Re: [PATCH v6 2/7] x86/kexec: Debugging support: load a GDT

[David Woodhouse]

On 23 February 2025 09:53:07 GMT, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>* David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>
>> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> 
>> 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@xxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/relocate_kernel_64.S | 27 +++++++++++++++++++++++++++
>>  1 file changed, 27 insertions(+)
>> 
>> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
>> index af2cd06ff318..c62f03808f18 100644
>> --- a/arch/x86/kernel/relocate_kernel_64.S
>> +++ b/arch/x86/kernel/relocate_kernel_64.S
>> @@ -39,6 +39,18 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
>>  SYM_DATA(kexec_pa_swap_page, .quad 0)
>>  SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
>>  
>> +#ifdef CONFIG_KEXEC_DEBUG
>> +	.balign 16
>> +SYM_DATA_START_LOCAL(kexec_debug_gdt)
>> +	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1
>> +	.long   0
>> +	.word   0
>> +	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
>> +	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
>> +	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
>> +SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
>> +#endif /* CONFIG_KEXEC_DEBUG */
>
>Yeah, so is there any reason (other than paranoia) why the early-early 
>GDT and IDT shouldn't be unconditional? There's many ways for such an 
>approach to bitrot, it's much better to not hide it behind a 
>default-disabled debug option...
>
>Some of the other bits, like the hard-coded serial debugging 
>assumptions, probably need to be behind the debug option - but much of 
>the new debug mechanism looks safe and generic and can be always-on, 
>IMHO.
>
>This would also throw regressions back into the face of whoever manages 
>to introduce them, ideally. ;-)
>
>Thanks,
>
>	Ingo

Makes sense to me. I was just trying to be as unobtrusive as possible. In a test branch where I was trying to fix up the objtool vs. CFI pain, I did move the IDT/GDT setup entirely into the ASM code and remove the C code which clears them (before the call into relocate_kernel() which might now trap if we remove the __nocfi hack). I never did get objtool to tolerate both clang and GCC builds though.

I think even the serial output (tied as it is to earlyprintk setup) could reasonably be enabled by default too.