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

[David Woodhouse]

On 17 December 2024 10:25:56 CET, Ard Biesheuvel <ardb at kernel.org> wrote:
>On Tue, 17 Dec 2024 at 10:17, David Woodhouse <dwmw2 at infradead.org> wrote:
>>
>> 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?
>
>Every absolute relocation produces an entry in the relocation table
>that needs to be applied at every boot when KASLR is in effect. Beyond
>that, it doesn't matter.
>
>I've looked into PIC codegen/PIE linking for the core kernel, which is
>why this caught my eye. If that effort ever advances, I'll need to
>revisit this code as well and apply the change I suggested.

Ack, I'll roll it in to the next revision if Ingo hasn't rounded this set up of fixes up already. (On which topic, since I took the easy way out of the CFO thing, this patch isn't strictly needed to fix a regression in tip/x86/boot so can be dropped for now too.)

Thanks.