[PATCH] selftests/powerpc: Fix try-run when source tree is not writable

[Michael Ellerman <mpe@ellerman.id.au>]

We added a usage of try-run to pmu/ebb/Makefile to detect if the
toolchain supported the -no-pie option.

This fails if we build out-of-tree and the source tree is not
writable, as try-run tries to write its temporary files to the current
directory. That leads to the -no-pie option being silently dropped,
which leads to broken executables with some toolchains.

If we remove the redirect to /dev/null in try-run, we see the error:

  make[3]: Entering directory '/linux/tools/testing/selftests/powerpc/pmu/ebb'
  /usr/bin/ld: cannot open output file .54.tmp: Read-only file system
  collect2: error: ld returned 1 exit status
  make[3]: Nothing to be done for 'all'.

And looking with strace we see it's trying to use a file that's in the
source tree:

  lstat("/linux/tools/testing/selftests/powerpc/pmu/ebb/.54.tmp", 0x7ffffc0f83c8)

We can fix it by setting TMPOUT to point to the $(OUTPUT) directory,
and we can verify with strace it's now trying to write to the output
directory:

  lstat("/output/kselftest/powerpc/pmu/ebb/.54.tmp", 0x7fffd1bf6bf8)

And also see that the -no-pie option is now correctly detected.

Fixes: 0695f8bca93e ("selftests/powerpc: Handle Makefile for unrecognized option")
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 tools/testing/selftests/powerpc/pmu/ebb/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/powerpc/pmu/ebb/Makefile b/tools/testing/selftests/powerpc/pmu/ebb/Makefile
index 417306353e07..ca35dd8848b0 100644
--- a/tools/testing/selftests/powerpc/pmu/ebb/Makefile
+++ b/tools/testing/selftests/powerpc/pmu/ebb/Makefile
@@ -7,6 +7,7 @@ include ../../../../../../scripts/Kbuild.include
 # The EBB handler is 64-bit code and everything links against it
 CFLAGS += -m64
 
+TMPOUT = $(OUTPUT)/
 # Toolchains may build PIE by default which breaks the assembly
 no-pie-option := $(call try-run, echo 'int main() { return 0; }' | \
         $(CC) -Werror $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -no-pie -x c - -o "$$TMP", -no-pie)

base-commit: 11a48a5a18c63fd7621bb050228cebf13566e4d8
prerequisite-patch-id: e460261e180666bfb63bcd0cc554874d73c3b71f
prerequisite-patch-id: 67db368cfdf8d3aefd78f140420281f9b4b53e07
prerequisite-patch-id: cf5d957a366998b4a3ce70a79b6e969eb98fca7d
prerequisite-patch-id: 3ace935c6ae425ad635eb38f906e790c3c9bf41f
-- 
2.25.1