Re: [PATCH 2/5] selftests: Fix the .c linking rule

Currently we can't build some tests, for example:

  $ make -C tools/testing/selftests/ TARGETS=vm
  ...
  gcc -Wall -I ../../../../usr/include   -lrt -lpthread ../../../../usr/include/linux/kernel.h userfaultfd.c -o tools/testing/selftests/vm/userfaultfd
  /tmp/ccmOkQSM.o: In function `stress':
  userfaultfd.c:(.text+0xc60): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xca5): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xcee): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xd30): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xd77): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xe7d): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xe9f): undefined reference to `pthread_cancel'
  userfaultfd.c:(.text+0xec6): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xf14): undefined reference to `pthread_join'
  /tmp/ccmOkQSM.o: In function `userfaultfd_stress':
  userfaultfd.c:(.text+0x13e2): undefined reference to `pthread_attr_setstacksize'
  collect2: error: ld returned 1 exit status

This is because the rule for linking .c files to binaries is incorrect.

The first bug is that it uses $< (first prerequisite) instead of $^ (all
preqrequisites), fix it by using ^$.

Secondly the ordering of the prerequisites vs $(LDLIBS) is wrong,
meaning on toolchains that use --as-needed we fail to link (as above).
Fix that by placing $(LDLIBS) *after* ^$.

Finally switch to using the default rule $(LINK.c), so that we get
$(CPPFLAGS) etc. included.

Fixes: a8ba798bc8ec ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 tools/testing/selftests/lib.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index 17ed4bbe3963..98841c54763a 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk

@@ -51,7 +51,7 @@ clean:
        $(RM) -r $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES) $(EXTRA_CLEAN)

 $(OUTPUT)/%:%.c
-       $(CC) $(CFLAGS) $(LDFLAGS) $(LDLIBS) $< -o $@
+       $(LINK.c) $^ $(LDLIBS) -o $@

 $(OUTPUT)/%.o:%.S
        $(CC) $(ASFLAGS) -c $< -o $@
--

2.7.4

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