Re: [PATCH v4 00/16] x86-64: Stack protector and percpu improvements

[Brian Gerst]

On Sat, Mar 23, 2024 at 10:25 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> > On Fri, Mar 22, 2024 at 5:52 PM Brian Gerst <brgerst@xxxxxxxxx> wrote:
> > >
> > > 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.
> >
> > The number of simplifications throughout the code, enabled by this
> > patch set, is really impressive, and it reflects the number of
> > workarounds to enable the feature that was originally not designed for
> > the kernel usage. As noted above, this issue was recognized in the GCC
> > compiler and the stack protector support was generalized by adding
> > configurable location for the stack protector value [1,2].
> >
> > The improved stack protector support was implemented in gcc-8.1,
> > released on May 2, 2018, when linux 4.17 was in development. In light
> > of this fact, and 5 (soon 6) GCC major releases later, I'd like to ask
> > if the objtool support to fixup earlier compilers is really necessary.
> > Please note that years ago x86_32 simply dropped stack protector
> > support with earlier compilers and IMO, we should follow this example
> > also with x86_64, because:
>
> Ack on raising the minimum version requirement for x86-64
> stackprotector to 8.1 or so - this causes no real pain on the distro
> side: when *this* new kernel of ours is picked by a distro, it almost
> always goes hand in hand with a compiler version upgrade.
>
> We should be careful with fixes marked for -stable backport, but other
> than that, new improvements like Brian's series are a fair game to
> tweak compiler version requirements.
>
> But please emit a (single) prominent build-time warning if a feature is
> disabled though, even if there are no functional side-effects, such as
> for hardening features.

Disabled for any reason or only if the compiler lacks support?

Brian Gerst