[PATCH v2] Kbuild: fix # escaping in .cmd files for future Make

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

I tried building using a freshly built Make (4.2.1-69-g8a731d1), but
already the objtool build broke with

orc_dump.c: In function ‘orc_dump’:
orc_dump.c:106:2: error: ‘elf_getshnum’ is deprecated [-Werror=deprecated-declarations]
  if (elf_getshdrnum(elf, &nr_sections)) {

Turns out that with that new Make, the backslash was not removed, so cpp
didn't see a #include directive, grep found nothing, and
-DLIBELF_USE_DEPRECATED was wrongly put in CFLAGS.

Now, that new Make behaviour is documented in their NEWS file:

  * WARNING: Backward-incompatibility!
    Number signs (#) appearing inside a macro reference or function invocation
    no longer introduce comments and should not be escaped with backslashes:
    thus a call such as:
      foo := $(shell echo '#')
    is legal.  Previously the number sign needed to be escaped, for example:
      foo := $(shell echo '\#')
    Now this latter will resolve to "\#".  If you want to write makefiles
    portable to both versions, assign the number sign to a variable:
      C := \#
      foo := $(shell echo '$C')
    This was claimed to be fixed in 3.81, but wasn't, for some reason.
    To detect this change search for 'nocomment' in the .FEATURES variable.

This also fixes up the two make-cmd instances to replace # with $(pound)
rather than with \#. There might very well be other places that need
similar fixup in preparation for whatever future Make release contains
the above change, but at least this builds an x86_64 defconfig with the
new make.

Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=197847
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
v2:

- squash the original objtool-only patch with the patch fixing up
  make-cmd, to avoid repeating the reference to the Make NEWS.

- use $(pound) in .cmd files to avoid backslash-counting issues (Masahiro).

- add bugzilla link (Randy).

 scripts/Kbuild.include         | 5 +++--
 tools/build/Build.include      | 5 +++--
 tools/objtool/Makefile         | 2 +-
 tools/scripts/Makefile.include | 2 ++
 4 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index cce31ee876b6..50cee534fd64 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -10,6 +10,7 @@ space   := $(empty) $(empty)
 space_escape := _-_SPACE_-_
 right_paren := )
 left_paren := (
+pound := \#
 
 ###
 # Name of target with a '.' as filename prefix. foo/bar.o => foo/.bar.o
@@ -322,11 +323,11 @@ endif
 
 # Replace >$< with >$$< to preserve $ when reloading the .cmd file
 # (needed for make)
-# Replace >#< with >\#< to avoid starting a comment in the .cmd file
+# Replace >#< with >$(pound)< to avoid starting a comment in the .cmd file
 # (needed for make)
 # Replace >'< with >'\''< to be able to enclose the whole string in '...'
 # (needed for the shell)
-make-cmd = $(call escsq,$(subst \#,\\\#,$(subst $$,$$$$,$(cmd_$(1)))))
+make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1)))))
 
 # Find any prerequisites that is newer than target or that does not exist.
 # PHONY targets skipped in both cases.
diff --git a/tools/build/Build.include b/tools/build/Build.include
index 418871d02ebf..a4bbb984941d 100644
--- a/tools/build/Build.include
+++ b/tools/build/Build.include
@@ -12,6 +12,7 @@
 # Convenient variables
 comma   := ,
 squote  := '
+pound   := \#
 
 ###
 # Name of target with a '.' as filename prefix. foo/bar.o => foo/.bar.o
@@ -43,11 +44,11 @@ echo-cmd = $(if $($(quiet)cmd_$(1)),\
 ###
 # Replace >$< with >$$< to preserve $ when reloading the .cmd file
 # (needed for make)
-# Replace >#< with >\#< to avoid starting a comment in the .cmd file
+# Replace >#< with >$(pound)< to avoid starting a comment in the .cmd file
 # (needed for make)
 # Replace >'< with >'\''< to be able to enclose the whole string in '...'
 # (needed for the shell)
-make-cmd = $(call escsq,$(subst \#,\\\#,$(subst $$,$$$$,$(cmd_$(1)))))
+make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1)))))
 
 ###
 # Find any prerequisites that is newer than target or that does not exist.
diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index e6acc281dd37..8ae824dbfca3 100644
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -35,7 +35,7 @@ CFLAGS   += -Wall -Werror $(WARNINGS) -fomit-frame-pointer -O2 -g $(INCLUDES)
 LDFLAGS  += -lelf $(LIBSUBCMD)
 
 # Allow old libelf to be used:
-elfshdr := $(shell echo '\#include <libelf.h>' | $(CC) $(CFLAGS) -x c -E - | grep elf_getshdr)
+elfshdr := $(shell echo '$(pound)include <libelf.h>' | $(CC) $(CFLAGS) -x c -E - | grep elf_getshdr)
 CFLAGS += $(if $(elfshdr),,-DLIBELF_USE_DEPRECATED)
 
 AWK = awk
diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index dd614463d4d6..495066bafbe3 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -120,3 +120,5 @@ ifneq ($(silent),1)
 	QUIET_UNINST   = @printf '  UNINST   %s\n' $1;
   endif
 endif
+
+pound := \#
-- 
2.15.1