linux-kernel - Re: [PATCH -tip] x86/stackprotector: Move stack canary to struct pcpu

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Message-ID: <CAFULd4ZGW-FYtEe-BJ53QVjHMLr1jyj_6vJhNEwqqwW6Z77+Tg@mail.gmail.com>
Date: Fri, 21 Feb 2025 15:02:18 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH -tip] x86/stackprotector: Move stack canary to struct pcpu_hot

On Fri, Feb 21, 2025 at 2:37 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Fri, Feb 21, 2025 at 8:25 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > On Fri, Feb 21, 2025 at 1:54 PM Brian Gerst <brgerst@...il.com> wrote:
> > >
> > > On Thu, Feb 20, 2025 at 3:04 PM Uros Bizjak <ubizjak@...il.com> 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@...il.com>
> > > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > > Cc: Ingo Molnar <mingo@...nel.org>
> > > > Cc: Borislav Petkov <bp@...en8.de>
> > > > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > > > Cc: "H. Peter Anvin" <hpa@...or.com>
> > > > Cc: Brian Gerst <brgerst@...il.com>
> > > > Cc: Ard Biesheuvel <ardb@...nel.org>
> > > > ---
> > > >  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.
>
> I got a warning from the relocs tool, but not a hard error.  What
> compiler/linker are you using?
>
> Does the attached patch build in your configuration?

Ah, the attached patch is similar to my previous approach, where the
build system *warned* on an offset (the patch was abandoned due to
Ard's request to put stack_canary to the *beginning* of struct
pcpu_hot, and this allowed for a simplified patch).

The attached patch builds for me without warning/error for both, SMP
and !SMP build.

You can put my Acked-by: on the patch, and if it is based on my
previous patch, I'd be grateful for a Co-developed-by tag.

Thanks.
Uros.