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Date: Tue, 18 Jul 2017 23:08:34 -0400
From: Brian Gerst <brgerst@...il.com>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>,
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Subject: Re: [RFC 16/22] x86/percpu: Adapt percpu for PIE support
On Tue, Jul 18, 2017 at 6:33 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> Perpcu uses a clever design where the .percu ELF section has a virtual
> address of zero and the relocation code avoid relocating specific
> symbols. It makes the code simple and easily adaptable with or without
> SMP support.
>
> This design is incompatible with PIE because generated code always try to
> access the zero virtual address relative to the default mapping address.
> It becomes impossible when KASLR is configured to go below -2G. This
> patch solves this problem by removing the zero mapping and adapting the GS
> base to be relative to the expected address. These changes are done only
> when PIE is enabled. The original implementation is kept as-is
> by default.
The reason the per-cpu section is zero-based on x86-64 is to
workaround GCC hardcoding the stack protector canary at %gs:40. So
this patch is incompatible with CONFIG_STACK_PROTECTOR.
--
Brian Gerst
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