linux-kernel - Re: [PATCH v6 00/15] x86-64: Stack protector and percpu improvements

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Message-ID: <CAFULd4ZQ8VoKvQ7aOgcfeLNROM4-rDYn=wHo=FYMO8ZkuQeSAQ@mail.gmail.com>
Date: Thu, 20 Feb 2025 10:51:03 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v6 00/15] x86-64: Stack protector and percpu improvements

On Wed, Feb 19, 2025 at 2:18 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Wed, Feb 19, 2025 at 6:47 AM Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > > > Thank you for doing this series - it all looks pretty good from my
> > > > side and I've applied it experimentally to tip:x86/asm. I fixed up
> > > > the trivial details other reviewers and me noticed.
> > > >
> > > > Note that the merge is tentative, it might still need a rebase if
> > > > some fundamental problem comes up - but let's see how testing goes
> > > > in -next.
> > >
> > > I wonder if there would be any benefit if stack canary is put into
> > > struct pcpu_hot?
> >
> > It should definitely be one of the hottest data structures on x86, so
> > moving it there makes sense even if it cannot be measured explicitly.
> >
>
> It would have to be done with linker tricks, since you can't make the
> compiler use a struct member directly.

Something like the attached patch?

It boots and runs without problems.

However, when building the kernel, I get "Absolute relocations
present" warning with thousands of locations:

  RELOCS  arch/x86/boot/compressed/vmlinux.relocs
WARNING: Absolute relocations present
Offset     Info     Type     Sym.Value Sym.Name
ffffffff81200826 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard
ffffffff81201493 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard
ffffffff81201714 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard
ffffffff81201d66 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard
...
ffffffff834e2a13 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard
ffffffff834e2a6a 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
__ref_stack_chk_guard

  RSTRIP  vmlinux

which I don't understand. Looking at the first one:

ffffffff8120081d <force_ibs_eilvt_setup.cold>:
ffffffff8120081d:    48 8b 44 24 08           mov    0x8(%rsp),%rax
ffffffff81200822:    65 48 2b 05 f6 ed 30     sub
%gs:0x230edf6(%rip),%rax        # ffffffff8350f620
<__ref_stack_chk_guard>
ffffffff81200829:    02

I don't think this is absolute relocation, see (%rip).

The kernel was compiled with gcc-14.2.1, so clang specific issue was not tested.

Uros.

View attachment "p.diff.txt" of type "text/plain" (3822 bytes)