RE: [PATCH v2 00/11] x86-64: Stack protector and percpu improvements

David Laight
2023-10-30T09:06:06Z

From: Uros Bizjak
> Sent: 30 October 2023 08:07
>
> On Sun, Oct 29, 2023 at 10:42 PM David Laight <David.Laight@aculab.com> wrote:
> >
> > From: Brian Gerst
> > > Sent: 26 October 2023 17:01
> > >
> > > Currently, x86-64 uses an unusual percpu layout, where the percpu section
> > > is linked at absolute address 0. The reason behind this is that older GCC
> > > versions placed the stack protector (if enabled) at a fixed offset from the
> > > GS segment base. Since the GS segement is also used for percpu variables,
> > > this forced the current layout.
> > >
> > > GCC since version 8.1 supports a configurable location for the stack
> > > protector value, which allows removal of the restriction on how the percpu
> > > section is linked. This allows the percpu section to be linked
> > > normally, like most other architectures. In turn, this allows removal
> > > of code that was needed to support the zero-based percpu section.
> >
> > I didn't think the minimum gcc version was anything like 8.1.
> > I'm using 7.5.0 and I don't think that is the oldest version.
>
> Please see previous discussion regarding modernizing stack protector
> on x86_64 [1]
>
> [1] https://lore.kernel.org/lkml/20211113124035.9180-1-brgerst@gmail.com/
>
> and x86_32 [2]
>
> [2] https://lore.kernel.org/lkml/cover.1601925251.git.luto@kernel.org/
>
> The conclusion in [2] is:
>
> "I'm all in favour of simply requiring GCC-8.1 to build a more secure
> x86_64 kernel. Gives people an incentive to not use ancient compilers.
>
> And if you do want to use your ancient compiler, we'll still build, you
> just don't get to have stackprotector."

I didn't see a patch that limited 'stackprotector' to gcc >= 8.1
Without that anyone who already has it enabled and is using an
older compiler will get very broken kernels.

David

>
> and in [1]:
>
> "Ack. We did this for 32-bit and got few complaints. Let’s finish the job."
>
> Uros.

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