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Message-ID: <dee56ade8f2841c0b276a0b9af221981@AcuMS.aculab.com>
Date: Sat, 9 Nov 2024 21:27:06 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Brian Gerst' <brgerst@...il.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, "H . Peter
Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov
<bp@...en8.de>, Ard Biesheuvel <ardb@...nel.org>, Uros Bizjak
<ubizjak@...il.com>
Subject: RE: [PATCH v5 00/16] x86-64: Stack protector and percpu improvements
From: Brian Gerst
> Sent: 09 November 2024 15:11
>
> On Sat, Nov 9, 2024 at 4:31 AM David Laight <David.Laight@...lab.com> wrote:
> >
> > From: Brian Gerst
> > > Sent: 05 November 2024 15:58
> > >
> > > 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 other architectures. In turn, this allows removal of code that was
> > > needed to support the zero-based percpu section.
> > >
> > > v5:
> > > - Added two patches from Ard Biesheuvel to make stack protector work
> > > properly when compiling with clang.
> > > - Raise minimum GCC version to 8.1 for x86.
> > > - Drop objtool conversion code.
> >
> > Is there any actual need to raise the GCC level?
> > Isn't it enough just to disable stack protection with older compilers?
> > The percpu layout can then always be the new (sane) one.
>
> Earlier versions of this series did make stack protector support
> conditional on newer compilers. That got rejected. I then added
> objtool support to convert the code old compilers produced. That also
> got rejected. I guess I can't please everyone.
I certainly wouldn't have bothered hacking objtool.
> > Is there even a selectable CONFIG_STACK_PROTECTOR?
> > Can than depend on gcc >= 8.1 for x86-64?
>
> Yes, stack protector support is optional, but practically all distro
> kernels enable it.
They include all sorts of stuff that slows things down :-)
But I'd rather be able to build and test kernels than have the stack protector.
> > I've a slight vested interest in that the system I test kernels on
> > has gcc 7.5.0 installed :-)
>
> What distro is on that system? Is it still actively supported?
The system in running Ubuntu 18.04 LTS - and still receives updates.
I do run locally build kernels on it, but I could just be building kernels.
Seems a shame to force an update for something I can just deselect.
For reference RHEL7 is still supported but has a 4.8.5 compiler.
So it is a long time since that has self-hosted kernels.
We build software for release on Centos-7 as an easy way to get an old glibc (etc),
although buildroot/busybox (x86-64) 'distribution' has to use a newer
compiler - the grub build fails well before you get to a kernel!
David
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