Message-ID: <47ea5a6a-025d-bcf2-d23a-be96cbcd36d3@zytor.com>
Date:   Sat, 13 Nov 2021 20:24:11 -0800
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Subject: Re: [PATCH 3/3] x86_64: Use relative per-cpu offsets
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To:     Andy Lutomirski <luto@kernel.org>, Brian Gerst <brgerst@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
Cc:     Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>
References: <20211113124035.9180-1-brgerst@gmail.com>
 <20211113124035.9180-4-brgerst@gmail.com>
 <719e0170-7645-4787-8c3a-676f34068c27@www.fastmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
In-Reply-To: <719e0170-7645-4787-8c3a-676f34068c27@www.fastmail.com>
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On 11/13/21 17:18, Andy Lutomirski wrote:
> 
> 
> On Sat, Nov 13, 2021, at 4:40 AM, Brian Gerst wrote:
>> The per-cpu section is currently linked at virtual address 0, because
>> older compilers hardcoded the stack protector canary value at a fixed
>> offset from the start of the GS segment.  Use a standard relative offset
>> as the GS base when the stack protector is disabled, or a newer compiler
>> is used that supports a configurable location for the stack canary.
> 
> Can you explain the benefit?  Also, I think we should consider dropping support for the fixed model like we did on x86_32.
> 

It would seem that UNLESS the fixed model is dropped, the benefit is 
probably negative.

	-hpa