Re: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

H.J. Lu
2020-01-30T17:52:17Z

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On Mon, Jan 27, 2020 at 3:34 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Jan 24, 2020 at 10:18:19AM -0800, H.J. Lu wrote:
> > With the command-line option, -mx86-used-note=yes, 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 x86 kernel linker script only contains a signle NOTE segment:
> >
> > PHDRS {
> > text PT_LOAD FLAGS(5);
> > data PT_LOAD FLAGS(6);
> > percpu PT_LOAD FLAGS(6);
> > init PT_LOAD FLAGS(7);
> > note PT_NOTE FLAGS(0);
> > }
> > SECTIONS
> > {
> > ...
> > .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
> > e.*)) __stop_notes = .; } :text :note
> > ...
> > }
> >
> > which may not be incompatible with note.gnu.property sections. Since
> > note.gnu.property section in kernel image is unused, this patch discards
> > .note.gnu.property sections in kernel linker script by adding
> >
> > /DISCARD/ : {
> > *(.note.gnu.property)
> > }
>
> I think this is happening in the wrong place? Shouldn't this be in the
> DISCARDS macro in include/asm-generic/vmlinux.lds.h instead?

Please read my commit message closely. We can't discard .note.gnu.property
sections by adding .note.gnu.property to default discarded sections
since default
discarded sections are placed AFTER .notes sections in x86 kernel
linker scripts.

> > before .notes sections. Since .exit.text and .exit.data sections are
> > discarded at runtime, it undefines EXIT_TEXT and EXIT_DATA to exclude
> > .exit.text and .exit.data sections from default discarded sections.
>
> This looks like a separate issue (though maybe related to DISCARDS)?

Here is the updated patch without EXIT_TEXT and EXIT_DATA change.
I will submit a separate patch for it.

Thanks.

--
H.J.

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From 6397008c2c295cd58a052acf9fc6411a3d68e46d Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 30 Jan 2020 09:25:54 -0800
Subject: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

With the command-line option, -mx86-used-note=yes, 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 x86 kernel linker script only contains a signle NOTE segment:

PHDRS {
text PT_LOAD FLAGS(5);
data PT_LOAD FLAGS(6);
percpu PT_LOAD FLAGS(6);
init PT_LOAD FLAGS(7);
note PT_NOTE FLAGS(0);
}
SECTIONS
{
...
.notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
e.*)) __stop_notes = .; } :text :note
...
}

which may not be incompatible with note.gnu.property sections. Since
note.gnu.property section in kernel image is never used, this patch
discards .note.gnu.property sections in kernel linker script by adding

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

before .notes sections. NB: We can't discard .note.gnu.property sections
by adding .note.gnu.property to default discarded sections since default
discarded sections are placed AFTER .notes sections in x86 kernel linker
scripts.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
---
arch/x86/kernel/vmlinux.lds.S | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index e3296aa028fe..d1b942365d27 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -150,6 +150,11 @@ SECTIONS
_etext = .;
. = ALIGN(PAGE_SIZE);

+ /* .note.gnu.property sections should be discarded */
+ /DISCARD/ : {
+ *(.note.gnu.property)
+ }
+
X86_ALIGN_RODATA_BEGIN
RO_DATA(PAGE_SIZE)
X86_ALIGN_RODATA_END
--
2.24.1