Re: [RFC PATCH 4/7] kconfig: support new special property shell=

[Ulf Magnusson]

On Tue, Feb 13, 2018 at 01:10:34AM +0900, Masahiro Yamada wrote:
> 2018-02-13 0:46 GMT+09:00 Kees Cook <keescook@xxxxxxxxxxxx>:
> > On Mon, Feb 12, 2018 at 2:44 AM, Masahiro Yamada
> > <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> >> Linus said:
> >>
> >>> But yes, I also reacted to your earlier " It can't silently rewrite it
> >>> to _REGULAR because the compiler support for _STRONG regressed."
> >>> Because it damn well can. If the compiler doesn't support
> >>> -fstack-protector-strong, we can just fall back on -fstack-protector.
> >>> Silently. No extra crazy complex logic for that either.
> >>
> >>
> >> If I understood his comment correctly,
> >> we do not need either WANT_* or _AUTO.
> >>
> >>
> >> Kees' comment:
> >>
> >>> 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.)
> >>
> >> No.  Kconfig will not do this silently.
> >>
> >> "make oldconfig" (or "make silentoldconfig" as well)
> >> always ask users about new symbols.
> >
> > The case I want to make sure can never happen is to have a config
> > setting that ends up not actually being true. For example, if
> > CONFIG_CC_STACKPROTECTOR appears in /proc/config.gz but the kernel
> > wasn't actually built with a stack protector, that's bad. We end up in
> > a place where the user can't trust the config to represent the actual
> > results of the build.
> >
> > So, as long as the Kconfig options are strongly tied to the compiler
> > capabilities, we're good on that front.
> >
> >> So, I can suggest to remove _REGULAR and _NONE.
> >>
> >> We have just two bool options, like follows.
> >>
> >> ------------------->8-----------------
> >> config CC_STACKPROTECTOR
> >>         bool "Use stack protector"
> >>         depends on CC_HAS_STACKPROTECTOR
> >>
> >> config CC_STACKPROTECTOR_STRONG
> >>         bool "Use strong strong stack protector"
> >>         depends on CC_STACKPROTECTOR
> >>         depends on CC_HAS_STACKPROTECTOR_STRONG
> >> -------------------->8------------------
> >>
> >> This will work well for all{yes,mod,no}config.
> >
> > This two-option arrangement is fine (though it assumes there won't be
> > another stack protector option in the future).
> >
> > The issue I have is this removes _AUTO, which I think can be solved in
> > the two-option arrangement too. The purpose of _AUTO is to effectively
> > enable stack-protector by default. As this option has been available
> > for over 10 years, and all distros enable it, it's an obvious
> > candidate to be enabled-by-default, especially since it kills a class
> > of exploits (as mentioned in my commit log: BlueBorne was trivially
> > defeated with stack-protector). So it should be possible to make these
> > two options "default y", yes?
> 
> 
> Yes.
> 
> Both should be "default y" to keep the equivalent behavior
> since the current default is _AUTO.
> 

Since the discussions in this thread are going in a million different
directions and making my head hurt, ping me if I'm needed re. the WANT_*
stuff or anything else. Masahiro's two-symbol version is much simpler
and neater if the caveats re. "fallback" are minor enough to not matter.
:)

Cheers,
Ulf