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

Brian Gerst
2024-03-24T12:34:14Z

On Sun, Mar 24, 2024 at 6:53 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
> > On Sun, Mar 24, 2024 at 12:05 AM Ingo Molnar <mingo@kernel.org> wrote:
> > >
> > >
> > > * Brian Gerst <brgerst@gmail.com> wrote:
> > >
> > > > On Sat, Mar 23, 2024 at 10:25 PM Ingo Molnar <mingo@kernel.org> wrote:
> > > > >
> > > > >
> > > > > * Uros Bizjak <ubizjak@gmail.com> wrote:
> > > > >
> > > > > > On Fri, Mar 22, 2024 at 5:52 PM Brian Gerst <brgerst@gmail.com> 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?
> > >
> > > Only if the user desires to have it enabled, but it's not possible due
> > > to compiler (or other build environment) reasons. Ie. if something
> > > unexpected happens from the user's perspective.
> > >
> > > The .config option is preserved even if the compiler doesn't support
> > > it, right?
> > >
> > > I suspect this should also cover features that get select-ed by a
> > > feature that the user enables. (Not sure about architecture level
> > > select-ed options.)
> > >
> > > Thanks,
> > >
> > > Ingo
> >
> > I could add something like:
> >
> > comment "Stack protector is not supported by the architecture or compiler"
> > depends on !HAVE_STACKPROTECTOR
> >
> > But, "make oldconfig" will still silently disable stack protector if
> > the compiler doesn't support the new options. It does put the
> > comment into the .config file though, so that may be enough.
>
> So I was thinking more along the lines of emitting an actual warning to
> the build log, every time the compiler check is executed and fails to
> detect [certain] essential or good-to-have compiler features.
>
> A bit like the red '[ OFF ]' build lines during the perf build:
>
> Auto-detecting system features:
>
> ... dwarf: [ on ]
> ... dwarf_getlocations: [ on ]
> ... glibc: [ on ]
> ... libbfd: [ on ]
> ... libbfd-buildid: [ on ]
> ... libcap: [ on ]
> ... libelf: [ on ]
> ... libnuma: [ on ]
> ... numa_num_possible_cpus: [ on ]
> ... libperl: [ on ]
> ... libpython: [ on ]
> ... libcrypto: [ on ]
> ... libunwind: [ on ]
> ... libdw-dwarf-unwind: [ on ]
> ... libcapstone: [ OFF ] <========
> ... zlib: [ on ]
> ... lzma: [ on ]
> ... get_cpuid: [ on ]
> ... bpf: [ on ]
> ... libaio: [ on ]
> ... libzstd: [ on ]
>
> ... or something like that.
>
> Thanks,
>
> Ingo

That list comes from the perf tool itself
(tools/perf/builtin-version.c), not the kernel config or build system.
Something like that could be added to the main kernel build. But it
should be a separate patch series as it will likely need a lot of
design iteration.

Brian Gerst