linux-kernel - [PATCH v3 2/3] tools build: Avoid circular .fixdep-in.o.cmd issues

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Message-ID: <20240709204203.1481851-3-briannorris@chromium.org>
Date: Tue,  9 Jul 2024 13:41:52 -0700
From: Brian Norris <briannorris@...omium.org>
To: Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Thomas Richter <tmricht@...ux.ibm.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc: linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org,
	Masahiro Yamada <masahiroy@...nel.org>,
	Brian Norris <briannorris@...omium.org>
Subject: [PATCH v3 2/3] tools build: Avoid circular .fixdep-in.o.cmd issues

The 'fixdep' tool is used to post-process dependency files for various
reasons, and it runs after every object file generation command. This
even includes 'fixdep' itself.

In Kbuild, this isn't actually a problem, because it uses a single
command to generate fixdep (a compile-and-link command on fixdep.c), and
afterward runs the fixdep command on the accompanying .fixdep.cmd file.

In tools/ builds (which notably is maintained separately from Kbuild),
fixdep is generated in several phases:

 1. fixdep.c -> fixdep-in.o
 2. fixdep-in.o -> fixdep

Thus, fixdep is not available in the post-processing for step 1, and
instead, we generate .cmd files that look like:

  ## from tools/objtool/libsubcmd/.fixdep.o.cmd
  # cannot find fixdep (/path/to/linux/tools/objtool/libsubcmd//fixdep)
  [...]

These invalid .cmd files are benign in some respects, but cause problems
in others (such as the linked reports).

Because the tools/ build system is rather complicated in its own right
(and pointedly different than Kbuild), I choose to simply open-code the
rule for building fixdep, and avoid the recursive-make indirection that
produces the problem in the first place.

Link: https://lore.kernel.org/all/Zk-C5Eg84yt6_nml@google.com/
Signed-off-by: Brian Norris <briannorris@...omium.org>
---

Changes in v3:
 - Drop unnecessary tools/build/Build

 tools/build/Build    |  3 ---
 tools/build/Makefile | 11 ++---------
 2 files changed, 2 insertions(+), 12 deletions(-)
 delete mode 100644 tools/build/Build

diff --git a/tools/build/Build b/tools/build/Build
deleted file mode 100644
index 76d1a4960973..000000000000
--- a/tools/build/Build
+++ /dev/null
@@ -1,3 +0,0 @@
-hostprogs := fixdep
-
-fixdep-y := fixdep.o
diff --git a/tools/build/Makefile b/tools/build/Makefile
index 17cdf01e29a0..fea3cf647f5b 100644
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -43,12 +43,5 @@ ifneq ($(wildcard $(TMP_O)),)
 	$(Q)$(MAKE) -C feature OUTPUT=$(TMP_O) clean >/dev/null
 endif

-$(OUTPUT)fixdep-in.o: FORCE
-	$(Q)$(MAKE) $(build)=fixdep
-
-$(OUTPUT)fixdep: $(OUTPUT)fixdep-in.o
-	$(QUIET_LINK)$(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ $<
-
-FORCE:
-
-.PHONY: FORCE
+$(OUTPUT)fixdep: $(srctree)/tools/build/fixdep.c
+	$(QUIET_CC)$(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ $<
-- 
2.45.2.803.g4e1b14247a-goog