[PATCH] Discard .note.gnu.property sections in generic NOTES

From: "H.J. Lu" <hjl.tools@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Kees Cook <keescook@chromium.org>,
	Thomas Lendacky <Thomas.Lendacky@amd.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Arnd Bergmann <arnd@arndb.de>,
	linux-arch@vger.kernel.org, Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: [PATCH] Discard .note.gnu.property sections in generic NOTES
Date: Fri, 27 Mar 2020 05:11:01 -0700	[thread overview]
Message-ID: <20200327121101.948934-1-hjl.tools@gmail.com> (raw)

**"this patch depends on patch Add RUNTIME_DISCARD_EXIT to generic DISCARDS"**

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:

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
...
}

The NOTE segment generated by kernel 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@gnu-skx-1 linux]$ readelf -n vmlinux

Displaying notes found in: .notes
  Owner                Data size Description
  Xen                  0x00000006 Unknown note type: (0x00000006)
   description data: 6c 69 6e 75 78 00
  Xen                  0x00000004 Unknown note type: (0x00000007)
   description data: 32 2e 36 00
  xen-3.0              0x00000005 Unknown note type: (0x006e6558)
   description data: 08 00 00 00 03
readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50
readelf: Warning:  type: 0xffffffff, namesize: 0x006e6558, descsize:
0x80000000, alignment: 8
[hjl@gnu-skx-1 linux]$

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)
}

before kernel NOTE segment in generic NOTES.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 include/asm-generic/vmlinux.lds.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 6b943fb8c5fd..6659a7c07c84 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -818,7 +818,14 @@
 #define TRACEDATA
 #endif
 
+/*
+ * Discard .note.gnu.property sections which are unused and have
+ * different alignment requirement from kernel note sections.
+ */
 #define NOTES								\
+	/DISCARD/ : {							\
+		*(.note.gnu.property)					\
+	}								\
 	.notes : AT(ADDR(.notes) - LOAD_OFFSET) {			\
 		__start_notes = .;					\
 		KEEP(*(.note.*))					\
-- 
2.25.1

