Re: [PATCH -tip] x86/stackprotector: Move stack canary to struct pcpu_hot

[Uros Bizjak]

On Fri, Feb 21, 2025 at 1:54 PM Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> On Thu, Feb 20, 2025 at 3:04 PM Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > Move stack canary from __stack_chk_guard to struct pcpu_hot and
> > alias __stack_chk_guard to point to the new location in the
> > linker script.
> >
> > __stack_chk_guard is one of the hottest data structures on x86, so
> > moving it there makes sense even if its benefit cannot be measured
> > explicitly.
> >
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Brian Gerst <brgerst@xxxxxxxxx>
> > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/current.h | 13 +++++++++++++
> >  arch/x86/kernel/cpu/common.c   |  1 -
> >  arch/x86/kernel/vmlinux.lds.S  |  2 ++
> >  3 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
> > index bf5953883ec3..e4ff1d15b465 100644
> > --- a/arch/x86/include/asm/current.h
> > +++ b/arch/x86/include/asm/current.h
> > @@ -15,6 +15,9 @@ struct task_struct;
> >  struct pcpu_hot {
> >         union {
> >                 struct {
> > +#ifdef CONFIG_STACKPROTECTOR
> > +                       unsigned long           stack_canary;
> > +#endif
> >                         struct task_struct      *current_task;
> >                         int                     preempt_count;
> >                         int                     cpu_number;
> > @@ -35,6 +38,16 @@ struct pcpu_hot {
> >  };
> >  static_assert(sizeof(struct pcpu_hot) == 64);
> >
> > +/*
> > + * stack_canary should be at the beginning of struct pcpu_hot to avoid:
> > + *
> > + * Invalid absolute R_X86_64_32S relocation: __stack_chk_guard
>
> This should be R_X86_64_PC32 relocations.

This is what the build system reports if any offset (including 0) is added to

PROVIDE(__stack_chk_guard = pcpu_hot);

It is not a warning, but an error that fails the build.

As was discussed in the previous thread, the above is required to
handle !SMP case, where mstack-protector-guard=global (used by !SMP
builds) ignores the
-mstack-protector-guard-symbol option and always uses __stack_chk_guard.

Uros.