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Subject: Re: [PATCH v5 1/2] powerpc/32: add stack protector support
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
References: <8e81b0647fea15e533a73ad4e9063c059fdfc6df.1537987712.git.christophe.leroy@c-s.fr>
From:   Samuel Holland <samuel@sholland.org>
Message-ID: <f5600a1d-71ca-f1d7-3973-026462990059@sholland.org>
Date:   Sat, 12 Jan 2019 17:32:40 -0600
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Hello all,

On 09/27/18 02:05, Christophe Leroy wrote:
[..snip..]
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index 07d9dce7eda6..45b8eb4d8fe7 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -112,6 +112,9 @@ KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
>  KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
>  endif
>  
> +cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
> +cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
> +
>  LDFLAGS_vmlinux-y := -Bstatic
>  LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
>  LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)
> @@ -404,6 +407,13 @@ archclean:
>  
>  archprepare: checkbin
>  
> +ifdef CONFIG_STACKPROTECTOR
> +prepare: stack_protector_prepare
> +
> +stack_protector_prepare: prepare0
> +	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
> +endif
> +

This breaks when building out-of-tree kernel modules. GCC is not getting passed
the -mstack-protector-guard-offset argument, so the default offset is used. The
kernel then panics the first time a function with stack protector is called.

I'm seeing this on powerpc64. It looks like it was reported for powerpc on
kernel bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201891

Linux 4.20 does not have a "prepare" target when KBUILD_EXTMOD is set. One is
added by:

  commit e07db28eea38ed4e332b3a89f3995c86b713cb5b
  Author: Masahiro Yamada <yamada.masahiro@socionext.com>
  Date:   Thu Nov 22 08:11:54 2018 +0900

      kbuild: fix single target build for external module

However, after cherry-picking that patch, the build fails because it's missing
prepare0. I applied the patch below and I successfully built an out-of-tree
module with CONFIG_STACKPROTECTOR=y.

diff --git a/Makefile b/Makefile
index 826826553085..f0a93e1ba1b6 100644
--- a/Makefile
+++ b/Makefile
@@ -1596,9 +1596,10 @@ help:
        @echo  ''

 # Dummies...
-PHONY += prepare scripts
-prepare:
+PHONY += prepare prepare0 scripts
+prepare: prepare0
        $(cmd_crmodverdir)
+prepare0: ;
 scripts: ;
 endif # KBUILD_EXTMOD

The context has been changed some in later patches, but I think a change like
this one should go into 5.0, and it e07db28eea38 should go into 4.20.y.

Thanks,
Samuel