Re: enabling COMPILE_TEST support for GCC plugins in v4.11

[Kees Cook]

On Fri, Dec 9, 2016 at 2:40 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Thursday, December 8, 2016 11:00:42 AM CET Kees Cook wrote:
>
>> If you have a moment, applying 215e2aa6c024[1] and reverting
>> a519167e753e for an allyesconfig/allmodconfig build should let you
>> know if things are working correctly with headers installed. If anyone
>> sees any problems, please let me know and I can queue up fixes.
>
> Using gcc-4.9.3 or gcc-5.3.1 for an ARM allmodconfig build, I get tons of
> errors such as this one:
>
> /git/arm-soc/init/initramfs.c: In function 'error':
> /git/arm-soc/init/initramfs.c:50:1: error: unrecognizable insn:
>  }
>  ^
> (insn 26 25 27 5 (set (reg:SI 111 [ local_entropy.243 ])
>         (rotatert:SI (reg:SI 116 [ local_entropy.243 ])
>             (const_int -30 [0xffffffffffffffe2]))) -1
>      (nil))
> *** WARNING *** there are active plugins, do not report this as a bug unless you can reproduce it without enabling any plugins.
> Event                            | Plugins
> PLUGIN_ATTRIBUTES                | latent_entropy_plugin
> PLUGIN_START_UNIT                | latent_entropy_plugin
> /git/arm-soc/init/initramfs.c:50:1: internal compiler error: in extract_insn, at recog.c:2202
> /git/arm-soc/arch/arm/vfp/vfpmodule.c: In function 'vfp_init':
> /git/arm-soc/arch/arm/vfp/vfpmodule.c:824:1: error: unrecognizable insn:
>  }
>  ^
> (insn 138 137 139 17 (set (reg:SI 165 [ local_entropy.93 ])
>         (rotatert:SI (reg:SI 150 [ local_entropy.93 ])
>             (const_int -9 [0xfffffffffffffff7]))) -1
>      (nil))
> *** WARNING *** there are active plugins, do not report this as a bug unless you can reproduce it without enabling any plugins.

Well that's exciting! :P

>
> Using gcc-6.1.1 or gcc-7.0.0, everything works fine as far as I
> can tell. With some older Ubuntu binary toolchains, I get this one:
>
> In file included from <stdin>:1:0:
> /git/arm-soc/scripts/gcc-plugins/gcc-common.h:4:22: fatal error: bversion.h: No such file or directory
> compilation terminated.
> Cannot use CONFIG_GCC_PLUGINS: your gcc installation does not support plugins, perhaps the necessary headers are missing?
>
> and I can't find the packages for the headers (I still get them for
> gcc-5, but not older versions). On very old toolchains (e.g. gcc-4.3),
> I get this one:
>
> Cannot use CONFIG_GCC_PLUGINS: your gcc version does not support plugins, you should upgrade it to at least gcc 4.5
> scripts/Makefile.gcc-plugins:54: recipe for target 'gcc-plugins-check' failed

I'm starting to wonder if we need to expose the compiler version to
Kconfig so that "all*config" builds for earlier compiler will
automatically leave things like plugins off. But I have no idea what
the right approach for that might be.

I'm not a fan of silently disabling stuff (like is done, for example,
for KCOV). If you've selected a Kconfig that your compiler can't
build, it should fail to build, but that's different from the
allyesconfig etc, since that's about compile tests not a sane kernel
image.

Perhaps if COMPILE_TEST is enabled, and there is something unsupported
by the compiler, only then would it warn and continue, instead of
killing build?

-Kees

-- 
Kees Cook
Nexus Security