[PATCH v1] kbuild: fix sub directory output build of kselftests

[Muhammad Usama Anjum]

Build of kselftests fail if kernel's top most Makefile is used for
running or building kselftests with separate output directory which is
sub-directory. srctree is set to .. erroneously.

make kselftest-all O=/linux_mainline/build
Makefile:1080: ../scripts/Makefile.extrawarn: No such file or directory

make kselftest-all O=build
Makefile:1080: ../scripts/Makefile.extrawarn: No such file or directory

Fix this by comparing abs_srctree with CURDIR instead of abs_objtree.
CURDIR changes based on from where the command has been run and it sets
the srctree correctly.

Fixes: 25b146c5b8cee("kbuild: allow Kbuild to start from any directory")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
---
Changes in V2:
- Correct the bugfix instead of workaround

V1: https://lore.kernel.org/lkml/20220223191016.1658728-1-usama.anjum@xxxxxxxxxxxxx/
---
 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index e66358b64ede..4090d7afcda4 100644
--- a/Makefile
+++ b/Makefile
@@ -238,12 +238,12 @@ ifeq ($(need-sub-make),)
 # so that IDEs/editors are able to understand relative filenames.
 MAKEFLAGS += --no-print-directory
 
-ifeq ($(abs_srctree),$(abs_objtree))
+ifeq ($(abs_srctree),$(CURDIR))
         # building in the source tree
         srctree := .
 	building_out_of_srctree :=
 else
-        ifeq ($(abs_srctree)/,$(dir $(abs_objtree)))
+        ifeq ($(abs_srctree)/,$(dir $(CURDIR)))
                 # building in a subdirectory of the source tree
                 srctree := ..
         else
-- 
2.30.2