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

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Message-ID: <CAFULd4aO6-1_FKXFyXW+QT5UL-3mU3Tr=yjjwt9-b1yM=TrN+w@mail.gmail.com>
Date: Fri, 21 Feb 2025 17:01:46 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, 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>
Subject: Re: [PATCH -tip] x86/stackprotector: Move stack canary to struct pcpu_hot

On Fri, Feb 21, 2025 at 4:54 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Fri, Feb 21, 2025 at 9:38 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > On Fri, 21 Feb 2025 at 15:33, Uros Bizjak <ubizjak@...il.com> wrote:
> > >
> > > On Fri, Feb 21, 2025 at 3:13 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > >
> > > > > > 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.
> > > > >
> > > >
> > > > Did you try building modules too?
> > >
> > > make -j 24 olddefconfig prepare modules_prepare bzImage modules
> > >
> > > for defconfig, SMP and !SMP.
> > >
> >
> > OK.
> >
> > I think I prefer Brian's approach - the only nit is that the placement
> > of stack_canary creates a padding hole in struct pcpu_hot. However,
> > that does not actually matter until we run out of space so I think it
> > is fine as is.
>
> Going off on a tangent, as struct pcpu_hot grows I think it can be
> done better as a subsection of percpu data (".data..percpu..hot").
> That way each variable has its own symbol again and it reduces header
> dependencies.

Please note an optimization in common.h:

/* const-qualified alias to pcpu_hot, aliased by linker. */
DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
            const_pcpu_hot);

struct pcpu_hot has its const alias, so we are able to use:

    if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
        return this_cpu_read_const(const_pcpu_hot.current_task);

where the compiler is able to eliminate a sequence of reads to the
const_pcpu_hot.current_task.

LTO does not like the approach, though, but clang does not use x86 seg
support, and gcc does not perform LTO.

Uros.