How to test preserve_context mode?

[David Woodhouse]

On Mon, 2024-11-18 at 09:54 +0800, Dave Young wrote:
> Hi David,
> 
> I did not see anyone around me test this.   The kexec jump feature
> was
> introduced by Ying [cced him] in below kernel comment, probably you
> can try the steps mentioned in the commit log:
> commit 3ab83521378268044a448113c6aa9a9e245f4d2f
> Author: Huang Ying <ying.huang at intel.com>
> Date:   Fri Jul 25 19:45:07 2008 -0700
> 
>     kexec jump

Thanks. I built an object which contains only a single 'ret'
instruction, but I couldn't get kexec-tools to load that without
loading a bunch of its own purgatory too, which breaks things. So I did
it manually:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <linux/kexec.h>
#include <linux/reboot.h>
#include <sys/reboot.h>
#include <sys/syscall.h>

int main (void)
{
        struct kexec_segment segment = {};
	unsigned char purgatory[] = {
		0x66, 0xba, 0xf8, 0x03,	// mov $0x3f8, %dx
		0xb0, 0x42,		// mov $0x42, %al
		0xee,			// outb %al, (%dx)
		0xc3,			// ret
	};
	int ret;

	segment.buf = &purgatory;
	segment.bufsz = sizeof(purgatory);
	segment.mem = (void *)0x400000;
	segment.memsz = 0x1000;
	ret = syscall(__NR_kexec_load, 0x400000, 1, &segment, KEXEC_PRESERVE_CONTEXT);
	if (ret) {
		perror("kexec_load");
		exit(1);
	}
	return 0;
}

That works, prints a 'B' to the serial port, then returns to the
kernel... which immediately crashes. Fixed thus, and now I can actually
test that my own changes are breaking it any *more*...

From: David Woodhouse <dwmw at amazon.co.uk>
Subject: [PATCH] x86/kexec: Restore GDT on return from preserve_context kexec

The restore_processor_state() function explicitly states that "the asm code
that gets us here will have restored a usable GDT". That wasn't true in the
case of returning from a preserve_context kexec. Make it so.

Without this, the kernel was depending on the called function to reload an
appropriate GDT.

Signed-off-by: David Woodhouse <dwmw at amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index ca01e3e2f097..ed2ae50535dd 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -252,6 +252,11 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
 	movq	CR0(%r8), %r8
 	movq	%rax, %cr3
 	movq	%r8, %cr0
+
+	/* Saved in save_processor_state. */
+	movq    $saved_context, %rax
+	lgdt    saved_context_gdt_desc(%rax)
+
 	movq	%rbp, %rax
 
 	popf
-- 
2.47.0

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