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 ? > 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="$CC -Werror -fno-stack-protector -c -x c /dev/null" config CC_STACKPROTECTOR bool choice prompt "Stack Protector buffer overflow detection" config CC_STACKPROTECTOR_AUTO bool "Auto" select CC_STACKPROTECTOR if (CC_HAS_STACKPROTECTOR || \ CC_HAS_STACKPROTECTOR_STRONG) config CC_STACKPROTECTOR_REGULAR bool "Regular" select CC_STACKPROTECTOR config CC_STACKPROTECTOR_STRONG bool "Strong" select CC_STACKPROTECTOR config CC_STACKPROTECTOR_NONE bool "None" endchoice comment "(WARNING) stackprotecter was chosen, but your compile does not support it. Build will fail" depends on CC_STACKPROTECTOR_REGULAR && \ !CC_HAS_STACKPROTECTOR comment "(WARNING) stackprotecter-strong was chosen, but your compile does not support it. Build will fail" depends on CC_STACKPROTECTOR_STRONG && \ !CC_HAS_STACKPROTECTOR_STRONG ------------------------->8--------------------------------- BTW, setting option flags in Makefile is dirty, like follows: ccflags-$(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 Makefile will become clean. Of course, this is at the cost of ugliness in Kconfig. -- Best Regards Masahiro Yamada -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html