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@xxxxxxxxx>
> 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@xxxxxxxxxxxx>
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@xxxxxxxxxxxx>
---
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
<<attachment: smime.p7s>>
