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.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

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."

and in [1]:

"Ack.  We did this for 32-bit and got few complaints. Let’s finish the job."

Uros.

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.

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

On Mon, Oct 30, 2023 at 10:05 AM David Laight <David.Laight@aculab.com> wrote:
>
> 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.

It's this part:

--cut here--
diff --git a/scripts/gcc-x86_32-has-stack-protector.sh
b/scripts/gcc-x86_32-has-stack-protector.sh
index f5c119495254..51f864d76bd6 100755
--- a/scripts/gcc-x86_32-has-stack-protector.sh
+++ b/scripts/gcc-x86_32-has-stack-protector.sh
@@ -1,4 +1,8 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0

-echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32
-O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
+# This requires GCC 8.1 or better.  Specifically, we require
+# -mstack-protector-guard-reg, added by
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
+
+echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32
-O0 -fstack-protector -mstack-protector-guard-reg=fs
-mstack-protector-guard-symbol=stack_canary - -o - 2> /dev/null | grep
-q "%fs"
--cut here--

Uros.

On Sun, Oct 29, 2023 at 5:42 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.

What distribution are you using that still relies on that old of a compiler?

Brian Gerst