From: David Woodhouse <dwmw@amazon.co.uk>
There are some failure modes which lead to triple-faults in the
relocate_kernel function, which is fairly much undebuggable for normal
mortals.
Adding a GDT in the relocate_kernel environment is step 1 towards being
able to catch faults and do something more useful.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/kernel/relocate_kernel_64.S | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 9b6ed08e00ea..2af4ce593645 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -131,6 +131,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
/* store the start address on the stack */
pushq %rdx
+#ifdef DEBUG
+ /* Create a GDTR (16 bits limit, 64 bits addr) on stack */
+ leaq .Lreloc_kernel_gdt(%rip), %r8
+ pushq %r8
+ pushw (%r8)
+
+ /* Load the GDT, put the stack back */
+ lgdt (%rsp)
+ addq $10, %rsp
+
+ /* Test that we can load segments */
+ movq %ds, %rax
+ movq %rax, %ds
+#endif /* DEBUG */
+
/*
* Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
* below.
@@ -331,5 +346,16 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
int3
SYM_CODE_END(swap_pages)
+#ifdef DEBUG
+.Lreloc_kernel_gdt:
+ .word 1f - .Lreloc_kernel_gdt - 1
+ .long 0
+ .word 0
+ .quad 0x00cf9a000000ffff /* __KERNEL32_CS */
+ .quad 0x00af9a000000ffff /* __KERNEL_CS */
+ .quad 0x00cf92000000ffff /* __KERNEL_DS */
+1:
+#endif /* DEBUG */
+
.skip KEXEC_CONTROL_CODE_MAX_SIZE - (. - relocate_kernel), 0xcc
SYM_CODE_END(relocate_range);
--
2.44.0