Re: [BUG] broken ebba638ae723d8a8fc2f7abce5ec18b688b791d7

["H. Peter Anvin" <hpa@zytor.com>]

[Adding Jeremy]

Jeremy, would this break Xen?  As far as I know, Xen is the main user of 
skipping segment register initialization.

	-hpa


On 01/27/2011 06:24 PM, H. Peter Anvin wrote:
> On 01/27/2011 03:00 PM, Kees Cook wrote:
>>
>> Yikes, good catch.
>>
>> arch/x86/kernel/trampoline_64.S uses:
>> movw $(trampoline_stack_end - r_base), %sp
>>
>> arch/x86/boot/compressed/head_64.S uses:
>> movl $boot_stack_end, %eax
>> addl %ebp, %eax
>> movl %eax, %esp
>>
>> what would be safe for arch/x86/kernel/head_32.S ? It uses "stack_start",
>> but later after paging set-up. Is the following sane to solve this?
>>
>
> To run it before paging is set up, you can't use stack, start; you have
> to use a pointer based on physical address. You have two problems with
> using stack_start: you're using a linear address to access stack_start,
> and stack_start itself contains a linear address.
>
> It's not entirely clear to me why we don't initialize %ss to __BOOT_DS
> with the other segment registers, but it would make most sense to me:
>
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index fc293dc..c10f9ba 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -99,7 +99,12 @@ ENTRY(startup_32)
> movl %eax,%es
> movl %eax,%fs
> movl %eax,%gs
> + movl %eax,%ss
> 2:
> +/*
> + * Set up an initial stack
> + */
> + movl $pa(init_thread_union+THREAD_SIZE), %esp
>
> /*
> * Clear BSS first so that there are no surprises...
>
> -hpa