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@xxxxxxxxx>:
> > > 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@xxxxxxxxx> 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
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