Re: [PATCH 8/9] x86/kexec: Cope with relocate_kernel() not being at the start of the page

[David Woodhouse]

On 17 December 2024 09:47:36 CET, Ard Biesheuvel <ardb at kernel.org> wrote:
>On Tue, 17 Dec 2024 at 00:37, David Woodhouse <dwmw2 at infradead.org> wrote:
>>
>> From: David Woodhouse <dwmw at amazon.co.uk>
>>
>> A few places in the kexec control code page make the assumption that the
>> first instruction of relocate_kernel is at the very start of the page.
>>
>> To allow for Clang CFI information to be added to relocate_kernel(), as
>> well as the general principle of removing unwarranted assumptions, fix
>> them to use the external __relocate_kernel_start symbol that the linker
>> adds. This means using a separate addq and subq for calculating offsets,
>> as the assembler can no longer calculate the delta directly for itself
>> and relocations aren't that versatile.
>>
>
>You can still avoid the absolute relocations though, ...
...
>> +       addq    $identity_mapped, %rsi
>> +       subq    $__relocate_kernel_start, %rsi
>
>... if you turn this into
>
>0:     addq    $identity_mapped - 0b, %rsi
>       subq    $__relocate_kernel_start - 0b, %rsi

Is there any benefit to doing so? Are absolute relocations problematic?