linux-kernel - Re: [BUG v5.2-rc1] ARM build broken

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Message-ID: <201905201142.CF71598A@keescook>
Date:   Mon, 20 May 2019 11:53:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>
Subject: Re: [BUG v5.2-rc1] ARM build broken

[Adding Chris and Ard, who might have more compiler versions that me...]

On Mon, May 20, 2019 at 07:08:39PM +0200, H. Nikolaus Schaller wrote:
> > Am 20.05.2019 um 17:59 schrieb Kees Cook <keescook@...omium.org>:
> > 
> > On Mon, May 20, 2019 at 05:15:02PM +0200, H. Nikolaus Schaller wrote:
> >> Hi,
> >> it seems as if ARM build is broken since ARM now hard enables CONFIG_HAVE_GCC_PLUGINS
> >> which indirectly enables CONFIG_GCC_PLUGIN_ARM_SSP_PER_TASK. Compiling this breaks
> >> on my system (Darwin build host) due to conflicts in system headers and Linux headers.
> >> 
> >> So how can I turn off all these GCC_PLUGINS?
> >> 
> >> The offending patch seems to be
> >> 
> >> 	security: Create "kernel hardening" config area
> >> 
> >> especially the new "default y" for GCC_PLUGINS. After removing that line from
> >> scripts/gcc-plugins/Kconfig makes my compile succeed.
> > 
> > The intention is to enable it _if_ the plugins are available as part of
> > the build environment. The "default y" on GCC_PLUGINS is mediated by:
> >        depends on HAVE_GCC_PLUGINS
> 
> HAVE_GCC_PLUGINS has the following description:
> 
> 	An arch should select this symbol if it supports building with
>           GCC plugins.
> 
> So an ARCH (ARM) selects it unconditionally of the build environment.
> 
> >        depends on PLUGIN_HOSTCC != ""
> 
> Well, we have it set to "g++" for ages and it was not a problem.
> So both conditions are true.

PLUGIN_HOSTCC should have passed the scripts/gcc-plugin.sh test, so
that's correct. And the result (CONFIG_GCC_PLUGINS) is correct: it
doesn't enable or disable anything itself.

What you want is to disable CONFIG_STACKPROTECTOR_PER_TASK, which
is the knob for the feature:

config STACKPROTECTOR_PER_TASK
        bool "Use a unique stack canary value for each task"
        depends on GCC_PLUGINS && STACKPROTECTOR && SMP && !XIP_DEFLATED_DATA
        select GCC_PLUGIN_ARM_SSP_PER_TASK
        default y

> Build error:
> 
>  HOSTCXX -fPIC scripts/gcc-plugins/arm_ssp_per_task_plugin.o - due to: scripts/gcc-plugins/gcc-common.h
> In file included from scripts/gcc-plugins/arm_ssp_per_task_plugin.c:3:0:
> scripts/gcc-plugins/gcc-common.h:153:0: warning: "__unused" redefined
> #define __unused __attribute__((__unused__))
> ^

Does the following patch fix your build? (I assume that line is just a
warning, but if not...)

diff --git a/scripts/gcc-plugins/gcc-common.h b/scripts/gcc-plugins/gcc-common.h
index 552d5efd7cb7..17f06079a712 100644
--- a/scripts/gcc-plugins/gcc-common.h
+++ b/scripts/gcc-plugins/gcc-common.h
@@ -150,8 +150,12 @@ void print_gimple_expr(FILE *, gimple, int, int);
 void dump_gimple_stmt(pretty_printer *, gimple, int, int);
 #endif
 
+#ifndef __unused
 #define __unused __attribute__((__unused__))
+#endif
+#ifndef __visible
 #define __visible __attribute__((visibility("default")))
+#endif
 
 #define DECL_NAME_POINTER(node) IDENTIFIER_POINTER(DECL_NAME(node))
 #define DECL_NAME_LENGTH(node) IDENTIFIER_LENGTH(DECL_NAME(node))

>  HOSTLLD -shared scripts/gcc-plugins/arm_ssp_per_task_plugin.so - due to target missing
> Undefined symbols for architecture x86_64:
>  "gen_reg_rtx(machine_mode)", referenced from:
>      (anonymous namespace)::arm_pertask_ssp_rtl_pass::execute() in arm_ssp_per_task_plugin.o

However, this part sounds more like what was fixed with
259799ea5a9a ("gcc-plugins: arm_ssp_per_task_plugin: Fix for older GCC < 6")

And maybe some additional fixes for 4.9 are needed?

> This is because CONFIG_GCC_PLUGIN_ARM_SSP_PER_TASK became automatically enabled and was never
> before. So the compiler may lack some library search path for building this plugin (which we
> did never miss).

Right -- maybe CONFIG_STACKPROTECTOR_PER_TASK doesn't work with old gcc
4.9.2? I'll see if I can find that compiler version...

-- 
Kees Cook