[PATCH 3/4] selftests/lib.mk: set PWD from CURDIR to avoid wrong extmod path

[Li Wang <liwang@redhat.com>]

When running selftests from the kernel top-level (e.g. make kselftest-all),
sub-makes might still inherit the caller's PWD from the environment.

Some selftests use $(PWD) in recursive kbuild invocations, which can
then incorrectly resolve to the kernel top directory instead of the
current test directory.

In that case, kbuild may generate an external-module wrapper Makefile in
the wrong location, potentially clobbering the top-level Makefile and
causing recursive include failures ("Too many open files").

Export PWD := $(CURDIR) in selftests/lib.mk so $(PWD) always matches the
actual current selftest directory.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Li Wang <liwang@redhat.com>
---
 tools/testing/selftests/lib.mk                | 2 ++
 tools/testing/selftests/mm/local_config.stamp | 0
 2 files changed, 2 insertions(+)
 create mode 100644 tools/testing/selftests/mm/local_config.stamp

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index f02cc8a2e4ae..2840e98c225c 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -60,6 +60,8 @@ endif
 selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
 top_srcdir = $(selfdir)/../../..
 
+export PWD := $(CURDIR)
+
 # msg: emit succinct information message describing current building step
 # $1 - generic step name (e.g., CC, LINK, etc);
 # $2 - optional "flavor" specifier; if provided, will be emitted as [flavor];
diff --git a/tools/testing/selftests/mm/local_config.stamp b/tools/testing/selftests/mm/local_config.stamp
new file mode 100644
index 000000000000..e69de29bb2d1
-- 
2.53.0