From 062e1fc008ded14a637ed9c8631fa31f57534dfc Mon Sep 17 00:00:00 2001
From: Quentin Monnet <quentin@isovalent.com>
Date: Sat, 9 Oct 2021 22:03:41 +0100
Subject: [PATCH] bpftool: Turn check on zlib from a phony target into a
 conditional error

One of bpftool's object files depends on zlib. To make sure we do not
attempt to build that object when the library is not available, commit
d66fa3c70e59 ("tools: bpftool: add feature check for zlib") introduced a
feature check to detect whether zlib is present.

This check comes as a rule for which the target ("zdep") is a
nonexistent file (phony target), which means that the Makefile always
attempts to rebuild it. It is mostly harmless. However, one side effect
is that, on running again once bpftool is already built, make considers
that "something" (the recipe for zdep) was executed, and does not print
the usual message "make: Nothing to be done for 'all'", which is a
user-friendly indicator that the build went fine.

Before, with some level of debugging information:

    $ make --debug=m
    [...]
    Reading makefiles...

    Auto-detecting system features:
    ...                        libbfd: [ on  ]
    ...        disassembler-four-args: [ on  ]
    ...                          zlib: [ on  ]
    ...                        libcap: [ on  ]
    ...               clang-bpf-co-re: [ on  ]

    Updating makefiles....
    Updating goal targets....
     File 'all' does not exist.
           File 'zdep' does not exist.
          Must remake target 'zdep'.
     File 'all' does not exist.
    Must remake target 'all'.
    Successfully remade target file 'all'.

After the patch:

    $ make --debug=m
    [...]

    Auto-detecting system features:
    ...                        libbfd: [ on  ]
    ...        disassembler-four-args: [ on  ]
    ...                          zlib: [ on  ]
    ...                        libcap: [ on  ]
    ...               clang-bpf-co-re: [ on  ]

    Updating makefiles....
    Updating goal targets....
     File 'all' does not exist.
    Must remake target 'all'.
    Successfully remade target file 'all'.
    make: Nothing to be done for 'all'.

(Note the last line, which is not part of make's debug information.)

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211009210341.6291-4-quentin@isovalent.com
---
 tools/bpf/bpftool/Makefile | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 331019f6d5b1..abcef1f72d65 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -198,7 +198,10 @@ $(BOOTSTRAP_OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
 $(OUTPUT)disasm.o: $(srctree)/kernel/bpf/disasm.c
 	$(QUIET_CC)$(CC) $(CFLAGS) -c -MMD -o $@ $<
 
-$(OUTPUT)feature.o: | zdep
+$(OUTPUT)feature.o:
+ifneq ($(feature-zlib), 1)
+	$(error "No zlib found")
+endif
 
 $(BPFTOOL_BOOTSTRAP): $(BOOTSTRAP_OBJS) $(LIBBPF_BOOTSTRAP)
 	$(QUIET_LINK)$(HOSTCC) $(CFLAGS) $(LDFLAGS) -o $@ $(BOOTSTRAP_OBJS) \
@@ -254,10 +257,7 @@ doc-uninstall:
 
 FORCE:
 
-zdep:
-	@if [ "$(feature-zlib)" != "1" ]; then echo "No zlib found"; exit 1 ; fi
-
 .SECONDARY:
-.PHONY: all FORCE clean install-bin install uninstall zdep
+.PHONY: all FORCE clean install-bin install uninstall
 .PHONY: doc doc-clean doc-install doc-uninstall
 .DEFAULT_GOAL := all