
    Reading makefile 'scripts/Makefile.gcc-plugins' (search path) (no ~ expansion)...
    Reading makefile 'scripts/Makefile.kasan' (search path) (no ~ expansion)...
    Reading makefile 'scripts/Makefile.extrawarn' (search path) (no ~ expansion)...
    Reading makefile 'scripts/Makefile.ubsan' (search path) (no ~ expansion)...
    Updating makefiles....
     Considering target file 'scripts/Makefile.ubsan'.
      Looking for an implicit rule for 'scripts/Makefile.ubsan'.
      Trying pattern rule with stem 'Makefile.ubsan'.
    [...]
      Trying pattern rule with stem 'Makefile.ubsan'.
      Trying implicit prerequisite 'scripts/Makefile.ubsan.o'.
      Looking for a rule with intermediate file 'scripts/Makefile.ubsan.o'.
       Avoiding implicit rule recursion.
       Trying pattern rule with stem 'Makefile.ubsan'.
       Trying rule prerequisite 'prepare'.
       Trying rule prerequisite 'FORCE'.
      Found an implicit rule for 'scripts/Makefile.ubsan'.
        Considering target file 'prepare'.
         File 'prepare' does not exist.
          Considering target file 'prepare0'.
           File 'prepare0' does not exist.
            Considering target file 'archprepare'.
             File 'archprepare' does not exist.
              Considering target file 'archheaders'.
               File 'archheaders' does not exist.
               Finished prerequisites of target file 'archheaders'.
              Must remake target 'archheaders'.
    Putting child 0x55976f4f6980 (archheaders) PID 31743 on the chain.

To avoid that, pass the -r and -R flags to eliminate the use of make
built-in rules (and while at it, built-in variables) when running
command "make kernelversion" from bpftool's Makefile.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/Makefile

@@ -24,7 +24,7 @@ endif
 
 LIBBPF = $(BPF_PATH)libbpf.a
 
-BPFTOOL_VERSION := $(shell make --no-print-directory -sC ../../.. kernelversion)
+BPFTOOL_VERSION := $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
 
 $(LIBBPF): FORCE
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(OUTPUT) $(OUTPUT)libbpf.a