[PATCH for-next v2] selftests/Makefile: override the srctree for out-of-tree builds

[Li Zhijian <lizhijian@xxxxxxxxxxx>]

Fixes an issue where out-of-tree kselftest builds fail when building
the BPF and bpftools components. The failure occurs because the top-level
Makefile passes a relative srctree path to its sub-Makefiles, which
leads to errors in locating necessary files.

For example, the following error is encountered:

```
$ make V=1 O=$build/ TARGETS=hid kselftest-all
...
make -C ../tools/testing/selftests all
make[4]: Entering directory '/path/to/linux/tools/testing/selftests/hid'
make  -C /path/to/linux/tools/testing/selftests/../../../tools/lib/bpf OUTPUT=/path/to/linux/O/kselftest/hid/tools/build/libbpf/ \
            EXTRA_CFLAGS='-g -O0'                                      \
            DESTDIR=/path/to/linux/O/kselftest/hid/tools prefix= all install_headers
make[5]: Entering directory '/path/to/linux/tools/lib/bpf'
...
make[5]: Entering directory '/path/to/linux/tools/bpf/bpftool'
Makefile:127: ../tools/build/Makefile.feature: No such file or directory
make[5]: *** No rule to make target '../tools/build/Makefile.feature'.  Stop.
```

To resolve this, override the srctree in the kselftests's top Makefile
when performing an out-of-tree build. This ensures that all sub-Makefiles
have the correct path to the source tree, preventing directory resolution
errors.

Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
---
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>

V2:
 - handle srctree in selftests itself rather than the linux' top Makefile # Masahiro Yamada <masahiroy@xxxxxxxxxx>

V1: https://lore.kernel.org/lkml/20241217031052.69744-1-lizhijian@xxxxxxxxxxx/
---
 tools/testing/selftests/Makefile | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 3d8a80abd4f0..ab82278353cf 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -154,15 +154,19 @@ override LDFLAGS =
 override MAKEFLAGS =
 endif
 
+top_srcdir ?= ../../..
+
 # Append kselftest to KBUILD_OUTPUT and O to avoid cluttering
 # KBUILD_OUTPUT with selftest objects and headers installed
 # by selftests Makefile or lib.mk.
+# Override the `srctree` variable to ensure it is correctly resolved in
+# sub-Makefiles, such as those within `bpf`, when managing targets like
+# `net` and `hid`.
 ifdef building_out_of_srctree
 override LDFLAGS =
+override srctree := $(top_srcdir)
 endif
 
-top_srcdir ?= ../../..
-
 ifeq ("$(origin O)", "command line")
   KBUILD_OUTPUT := $(O)
 endif
-- 
2.44.0