linux-kernel - Re: [PATCH] kbuild: escape single backslashes in macro make-cmd

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Date:	Thu, 7 Aug 2014 18:07:39 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kbuild: escape single backslashes in macro make-cmd

On Wed, Aug 06, 2014 at 05:01:57PM +0200, Michal Marek wrote:
> On 2014-08-06 14:19, Konstantin Khlebnikov wrote:
> > On Wed, Aug 6, 2014 at 3:45 PM, Michal Marek <mmarek@...e.cz> wrote:
> >> On 2014-07-26 18:35, Konstantin Khlebnikov wrote:
> >>> This already has been fixed in commit c353acba28fb3fa1fd05fd
> >>> ("kbuild: make: fix if_changed when command contains backslashes")
> >>> but escaping still isn't perfect and triggers false-positive rebuilds.
> >>>
> >>> For x86 problem happens every time, because rules in arch/x86/realmode/rm/
> >>> and arch/x86/boot/ contains commands like sed -n -e 's/foo\(.*\)/\1/p'.
> >>> Backslash in \1 isn't escaped and turns into ascii symbol with code 1.
> >>> Macro if_changed detects command change and rebuilds target again and again.
> >>>
> >>> Backslash escaping conflicts with other passes because it's used for escaping
> >>> other symbols. To avoid that current macro handles only double backslashes.
> >>> Obviously this doesn't work for \1 like above.
> >>>
> >>> This patch reorders passes. It doubles all backslashes before escaping # and '
> >>>
> >>> Visible effect in rebuilding x86/defconfig without changes, before patch:
> >>>
> >>> blind@...g:~/src/linux$ make V=2
> >>>   CHK     include/config/kernel.release
> >>>   CHK     include/generated/uapi/linux/version.h
> >>>   CHK     include/generated/utsrelease.h
> >>>   CALL    scripts/checksyscalls.sh - due to target missing
> >>>   CHK     include/generated/compile.h
> >>>   PASYMS  arch/x86/realmode/rm/pasyms.h - due to command line change
> >>
> >> With which make and shell version are you seeing this? While the patch
> >> looks correct, I can't reproduce the error here:
> > 
> > /bin/sh points to dash (debian default setup).
> > 
> > I cannot reproduce this using bash. That explains why this bug is still here.
> 
> So the difference between the shells is that their 'echo' builtin treats
> \<number> differently:
> $ ./dash -c "echo '\2'"
> ..
> $ ./dash -c "echo '\2'" | xxd
> 0000000: 020a                                     ..
> $ /bin/bash -c "echo '\2'"
> \2

The problem is that the bash echo does not interpred _any_ \-sequences
without -e. So the nicely escaped \\ stays a \\ and we get spurious
rebuilds with bash instead. It previously worked with bash, because the
backslash escaping was completely broken. You not only moved it up in
the chain, but also fixed it by removing one superflous level of
escaping.

I'm now testing this, i.e. dropping the backslash escaping completely
and doing a printf '%s\n' instead. Can you try if it works for you?


diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 122f95c..8a9a4e1 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -215,11 +215,13 @@ else
 arg-check = $(if $(strip $(cmd_$@)),,1)
 endif
 
-# >'< substitution is for echo to work,
-# >$< substitution to preserve $ when reloading .cmd file
-# note: when using inline perl scripts [perl -e '...$$t=1;...']
-# in $(cmd_xxx) double $$ your perl vars
-make-cmd = $(subst \\,\\\\,$(subst \#,\\\#,$(subst $$,$$$$,$(call escsq,$(cmd_$(1))))))
+# Replace >$< with >$$< to preserve $ when reloading the .cmd file
+# (needed for make)
+# Replace >#< with >\#< to avoid starting a comment in the .cmd file
+# (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)))))
 
 # Find any prerequisites that is newer than target or that does not exist.
 # PHONY targets skipped in both cases.
@@ -230,7 +232,7 @@ any-prereq = $(filter-out $(PHONY),$?) $(filter-out $(PHONY) $(wildcard $^),$^)
 if_changed = $(if $(strip $(any-prereq) $(arg-check)),                       \
 	@set -e;                                                             \
 	$(echo-cmd) $(cmd_$(1));                                             \
-	echo 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd)
+	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd)
 
 # Execute the command and also postprocess generated .d dependencies file.
 if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ),                  \
--
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