[PATCH v3 1/3] tools build: Correct libsubcmd fixdep dependencies

[Brian Norris]

All built targets need fixdep to be built first, before handling object
dependencies [1]. We're missing one such dependency before the libsubcmd
target.

This resolves .cmd file generation issues such that the following
sequence produces many fewer results:

  $ git clean -xfd tools/
  $ make tools/objtool
  $ grep "cannot find fixdep" $(find tools/objtool -name '*.cmd')

In particular, only a buggy tools/objtool/libsubcmd/.fixdep.o.cmd
remains, due to circular dependencies of fixdep on itself.

Such incomplete .cmd files don't usually cause a direct problem, since
they're designed to fail "open", but they can cause some subtle problems
that would otherwise be handled by proper fixdep'd dependency files. [2]

[1] This problem is better described in commit abb26210a395 ("perf
tools: Force fixdep compilation at the start of the build"). I don't
apply its solution here, because additional recursive make can be a bit
of overkill.

[2] Example failure case:

  cp -arl linux-src linux-src2
  cd linux-src2
  make O=/path/to/out
  cd ../linux-src
  rm -rf ../linux-src2
  make O=/path/to/out

Previously, we'd see errors like:

  make[6]: *** No rule to make target
  '/path/to/linux-src2/tools/include/linux/compiler.h', needed by
  '/path/to/out/tools/bpf/resolve_btfids/libsubcmd/exec-cmd.o'.  Stop.

Now, the properly-fixdep'd .cmd files will ignore a missing
/path/to/linux-src2/...

Link: https://lore.kernel.org/all/ZGVi9HbI43R5trN8@bhelgaas/
Link: https://lore.kernel.org/all/Zk-C5Eg84yt6_nml@xxxxxxxxxx/
Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---

Changes in v3:
 - update notes about failure cases
 - add Jiri's Acked-by

 tools/lib/subcmd/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/subcmd/Makefile b/tools/lib/subcmd/Makefile
index b87213263a5e..59b09f280e49 100644
--- a/tools/lib/subcmd/Makefile
+++ b/tools/lib/subcmd/Makefile
@@ -76,7 +76,7 @@ include $(srctree)/tools/build/Makefile.include
 
 all: fixdep $(LIBFILE)
 
-$(SUBCMD_IN): FORCE
+$(SUBCMD_IN): fixdep FORCE
 	@$(MAKE) $(build)=libsubcmd
 
 $(LIBFILE): $(SUBCMD_IN)
-- 
2.45.2.803.g4e1b14247a-goog