linux-kernel - Re: [PATCH] powerpc/vdso32: mark __kernel_datapage_offset as STV

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Date:   Wed, 5 Feb 2020 07:25:59 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Fangrui Song <maskray@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] powerpc/vdso32: mark __kernel_datapage_offset as
 STV_PROTECTED



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@...gle.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
>   
>