Re: [PATCH v5 00/16] x86-64: Stack protector and percpu improvements

[Brian Gerst]

On Sat, Nov 9, 2024 at 4:31 AM David Laight <David.Laight@xxxxxxxxxx> 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.

> 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.

> 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?

Brian Gerst