[PATCH] x86: Add a "x86" ELF note namespace

[Borislav Petkov <bp@alien8.de>]

On Wed, Nov 22, 2023 at 12:58:26PM +0100, Borislav Petkov wrote:
> On Wed, Nov 22, 2023 at 10:15:42AM +0100, Linux regression tracking (Thorsten Leemhuis) wrote:
> > [1] unless you fiddle with things obviously internal; not sure if this
> > case would qualify for him, but somehow I doubt it -- but I might be
> > wrong there.
> 
> Well, think about it - by that logic, if CONFIG_* items are an ABI, we
> will never ever be able to change any of them. Now that would be awful.
> 
> > Any progress on this?
> 
> We're thinking...

Turns out this is easier than I think and people have solved this
problem already - all I need to do is use it. Wonderful.

Lemme Cc initramfs@vger.kernel.org as an FYI and see whether dracut
folks would have any comments about this.

---
From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Wed, 22 Nov 2023 13:59:40 +0100

Add a "x86" ELF note namespace to put ELF note structures with which to
communicate in a ABI-compliant manner what a kernel image supports.

Also, add the first note type of this - X86_ELFNOTE_MICROCODE - which
denotes that microcode support is built into the kernel image and thus
initrd-generating tools like dracut can parse the ELF .notes section for
this.

$ readelf -n vmlinux

Displaying notes found in: .notes
  Owner                Data size        Description
  ...
  x86                  0x00000004       Unknown note type: (0x00000000)
  description data: 01 00 00 00
  ^^^^^^^^^^^^^^

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
 arch/x86/include/uapi/elfnote.h      | 18 ++++++++++++++++++
 arch/x86/kernel/cpu/microcode/core.c |  5 +++++
 2 files changed, 23 insertions(+)
 create mode 100644 arch/x86/include/uapi/elfnote.h

diff --git a/arch/x86/include/uapi/elfnote.h b/arch/x86/include/uapi/elfnote.h
new file mode 100644
index 000000000000..bef26c4944e8
--- /dev/null
+++ b/arch/x86/include/uapi/elfnote.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _X86_UAPI_ELFNOTE_H_
+#define _X86_UAPI_ELFNOTE_H_
+
+/*
+ * "x86" namespaced ELF note structures to communicate features
+ * supported by the kernel binary to external utilities which need that
+ * info in order to do additional preparatory work based on the target
+ * kernel image.
+ */
+
+/*
+ * Used by the microcode loader to communicate support to external
+ * initrd generators like dracut.
+ */
+#define X86_ELFNOTE_MICROCODE	0
+
+#endif /* _X86_UAPI_ELFNOTE_H_ */
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 232026a239a6..f35444bafbbc 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -24,6 +24,7 @@
 #include <linux/capability.h>
 #include <linux/firmware.h>
 #include <linux/cpumask.h>
+#include <linux/elfnote.h>
 #include <linux/kernel.h>
 #include <linux/delay.h>
 #include <linux/mutex.h>
@@ -32,6 +33,8 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 
+#include <uapi/elfnote.h>
+
 #include <asm/apic.h>
 #include <asm/cpu_device_id.h>
 #include <asm/perf_event.h>
@@ -859,3 +862,5 @@ static int __init microcode_init(void)
 
 }
 late_initcall(microcode_init);
+
+ELFNOTE32("x86", X86_ELFNOTE_MICROCODE, CONFIG_MICROCODE);
-- 
2.42.0.rc0.25.ga82fb66fed25

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette