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Date:   Sat, 10 Feb 2018 09:55:19 +0100
From:   Ulf Magnusson <ulfalizer@gmail.com>
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "Luis R . Rodriguez" <mcgrof@suse.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Pavel Machek <pavel@ucw.cz>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 4/7] kconfig: support new special property shell=
Message-ID: <20180210085519.737ckf4bcl57h4g2@huvuddator>
References: <1518106752-29228-1-git-send-email-yamada.masahiro@socionext.com>
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On Sat, Feb 10, 2018 at 09:05:56AM +0100, Ulf Magnusson wrote:
> On Sat, Feb 10, 2018 at 08:49:24AM +0100, Ulf Magnusson wrote:
> > On Sat, Feb 10, 2018 at 04:12:13PM +0900, Masahiro Yamada wrote:
> > > 2018-02-10 14:48 GMT+09:00 Ulf Magnusson <ulfalizer@gmail.com>:
> > > > On Fri, Feb 09, 2018 at 12:46:54PM -0800, Kees Cook wrote:
> > > >> On Fri, Feb 9, 2018 at 4:46 AM, Ulf Magnusson <ulfalizer@gmail.com> wrote:
> > > >> > One thing that makes Kconfig confusing (though it works well enough in
> > > >> > practice) is that .config files both record user selections (the saved
> > > >> > configuration) and serve as a configuration output format for make.
> > > >> >
> > > >> > It becomes easier to think about .config files once you realize that
> > > >> > assignments to promptless symbols never have an effect on Kconfig
> > > >> > itself: They're just configuration output, intermixed with the saved
> > > >> > user selections.
> > > >> >
> > > >> > Assume 'option env' symbols got written out for example:
> > > >> >
> > > >> >         - For a non-user-assignable symbol, the entry in the .config
> > > >> >           file is just configuration output and ignored by Kconfig,
> > > >> >           which will fetch the value from the environment instead.
> > > >> >
> > > >> >         - For an assignable 'option env' symbol, the entry in the
> > > >> >           .config file is a saved user selection (as well as
> > > >> >           configuration output), and will be respected by Kconfig.
> > > >>
> > > >> In the stack-protector case, this becomes quite important, since the
> > > >> goal is to record the user's selection regardless of compiler
> > > >> capability. For example, if someone selects _REGULAR, it shouldn't
> > > >> "upgrade" to _STRONG. (Similarly for _NONE.) Having _AUTO provides a
> > > >> way to pick "best possible for this compiler", though. If a user had
> > > >> previously selected _STRONG but they're doing builds with an older
> > > >> compiler (or a misconfigured newer compiler) without support, the goal
> > > >> is to _fail_ to build, not silently select _REGULAR.
> > > >>
> > > >> So, in this case, what's gained is the logic for _AUTO, and the logic
> > > >> to not show, say, _STRONG when it's not available in the compiler. But
> > > >> we must still fail to build if _STRONG was in the .config. It can't
> > > >> silently rewrite it to _REGULAR because the compiler support for
> > > >> _STRONG regressed.
> > > >>
> > > >> -Kees
> > > >>
> > > >> --
> > > >> Kees Cook
> > > >> Pixel Security
> > > >
> > > > Provided that would be the desired behavior:
> > > >
> > > > What about changing the meaning of the choice symbols from e.g. "select
> > > > -fstack-protector-strong" to "want -fstack-protector-strong"? Then the
> > > > user preference would always be remembered, regardless of what's
> > > > available.
> > > >
> > > > Here's a proof-of-concept. I realized that the fancy new 'imply' keyword
> > > > fits pretty well here, since it works like a dependency-respecting
> > > > select.
> > > >
> > > >         config CC_HAS_STACKPROTECTOR_STRONG
> > > >                 bool
> > > >                 option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"
> > > >
> > > >         config CC_HAS_STACKPROTECTOR
> > > >                 bool
> > > >                 option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
> > > >
> > > >
> > > >         choice
> > > >                 prompt "Stack Protector buffer overflow detection"
> > > >                 default WANT_CC_STACKPROTECTOR_STRONG
> > > >
> > > >         config WANT_CC_STACKPROTECTOR_STRONG
> > > >                 bool "Strong"
> > > >                 imply CC_STACKPROTECTOR_STRONG
> > > >
> > > >         config WANT_CC_STACKPROTECTOR_REGULAR
> > > >                 bool "Regular"
> > > >                 imply CC_STACKPROTECTOR_REGULAR
> > > >
> > > >         config WANT_CC_STACKPROTECTOR_NONE
> > > >                 bool "None"
> > > >                 imply CC_STACKPROTECTOR_NONE
> > > >
> > > >         endchoice
> > > >
> > > >
> > > >         config CC_STACKPROTECTOR_STRONG
> > > >                 bool
> > > >                 depends on CC_HAS_STACKPROTECTOR_STRONG
> > > 
> > > 
> > > Do you mean
> > > 
> > >          config CC_STACKPROTECTOR_STRONG
> > >                  bool
> > >                  depends on CC_HAS_STACKPROTECTOR_STRONG && \
> > >                             WANT_CC_STACKPROTECTOR_STRONG
> > > 
> > > or, maybe
> > > 
> > > 
> > >          config CC_STACKPROTECTOR_STRONG
> > >                  bool
> > >                  depends on CC_HAS_STACKPROTECTOR_STRONG
> > >                  default WANT_CC_STACKPROTECTOR_STRONG
> > > 
> > > ?
> > 
> > With the 'imply', it should work with just the 'depends on'. I had your
> > last version earlier though, and it works too.
> > 
> > 'imply' kinda makes sense, as in "turn on the strong stack protector if
> > its dependencies are satisfied".
> > 
> > > 
> > > 
> > > 
> > > 
> > > 
> > > >         config CC_STACKPROTECTOR_REGULAR
> > > >                 bool
> > > >                 depends on CC_HAS_STACKPROTECTOR_REGULAR
> > > >
> > > >         config CC_STACKPROTECTOR_NONE
> > > >                 bool
> > > >
> > > > This version has the drawback of always showing all the options, even if
> > > > some they wouldn't be available. Kconfig comments could be added to warn
> > > > if an option isn't available at least:
> > > >
> > > >         comment "Warning: Your compiler does not support -fstack-protector-strong"
> > > >                 depends on !CC_HAS_STACKPROTECTOR_STRONG
> > > >
> > > >         config WANT_CC_STACKPROTECTOR_STRONG
> > > >                 ...
> > > >
> > > >
> > > >         comment "Warning: Your compiler does not support -fstack-protector"
> > > >                 depends on !CC_HAS_STACKPROTECTOR_REGULAR
> > > >
> > > >         config WANT_CC_STACKPROTECTOR_REGULAR
> > > >                 ...
> > > >
> > > > This final comment might be nice to have too:
> > > >
> > > >         comment "Warning: Selected stack protector not available"
> > > >                 depends on !(CC_STACKPROTECTOR_STRONG ||
> > > >                              CC_STACKPROTECTOR_REGULAR ||
> > > >                              CC_STACKPROTECTOR_NONE)
> > > >
> > > > Should probably introduce a clear warning that tells the user what they
> > > > need to change in Kconfig if they build with a broken selection too.
> > > >
> > > >
> > > > CC_STACKPROTECTOR_AUTO could be added to the choice in a slightly kludgy
> > > > way too. Maybe there's something neater.
> > > >
> > > >         config CC_STACKPROTECTOR_AUTO
> > > >                 bool "Automatic"
> > > >                 imply CC_STACKPROTECTOR_STRONG
> > > >                 imply CC_STACKPROTECTOR_REGULAR if !CC_HAS_STACKPROTECTOR_STRONG
> > > >                 imply CC_STACKPROTECTOR_NONE    if !CC_HAS_STACKPROTECTOR_STRONG && \
> > > >                                                    !CC_HAS_STACKPROTECTOR_REGULAR
> > > >
> > > >
> > > > Another drawback of this approach is that it breaks existing .config
> > > > files (the CC_STACKPROTECTOR_* settings are ignored, since they just
> > > > look like "configuration output" to Kconfig now). If that'd be a
> > > > problem, the old names could be used instead of
> > > > WANT_CC_STACKPROTECTOR_STRONG, etc., and new names introduced instead,
> > > > though it'd look a bit cryptic.
> > > >
> > > > Ideas?
> > > >
> > > 
> > > 
> > > 
> > > FWIW, the following is what I was playing with.
> > > (The idea for emitting warnings is Ulf's idea)
> > > 
> > > 
> > > ------------------>8-------------------
> > > config CC
> > >         string
> > >         option env="CC"
> > > 
> > > config CC_HAS_STACKPROTECTOR
> > >         bool
> > >         option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
> > > 
> > > config CC_HAS_STACKPROTECTOR_STRONG
> > >         bool
> > >         option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"
> > > 
> > > config CC_HAS_STACKPROTECTOR_NONE
> > >         bool
> > >         option shell="$CC -Werror -fno-stack-protector -c -x c /dev/null"
> > > 
> > > config CC_STACKPROTECTOR
> > >         bool
> > > 
> > > choice
> > >         prompt "Stack Protector buffer overflow detection"
> > > 
> > > config CC_STACKPROTECTOR_AUTO
> > >         bool "Auto"
> > >         select CC_STACKPROTECTOR if (CC_HAS_STACKPROTECTOR || \
> > >                                      CC_HAS_STACKPROTECTOR_STRONG)
> > 
> > With this approach, I guess you would still need to handle the
> > CC_STACKPROTECTOR_AUTO logic outside of Kconfig, since e.g.
> > CC_STACKPROTECTOR_STRONG won't get enabled automatically if supported.
> > 
> > The idea above was to make it "internal" to the Kconfig files (though it
> > still gets written out), with the
> > CC_STACKPROTECTOR_{REGULAR,STRONG,NONE} variables automatically getting
> > set as appropriate.
> 
> That was a confusing way of putting it -- sorry about that.
> 
> What I meant was that it would just be a user selection, with all the
> logic of selecting one of CC_STACKPROTECTOR_{REGULAR,STRONG,NONE} being
> handled internally in the Kconfig files, even in the
> CC_STACKPROTECTOR_AUTO case.
> 
> Nothing outside of Kconfig would need to check CC_STACKPROTECTOR_AUTO
> then.
> 
> > 
> > The build could then the detect if none of
> > CC_STACKPROTECTOR_{REGULAR,STRONG,NONE} are set and do what's
> > appropriate (error out in some semi-helpful way or whatever... not
> > deeply familiar with kernel policy here :).
> > 
> > > 
> > > config CC_STACKPROTECTOR_REGULAR
> > >         bool "Regular"
> > >         select CC_STACKPROTECTOR
> > > 
> > > config CC_STACKPROTECTOR_STRONG
> > >         bool "Strong"
> > >         select CC_STACKPROTECTOR
> > > 
> > > config CC_STACKPROTECTOR_NONE
> > >         bool "None"
> > > 
> > > endchoice
> > > 
> > > 
> > > comment "(WARNING) stackprotecter was chosen, but your compile does
> > > not support it.  Build will fail"
> > >         depends on CC_STACKPROTECTOR_REGULAR && \
> > >                    !CC_HAS_STACKPROTECTOR
> > > 
> > > comment "(WARNING) stackprotecter-strong was chosen, but your compile
> > > does not support it.  Build will fail"
> > >         depends on CC_STACKPROTECTOR_STRONG && \
> > >                    !CC_HAS_STACKPROTECTOR_STRONG
> > > ------------------------->8---------------------------------
> > > 
> > > 
> > > 
> > > 
> > > 
> > > BTW, setting option flags in Makefile is dirty, like follows:
> > > 
> > > 
> > > ccflags-$(CONFIG_CC_STACKPROTECTOR_STRONG)  += -fstack-protector-strong
> > > ccflags-$(CONFIG_CC_STACKPROTECTOR_REGULAR) += -fstack-protector
> > > 
> > > if ($(CONFIG_CC_STACKPROTECTOR_AUTO),y)
> > > ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR)        += -fstack-protector
> > > ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_STRONG) += -fstack-protector-strong
> > > ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE)   += -fno-stack-protector
> > > endif
> > > 
> > > if ($(CONFIG_CC_STACKPROTECTOR_NONE),y)
> > > ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE)   += -fno-stack-protector
> > > endif
> > > 
> > > 
> > > 
> > > 
> > > One idea could be to calculate the compiler option in Kconfig.
> > > 
> > > config CC_OPT_STACKPROTECTOR
> > >         string
> > >         default "-fstack-protector-strong" if CC_STACKPROTECTOR_STRONG || \
> > >                                              (CC_STACKPROTECTOR_AUTO && \
> > >                                               CC_HAS_STACKPROTECTOR_STRONG)
> > >         default "-fstack-protector"        if CC_STACKPROTECTOR_REGULAR || \
> > >                                               (CC_STACKPROTECTOR_AUTO && \
> > >                                                CC_HAS_STACKPROTECTOR)
> > >         default "-fno-stack-protector"        if CC_HAS_STACKPROTECTOR_NONE
> > 
> > If CC_STACKPROTECTOR_AUTO is made "internal", this could be simplified
> > to something like
> > 
> > 	config CC_OPT_STACKPROTECTOR
> > 		string
> > 		default "-fstack-protector-strong" if CC_STACKPROTECTOR_STRONG
> > 		default "-fstack-protector"        if CC_STACKPROTECTOR_REGULAR
> > 		default "-fno-stack-protector"     if CC_HAS_STACKPROTECTOR_NONE
> > 		# If the compiler doesn't even support
> > 		# -fno-stack-protector
> > 		default ""
> > 
> > (Last default is just to make the empty string explicit. That's the
> > value it would get anyway.)
> > 
> > > 
> > > 
> > > 
> > > Makefile will become clean.
> > > Of course, this is at the cost of ugliness in Kconfig.
> > > 
> > > 
> > > 
> > > 
> > > -- 
> > > Best Regards
> > > Masahiro Yamada
> > 
> > Please tell me if I've misunderstood some aspect of the old behavior.
> > 
> > Cheers,
> > Ulf
> 
> Cheers,
> Ulf

Here's a complete updated example, with some stuff from Masahiro added.

Turns out warnings inside choices get cut off easily in menuconfig, so I
went with just a single warning instead (which should be enough anyway).

With this version, the only "outputs" that the Makefiles needs to look
at are CC_STACKPROTECTOR_{STRONG,REGULAR,NONE} (and
CC_OPT_STACKPROTECTOR). WANT_CC_OPT_STACKPROTECTOR_AUTO is handled
automatically.

The caveat related to old .config files mentioned above still applies.

How many compilers don't support -fno-stack-protector by the way?

	config CC_HAS_STACKPROTECTOR_STRONG
		bool
		option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"
	
	config CC_HAS_STACKPROTECTOR_REGULAR
		bool
		option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
	
	config CC_HAS_STACKPROTECTOR_NONE
		bool
		default y
		option shell="$CC -Werror -fno-stack-protector -c -x c /dev/null"
	
	
	choice
		prompt "Stack Protector buffer overflow detection"
		default WANT_CC_STACKPROTECTOR_AUTO
	
	config WANT_CC_STACKPROTECTOR_AUTO
		bool "Automatic"
		imply CC_STACKPROTECTOR_STRONG
		imply CC_STACKPROTECTOR_REGULAR if !CC_HAS_STACKPROTECTOR_STRONG
		imply CC_STACKPROTECTOR_NONE    if !CC_HAS_STACKPROTECTOR_STRONG && \
						   !CC_HAS_STACKPROTECTOR_REGULAR
	
	config WANT_CC_STACKPROTECTOR_STRONG
		bool "Strong"
		imply CC_STACKPROTECTOR_STRONG
	
	config WANT_CC_STACKPROTECTOR_REGULAR
		bool "Regular"
		imply CC_STACKPROTECTOR_REGULAR
	
	config WANT_CC_STACKPROTECTOR_NONE
		bool "None"
		imply CC_STACKPROTECTOR_NONE
	
	endchoice
	
	comment "Warning: Selected stack protector not available"
		depends on !(CC_STACKPROTECTOR_STRONG || \
			     CC_STACKPROTECTOR_REGULAR || \
			     CC_STACKPROTECTOR_NONE)
	
	
	config CC_STACKPROTECTOR_STRONG
		bool
		depends on CC_HAS_STACKPROTECTOR_STRONG
	
	config CC_STACKPROTECTOR_REGULAR
		bool
		depends on CC_HAS_STACKPROTECTOR_REGULAR
	
	config CC_STACKPROTECTOR_NONE
		bool
	
	
	config CC_OPT_STACKPROTECTOR
		string
		default "-fstack-protector-strong" if CC_STACKPROTECTOR_STRONG
		default "-fstack-protector"        if CC_STACKPROTECTOR_REGULAR
		default "-fno-stack-protector"     if CC_HAS_STACKPROTECTOR_NONE
		# If the compiler doesn't even support
		# -fno-stack-protector
		default ""

Of course, at some point you're just moving complexity from one place to
another. Maybe this all-Kconfig approach isn't worth it if people find
it harder to understand. I don't know how bad the Makefiles are here at
the moment.

Cheers,
Ulf