[RFC PATCH 3/3] kbuild: use bash as the default shell for Make and Kconfig

[Masahiro Yamada <masahiroy@kernel.org>]

GNU Make uses /bin/sh by default for running recipe lines and $(shell ),
and so does Kconfig for $(shell,...).

We are sometimes hit by subtle portability issues like [1] because the
code was only tested on systems where /bin/sh is a symlink to bash.

One way is to continue whack-a-mole game as somebody will eventually
notice that the code is not working on their build environments.

Another is a big hammer approach; force bash in the build system so that
it works in the same way for everyone. In fact, there exists such a
guideline that permits only Bash as the shell scripting language. [2]
The kbuild test robot also specifies SHELL=/bin/bash to eliminate the
shell portability issue.

This commit changes the shell for Make and Kconfig to bash. As the GNU
Make manual [3] says, the 'SHELL' is never set from the environment.
You cannot do 'export SHELL:=/bin/bash' to propagate it to sub-makes.
Instead, use MAKEOVERRIDES to make SHELL:=/bin/bash globally effective.

[1]: https://lore.kernel.org/all/20220804190320.262510-1-alexandre.belloni@bootlin.com/
[2]: https://google.github.io/styleguide/shellguide.html
[3]: https://www.gnu.org/software/make/manual/html_node/Choosing-the-Shell.html

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 Makefile                | 7 +++++++
 scripts/Kconfig.include | 3 +++
 2 files changed, 10 insertions(+)

diff --git a/Makefile b/Makefile
index f09673b6c11d..cc01987560b3 100644
--- a/Makefile
+++ b/Makefile
@@ -36,6 +36,13 @@ __all:
 
 ifneq ($(sub_make_done),1)
 
+# Use /bin/bash for repices and $(shell ) invocations.
+SHELL := /bin/bash
+
+# Unlike the other variables, the 'SHELL' is never set from the environment.
+# Use MAKEOVERRIDES to pass 'SHELL' down to sub-makes.
+MAKEOVERRIDES += SHELL=$(SHELL)
+
 # Do not use make's built-in rules and variables
 # (this increases performance and avoids hard-to-debug behaviour)
 MAKEFLAGS += -rR
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index a0ccceb22cf8..d37b864ba1f2 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -1,6 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
 # Kconfig helper macros
 
+# Use /bin/bash to evaluate $(shell,) functions
+SHELL := /bin/bash
+
 # Convenient variables
 comma       := ,
 quote       := "
-- 
2.34.1