linux-kernel - Re: [PATCH v2 2/6] x86-64: Convert stack protector to normal percpu variable

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Message-ID: <20211202225107.GF16608@worktop.programming.kicks-ass.net>
Date:   Thu, 2 Dec 2021 23:51:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Brian Gerst' <brgerst@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 2/6] x86-64: Convert stack protector to normal percpu
 variable

On Wed, Dec 01, 2021 at 09:50:57AM +0000, David Laight wrote:
> From: Brian Gerst
> > Sent: 30 November 2021 20:56
> > 
> > Older versions of GCC fixed the location of the stack protector canary
> > at %gs:40.  This constraint forced the percpu section to be linked at
> > virtual address 0 so that the canary could be the first data object in
> > the percpu section.  Supporting the zero-based percpu section requires
> > additional code to handle relocations for RIP-relative references to
> > percpu data, extra complexity to kallsyms, and workarounds for linker
> > bugs due to the use of absolute symbols.
> > 
> > Since version 8.1, GCC has options to configure the location of the
> > canary value.  This allows the canary to be turned into a normal
> > percpu variable and removes the constraint that the percpu section
> > be zero-based.
> 
> I didn't think the minimum gcc version has been raised as far as 8.1?

Older GCC can still build a kernel, just not with stack protector on.
And 8.1 is already 3 years old. If you run ancient distros, you can run
ancient kernels too.