linux-kernel - Re: [PATCH 2/1] Kbuild: fix # escaping in .cmd files for future Make

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Date:   Mon, 26 Mar 2018 20:48:00 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/1] Kbuild: fix # escaping in .cmd files for future Make

2018-03-26 8:09 GMT+09:00 Rasmus Villemoes <linux@...musvillemoes.dk>:
> The latest official Make release is 4.2.1 from mid-2016, but the current
> git release has this relevant note in the 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.
>
> Prepare for whatever future Make release contains that change by fixing
> up the .cmd file escaping - without this, make always thinks the command
> string has changed and hence rebuilds everything.
>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
> So the previous patch made everything build, but building again
> revealed that this central place very much also needed fixing.
>
>  scripts/Kbuild.include | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index 065324a8046f..7a926e4688f4 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
> @@ -328,7 +329,7 @@ endif
>  # (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)))))
>

Thanks for the patch, but this changes the behavior.
With '#' replaced with $(pound), '\\' does not escape anything,
so it is treated as '\\'.


The following keeps the current behavior:

make-cmd = $(call escsq,$(subst $(pound),\$(pound),$(subst
$$,$$$$,$(cmd_$(1)))))



But, I think the following is an even better fix:

make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst
$$,$$$$,$(cmd_$(1)))))



The following 4 test cases produce the same foo.txt,
but insane .cmd file is produced for some case.
Adjusting the number of back-slashes is not a perfect solution.

Replacing # with $(pound) works for all the cases.


[test code 1]
extra-y := foo.txt

quiet_cmd_foo = DEFINE   $@
      cmd_foo = echo '\#define FOO' >$@

$(obj)/foo.txt: FORCE
        $(call if_changed,foo)


[test code 2]
extra-y := foo.txt

quiet_cmd_foo = DEFINE   $@
      cmd_foo = echo \\\#define FOO >$@

$(obj)/foo.txt: FORCE
        $(call if_changed,foo)


[test code3]
extra-y := foo.txt

quiet_cmd_foo = DEFINE   $@
      cmd_foo = echo '$(pound)define FOO' >$@

$(obj)/foo.txt: FORCE
        $(call if_changed,foo)



[test code4]
extra-y := foo.txt

quiet_cmd_foo = DEFINE   $@
      cmd_foo = echo \$(pound)define FOO >$@

$(obj)/foo.txt: FORCE
        $(call if_changed,foo)





           | current | \$(pound) | \\$(pound) | $$(pound)
----------------------------------------------------------
test code1 |    OK   |    OK     |     NG     |   OK
test code2 |    NG   |    NG     |     OK     |   OK
test code3 |    OK   |    OK     |     NG     |   OK
test code4 |    NG   |    NG     |     OK     |   OK




-- 
Best Regards
Masahiro Yamada