Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] powerpc/vdso32: mark __kernel_datapage_offset as
 STV_PROTECTED
To:     Fangrui Song <maskray@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
References: <20200205005054.k72fuikf6rwrgfe4@google.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <10e3d362-ec29-3816-88ff-8415d5c78e3b@c-s.fr>
Date:   Wed, 5 Feb 2020 07:25:59 +0100
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.2
MIME-Version: 1.0
In-Reply-To: <20200205005054.k72fuikf6rwrgfe4@google.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: fr
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


Le 05/02/2020 à 01:50, Fangrui Song a écrit :
> A PC-relative relocation (R_PPC_REL16_LO in this case) referencing a
> preemptible symbol in a -shared link is not allowed.  GNU ld's powerpc
> port is permissive and allows it [1], but lld will report an error after
> https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?id=ec0895f08f99515194e9fcfe1338becf6f759d38

Note that there is a series whose first two patches aim at dropping 
__kernel_datapage_offset . See 
https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=156045 
and especially patches https://patchwork.ozlabs.org/patch/1231467/ and 
https://patchwork.ozlabs.org/patch/1231461/

Those patches can be applied independentely of the rest.

Christophe

> 
> Make the symbol protected so that it is non-preemptible but still
> exported.
> 
> [1]: https://sourceware.org/bugzilla/show_bug.cgi?id=25500
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/851
> Signed-off-by: Fangrui Song <maskray@google.com>
> ---
>   arch/powerpc/kernel/vdso32/datapage.S | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/vdso32/datapage.S b/arch/powerpc/kernel/vdso32/datapage.S
> index 217bb630f8f9..2831a8676365 100644
> --- a/arch/powerpc/kernel/vdso32/datapage.S
> +++ b/arch/powerpc/kernel/vdso32/datapage.S
> @@ -13,7 +13,8 @@
>   #include <asm/vdso_datapage.h>
>   
>   	.text
> -	.global	__kernel_datapage_offset;
> +	.global	__kernel_datapage_offset
> +	.protected	__kernel_datapage_offset
>   __kernel_datapage_offset:
>   	.long	0
>   
>