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

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Date:	Wed, 6 Aug 2014 16:19:57 +0400
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Michal Marek <mmarek@...e.cz>
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 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.

>
> $ 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
> Kernel: arch/x86/boot/bzImage is ready  (#1)
>   Building modules, stage 2.
>   MODPOST 12 modules - due to target is PHONY
> $ cat arch/x86/realmode/rm/.pasyms.h.cmd
> cmd_arch/x86/realmode/rm/pasyms.h := nm arch/x86/realmode/rm/header.o
> arch/x86/realmode/rm/trampoline_64.o arch/x86/realmode/rm/stack.o
> arch/x86/realmode/rm/reboot.o arch/x86/realmode/rm/wakeup_asm.o
> arch/x86/realmode/rm/wakemain.o arch/x86/realmode/rm/video-mode.o
> arch/x86/realmode/rm/copy.o arch/x86/realmode/rm/bioscall.o
> arch/x86/realmode/rm/regs.o arch/x86/realmode/rm/video-vga.o
> arch/x86/realmode/rm/video-vesa.o arch/x86/realmode/rm/video-bios.o |
> sed -n -r -e 's/^([0-9a-fA-F]+) [ABCDGRSTVW] (.+)$$/pa_\2 = \2;/p' |
> sort | uniq > arch/x86/realmode/rm/pasyms.h
>
> This is with both 3.16 and my kbuild branch, which is based on 3.16-rc1.
> Also, in the description, you say that backslash escaping conflicts with
> escaping # and ', but this has nothing to do with the \2 backreference
> in the sed command.

I'm not quite understand what the difference between escaping standalone
backslash ( ' \ ' -> \' \\ \' ) that backreference (  ' \2 ' -> \' \\2 \' )

We have to double backslashes, but this must be done before escaping # and ',
otherwise doubling undoes their escaping.

>
> Michal
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