kbuild: doc: describe proper script invocation

During an investigation to fix up the execute bits of scripts in the
kernel repository, Andrew Morton and Kees Cook pointed out that the
execute bit should not matter, and that build scripts cannot rely on that.
Kees could not point to any documentation, though.

Masahiro Yamada explained the convention of setting execute bits to make
it easier for manual script invocation.

Provide some basic documentation how the build shall invoke scripts, such
that the execute bits do not matter, and acknowledge that execute bits are
useful nonetheless.

This serves as reference for further clean-up patches in the future.
Suggested-by: NAndrew Morton <akpm@linux-foundation.org>
Suggested-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NLukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ujjwal Kumar <ujjwalkumar0501@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/lkml/20200830174409.c24c3f67addcce0cea9a9d4c@linux-foundation.org/
Link: https://lore.kernel.org/lkml/202008271102.FEB906C88@keescook/
Link: https://lore.kernel.org/linux-kbuild/CAK7LNAQdrvMkDA6ApDJCGr+5db8SiPo=G+p8EiOvnnGvEN80gA@mail.gmail.com/
Link: https://lkml.kernel.org/r/20201001075723.24246-1-lukas.bulwahn@gmail.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/kbuild/makefiles.rst

@@ -21,6 +21,7 @@ This document describes the Linux kernel Makefiles.
 	   --- 3.10 Special Rules
 	   --- 3.11 $(CC) support functions
 	   --- 3.12 $(LD) support functions
+	   --- 3.13 Script Invocation
 
 	=== 4 Host Program support
 	   --- 4.1 Simple Host Program
@@ -605,6 +606,25 @@ more details, with real examples.
 		#Makefile
 		LDFLAGS_vmlinux += $(call ld-option, -X)
 
+3.13 Script invocation
+----------------------
+
+	Make rules may invoke scripts to build the kernel. The rules shall
+	always provide the appropriate interpreter to execute the script. They
+	shall not rely on the execute bits being set, and shall not invoke the
+	script directly. For the convenience of manual script invocation, such
+	as invoking ./scripts/checkpatch.pl, it is recommended to set execute
+	bits on the scripts nonetheless.
+
+	Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
+	$(PYTHON) and $(PYTHON3) to refer to interpreters for the respective
+	scripts.
+
+	Example::
+
+		#Makefile
+		cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
+			     $(KERNELRELEASE)
 
 4 Host Program support
 ======================