linux-kernel - Re: [PATCH] x86: Discard .note.gnu.property sections in vDSO

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Date:   Fri, 27 Mar 2020 05:07:41 -0700
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH] x86: Discard .note.gnu.property sections in vDSO

On Fri, Mar 27, 2020 at 3:44 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Mar 26, 2020 at 10:43:14AM -0700, H.J. Lu wrote:
> > With the command-line option, -mx86-used-note=yes, the x86 assembler
>
> I see:
>
>        -mx86-used-note=no
>        -mx86-used-note=yes
>            These options control whether the assembler should generate
>            GNU_PROPERTY_X86_ISA_1_USED and GNU_PROPERTY_X86_FEATURE_2_USED GNU property
>            notes.  The default can be controlled by the --enable-x86-used-note
>            configure option.
>
> Is there a plan to use this build option in the kernel in the future or
> all binutils will have it enabled or what's the story here?
>
> Because I don't see -mx86-used-note used anywhere in the kernel...

-mx86-used-note=yes can be enabled by default at binutils configure time:

[hjl@...-cfl-2 ~]$ as --help | grep mx86-used-note
  -mx86-used-note=[no|yes] (default: yes)
[hjl@...-cfl-2 ~]$

I am changing the commit log to

---
With the command-line option, -mx86-used-note=yes, which can also be
enabled at binutils build time with

  --enable-x86-used-note  generate GNU x86 used ISA and feature properties

the x86 assembler in binutils 2.32 and above generates a program property
note in a note section, .note.gnu.property, to encode used x86 ISAs and
features.  But kernel linker script only contains a single NOTE segment:
---

> > in binutils 2.32 and above generates a program property note in a note
> > section, .note.gnu.property, to encode used x86 ISAs and features.  But
> > x86 kernel vDSO linker script only contains a single NOTE segment:
> >
> > PHDRS
> > {
> >  text PT_LOAD FLAGS(5) FILEHDR PHDRS; /* PF_R|PF_X */
> >  dynamic PT_DYNAMIC FLAGS(4); /* PF_R */
> >  note PT_NOTE FLAGS(4); /* PF_R */
> >  eh_frame_hdr 0x6474e550;
> > }
> >
> > The NOTE segment generated by vDSO linker script is aligned to 4 bytes.
> > But .note.gnu.property section must be aligned to 8 bytes on x86-64 and
> > we get
> >
> > [hjl@...-skx-1 vdso]$ readelf -n vdso64.so
> >
> > Displaying notes found in: .note
> >   Owner                Data size      Description
> >   Linux                0x00000004     Unknown note type: (0x00000000)
> >    description data: 06 00 00 00
> > readelf: Warning: note with invalid namesz and/or descsz found at offset 0x20
> > readelf: Warning:  type: 0x78, namesize: 0x00000100, descsize: 0x756e694c, alignment: 8
> > [hjl@...-skx-1 vdso]$
> >
> > Since note.gnu.property section in vDSO is not checked by dynamic linker,
> > this patch discards .note.gnu.property sections in vDSO by adding
>
> Avoid having "This patch" or "This commit" in the commit message. It is
> tautologically useless.

I am changing it to

Since note.gnu.property section in kernel image is never used, discard
.note.gnu.property sections in kernel linker script by adding

/DISCARD/ : {
  *(.note.gnu.property)
}

> Also, do
>
> $ git grep 'This patch' Documentation/process
>
> for more details.

Thanks.

-- 
H.J.